C recursive variadic macro. Hot Network Questions I haven't tested it, though.
C recursive variadic macro What it apparently does is create an end-recursive variadic macro. : void COMMON_Print(const char* fmt, ) Then you can define a va_list and use va_start to set it:. Plugging the argument count into a dispatcher produces an overloaded macro. 4. chqrlie. I also have a variant that recognizes multiple different subsystems, each of which is given a name and its own debugging level, so that I can use -D input=4,macros=9,rules=2 to set the debug level of the input system to 4, the macros system to 9 (undergoing intense scrutiny) and the rules system to 2. /* * The PP_NARG macro evaluates to the number of arguments that have been * passed to it. Stack Overflow. I want to implement a variadice macro in C/C++, which could be unrolled as a piece of code. Variadic macros in C. C Function that calculates total sizeof arguments. So it can be done. The problem with this is that CALL is a macro, and you cannot nest variadic macro invocations within a ##__VA_ARGS__ list. 4 paragraph 2. Given a variadic macro like: #define FIELD_DECLARATION(NAME, OTHER_FIELD, ) FIELD_DECLARATION(First, Thing) FIELD_DECLARATION(Second, Thing, Thing, Nothing) The code below is working for what you've asked for with up to 1024 arguments and without using additional stuff like boost. An object-like macro is a simple identifier that will be replaced by a code fragment. Only after the substitution into EXPAND is the macro called, by which point the variadic arguments are already substituted. Passing template parameters into macros in MSVC. The major compilers all support it so IMHO just use it, and if the situation ever arises where you have to mangle your code to support some rare but conforming compiler, cross that bridge when you come to it. CHAIN_COMMA(MY_CHAIN) // alpha, beta, gamma In fact, in current a recursive use of a macro never expands but keeps the name of the macro intact. Improve this answer. The reason why it works when the inner macro is passed as a named argument is So I've got a macro that works nicely in GCC, but not in Microsoft's C++ Compiler. Unfortunately, the inner FOR_EACH did not get expanded. I would like to make a macro F taking a variable number of parameters, that expands to the parameters each separated by a ||. All macro replacement takes place when a macro is invoked. Here is GCC’s page on self-referential macros. So it's literally impossible to use a recursive call, since the actual implementation that will house all this will be expanded at compile time; and you cannot recruse a macro. Code: If you need to pass on variadic arguments, If this is a problem for your logic, you can copy the whole argument list with the va_copy macro. A comma that is not inside such A well-known and portable way to suppress C compiler warnings about unused variables is (see unused parameter warnings in C code):. h A set of macros building on those defined in pp_iter. In particular, they rely on several hundred numbered macros which compensate for the fact that the C preprocessor does not allow recursive macro expansion. You need to use va_start, va_arg and va_end from stdarg. Fusion? 4. What you have will expand to: X,Y::do(); And not to. You need to unfold the recursion yourself and use two different macros (or whatever your desired recursion depth is). S. 3 Macro Replacement, ¶4, ¶12). And another problem with that is stop condition of recursion. F(a, b, c) should expand to a || b || c, F(a) should expand to just a, etc. Continuing on the theme are these macros: /* * Turn A into a string literal without expanding macro definitions * (however, if invoked from a macro, macro arguments are expanded). ) providing the ability to swap out which variadic macro is utilized for this recursion process So CPP retains the comma when conforming to a specific C standard, and drops it otherwise. Note, that I need a solution, where b and <some_parameters> are evaluated only when bool(a) is true . It seems that they will be in the next version. Surprising expansion of variadic GNU C preprocessor macros in the presence of the ## operator. you can't call it via func<>();. Scope. Unpacking that pack is more difficult than it should be, so we use a recursive template to iterate over all the parameters one by one. There is no way with C-Macros to wrap each indiviual argument from a __VA_ARGS__ list with another macro and especially no way to split them into a list of pairs. Follow edited May 23, 2017 at 12:17. #define MY_CHAIN (alpha) (beta) (gamma) into Consider what the invocation of the recursive call func<B>(); looks like when B is empty. Generally speaking, "write it in C++" is not a useful answer __VA_ARGS__ is only for macros; variadic functions are rather different. with ##, zero argument is OK but macro cannot be evaluated recursively. I assume you had pretty much the same problem. c; macros; variadic-macros; Share. , passing a variable number of arguments to functions such as printf Is it possible to iterate over arguments in variadic macros?, which explains how to iteratively apply a macro to each of the arguments of the without ##, I have to supply at east one argument, but macro can be recursively expanded. 3. Macros I'm trying to make a recursive function with variable args in C, and I can't seem to pass the args when writing the recursive call. CREATE_CLASS(Person, (const char*, name), (int, age), (float, height)) This is going to be the easier option to work with, since the preprocessor syntax handles balanced parentheses, in a way so that e. The string you supplied for %s can't itself contain format specifiers (that will be interpreted). With the use of that, your apply() looks like: #define apply()EVAL(MAP(apply_, With little syntax change (MACRO can stringify, but cannot unstringify), your usage might be: def_name(, a, b) def_name(_, a, b, c) You might do, with some upper bound limit: Because forming function types ignores extra parentheses, you can use this macro with or without parentheses where the type name doesn't include a comma: FOO((int), int_var); FOO(int, int_var2); In C, of course, this isn't necessary because type names can't contain commas outside parentheses. 2. Sadly, i didn't found a way to make it recursive so that a limited number of macros can be used for an arbitrary number of arguments, the only way i found was to define a macro for each possible number of arguments. When using variadic macros you need __VA_ARGS__ to expand all the arguments. The below example defines a function similar to std::printf, that replace each occurrence of the character % in the format string with a value. 7. Commented Nov 2, 2022 at 23:00. Presumabely it just separates arguments to a function call, but since macros works with just text you can actually use __VA_ARGS__ in other contexts as well, where a comma-separated list makes sense. The input: NEST_RECURSIVE(A, B, C) There were several questions here regarding variadic macros in C. Macro using variable arguments. The value of N depends on the number of arguments you have passed because N becomes the 11th element in the macro expansion (because of the 10 place holders). The preprocessor is dumb and doesn't understand C++ syntax or semantics. Every serious programmer knows it. It's a bit of work, but you can accomplish some fairly impressive things using it. Otherwise the comma is dropped as an extension to the standard. People will rightly curse at you if you do. I know there's a standard trick but I can't seem to find it. #define PRINT () CAT (CT (__VA_ARGS__)) (__VA_ARGS__) #define CT () VALS (__VA_ARGS__, 5, 4, 3, 2, 1) #define VALS (N1, N2, How do I make a recursive variadic macro, with the condition that the recursion ends when only one argument remains? The code so far is: #define Iterates over a set of variadic macro arguments and applies a provided transformation macro TF(ARG, IDX) to each argument ARG and index IDX. In GCC they are included simply because since they are part of C99, the logic is already implemented, so it must have seemed a good idea to let the C++ folk benefit :) "This is a language constraint, so not only does C not define the behavior of a program containing an invocation of a variadic macro with an empty VA list, but a conforming implementation will issue a diagnostic about it. For example, the recursive macros uses a recursion state so only the number of scans needed are applied, which is more efficient than the EVAL macro which always applies lots of scans even if they are not needed. See this answer for a hint on how to mix variadic macros with variadic templates; you will need a macro to expand __LINE__ at the correct time. This will help you to achieve what you need. As you want to hide the platform specific macro call from the API header you can process the function arguments with vsnprintf instead of vprintf and call the logging macro with format "%s" and the resulting string buffer. P. Invalid argument count in a variadic macro. Detecting 0 may indeed be handy in some cases, but the solutions seem to either be less general (requiring that first token to be pasteable; which may or may not be okay depending on what you're using it for), or Edit: I saw this post about variadic macro- but It's different from what I'm asking here since the argument number needs to be a recognized variable, which will be treated in the implementation of PRINT as -1 if the call to PRINT doesn't contain the number argument (-1 will be an indicator for printing no number) and otherwise it will print the number prefix. – n. The ## operator does exact token substitution, so in that case it's trying to send the token "CALL(func1, false)" as the last argument to the func1 C function. " But it compiles C - Variadic macro which expands into set of macro calls on each argument. ) Now what if I wanted to change my STATIC_ASSERT macro to take a message like the standard C++ static_assert, but adding a prefix to the message? C++11 adds variadic argument lists for macros. – Weather Vane. I've been studying variadic classes and methods in C++ and am trying to figure out the recursion. e. c -o rootTESTOBJECT I get the compiler warning root. Is there a way to extract every second parameter from a list of variadic parameters in C/C++ preprocessor? I would like to write a macro to generate boilerplate code for interface methods in the following way: # In C, is it possible to concatenate each of the variable arguments in a a variadic macro? Example: MY_MACRO(A, B, C) // will yield HDR_A, HDR_B, HDR_C MY_MACRO(X, Y) // will yield HDR_X, HDR_Y The normal ## operator has special meaning for variadic macros (avoiding the comma for empty argument list). Note that the loop is unrolled anyway, so it's functionally equal to a recursive tail-call-optimized function or a recursively expanded macro. The Overflow Blog Developers want more, more, more: the 2024 results from Stack Overflow’s MSVC doesn't seem to resolve recursive macros the same way GCC does. The problem however is that those arguments are comma-separated. First, your function needs at least one named parameter, e. Coord that contains two static member variables, then pass it as an argument of variadic template function variadic_print_coord(), how do I unpack the variadic expressions, to call the print_pair() function that shown below. How can I create simple macro definitions (name + value) that has a common prefix for all the macros? The goal is to be able to make multiple copies of the source file and compile them with different flags so all versions can be linked together into the same final binary without symbol/macro name collisions (the macros will later be moved into header files). buildsareplacementtokenlistforeachofT’sarguments. Now taking the approach from https:// If you do want the G(a, b, c) syntax, then because macro replacement is not recursive, I think you'll likely need one macro per number of arguments that you are going to pass. Recursive macros are a powerful feature of the C preprocessor. I'm sure t Generating code in batch by using macro in C/C++. A simple workaround is to Types Of Macros in C. It should be fully compatible with C. There are no recursive macros in C. 245. replace]/4:. 34. I think I know what I need, but I couldn't come up with a working solution just yet. I'd recommend looking at using the Boost. Variadic macros with zero arguments doesn't compile even with ##__VA_ARGS__ 0. But if you are going to do something GCC specific anyway, then why not use the old-fashioned GCC extension: Some C macros (and an associated code generator) to count and iterate over variadic macro arguments. I call it a variadic X-macro. For instance : MACRO(x) would print x = 123 MACRO(x,y) would print x,y = 123,666 A better macro would be more readable C preprocessor, recursive macros. It defines an EVAL() and also a MAP(m, first, ) macro to do recursion and to use for each iteration the macro m with the next parameter first. Exceptions arise from the rules of scanning and replacement. C variadic macro. While this might sound confusing, with well-constructed examples, their utility and simplicity become evident. As of variadic macros with zero arguments, there are tricks à la boost to detect this and to macro-program around it. I know that variadic macros have been added in C99 (and via GNU extensions). 0. This is because if the expansion of a macro is self-referential, the inner occurence of the macro is not expanded. h> #define Option_VariadicMacro(f (C) variadic functions in general, not limited to printf-like functions such as their example. CpptakesT’ssubstitutionlistand,if T I want to create recursive variadic macro UNREFERENCED_PARAMETERS. However, @Ise Wisteria suggested the Variadic macros are function-like macros that contain a variable number of arguments. It may not be used as a macro name, macro argument name, or within a different type of I would like to define a macro with a variable number of parameters which prints the name and value of each given parameter. could be an AI. Wrapping variadic arguments in a c macro. Standard C, as of C99, supports only the form where the parameter is called __VA_ARGS__. You can't "nest" format specifiers: it isn't recursive. I'm wrapping it with a macro that counts the number of arguments, as suggested here. ) What's the trick to create a variadic macro FOO(a1, a2, a3,, an) such that it expands to FOOn(a1, a2, a3,, an) for values of n in whatever preselected bounded range you choose? That is, FOO(a) should expand to FOO1(a), FOO(a, b, c) to FOO3(a, b, c), etc. Hello, Boost/Preprocessor!↩︎. Assuming the enum entries have default values, they would then map as indices in the vector. This is called pattern matching, a technique to destructure a sum type (tagged union) into its respective components. A recursive macro is a macro that re-expands itself during its own replacement process. Preprocessor library for automatic code generation. Recursive Macros. ↩︎. ,##____VA_ARGS____ as answered in Variadic macros with zero arguments. Note that with this particular code you can count only up to 10 I am having trouble doing this, as doing recursive variadic macros is not exactly legal. Examples can also be seen here and here. The advisory makes it clear that functions are nicer than macro functions, and cites the section in C Traps and Pitfalls, Andrew Koenig (1988) on comparing macro functions with functions, but describes it as a preference, and specifically over the short macro functions designed for "speed advantage". The syntax for defining the macro is similar to that of a function. In your \Tuple example, \ensuremath would read the following argument \bgroup and make sure that this argument Using Paul Fultz II's solution in the post C-preprocessor recursive macro, I'd like to expand an unlimited number of parenthesized macro arguments, e. For instance, you could do what you want this way: Feature-test macro Value Std Feature __cpp_variadic_templates: 200704L (C++11) Variadic templates Example. Instantly share code, notes, and snippets. This is due to the fact that the C preprocessor always expands arguments. If you’d pass a list A variadic macro is a feature of some computer programming languages, especially the C preprocessor, whereby a macro may be declared to accept a varying number of arguments. c++; c; macros; foreach; Share. – Paul Fultz II. Object-Like Macros. HAS_COMMA(_TRIGGER_PARENTHESIS_ __VA_ARGS__ (/*empty*/) is the main check, did you notice that this is TEST() ? In fact, the remaining three macro are There isn't any way to expand a macro into a replacement list. Another useful explanation of how to do iteration in the C preprocessor is found in the documentation for the Boost Preprocessor Library, particularly the topic on reentrancy. Remarks To use variadic macros, the ellipsis may be specified as the final formal argument in a macro definition, and the replacement identifier __VA_ARGS__ may be used in the definition to insert the extra arguments. h> #include <stdarg. Macros are defined using the #define directive and are processed by the preprocessor before the actual compilation begins. c; macros; c-preprocessor; Share. How to make a variadic macro (variable number of arguments) 5. I am using variadic macros along with variadic template parameters to achieve this. Italsorememberstheoriginal, non-macro-expandedargumentsforusewith# and##. Understanding Macro Basics. We're using GCC (G++) 4. This is a recursive macro, which is illegal. Variadic macros. I was looking at those, that saves me from having to add a wrapper around the function that actually does the write. Macro recursive expansion to Macro expansion does not work like argument pack expansion with variadic templates. I need to write a variadic macro in C which must take zero or more arguments. I know C doesn't support recursive macros. Skip to content. I've come up with something like this, but it's still kin Now the way the thing is designed is the function itself is constructed using a macro, despite the evil intentions, it's required, as this is an experiment. Only one comment: One of the most important features that comes with C++11 variadic templates are variadic template functions. – A common theme in these answers is that we need a GCC specific hack. Although it doesn't work for zero arguments. What is good practice is to get away from using macros for defining constants and doing some crazy C level coding stuff to create container templates. " Recursion and corecursion are forbidden by a specifically defined mechanism. ") Please, correct me, if I I am trying to learn preprocessor tricks that I found not so easy (Can we have recursive macros?, Is there a way to use C++ preprocessor stringification on variadic macro arguments?, C++ preprocessor Good, flexible ways to unpack the VA_ARGS from a variadic macro. But GENERATE_BUILDER only takes two arguments. qrdl qrdl. It requires that a block returns a value (which is true in c, on which glsl is based). 14159 # define SQUARE(x) ((x) * (x)) int main { double radius = 5. So, after playing around a bit more with the macros from Paul's answers and github wiki, I actually managed to produce a working BITS() macro which does what I intended. Variadic macros in C Hot Network Questions Why do most SAS troops keep wearing their new red berets even after being given permission to use their old beige ones? A variadic macro is a macro that can accept an unbounded sequence of arguments. This method will help you print variable's value along with it's name. When the preprocessor scans Variadic macros tricks. (See the GCC Manual Section 3. "It is really silly to compile your C code with a C++ compiler" => Not considered so by everyone (including me). In gcc, that can be achieved by adding "##" after the comma, e. It is a recursive macro that is scanned You can use the NARGS macro, which counts the arguments of variadic macros, to create recursive macros that apply a prefix to all your aguments: How to make a variadic macro (variable number of arguments) 0. h to handle them. Each opening parenthesis (must match a ). C++ Variadic Macros in Boost. Why does variadic macro fail with expected primary-expression before ‘)’ token unless preceded by a named argument? Hot Network Questions I know that "macro parameters are fully expanded before inserted in the macro body" but this does not seem to help me here as I don't understand what this means in the context of and __VA_ARGS__ Obviously __VA_ARGS__ binds to N and is only evaluated later. Hot Network Questions Will the first Mars mission force the space laundry question? Question. Optional Parameters with C++ Macros. 1: Prefer C++ to C, CPL. DO NOT USE IN PRODUCTION CODE. Is this type of feature only available in more high-level kind of languages such as Javascript? Edit: So far, this is what I How can I write a for/while loop inside a #define directive in C? Skip to main content. Any number of parameters could be passed into the macro, and the macro will be unrolled as the following examle: Variadic recursive preprocessor macros - is it possible? 146. To actually select between the two macros, the following MAP_NEXT macro compares a single list item against the special end-of-list marker (). (const char*, name) is a single argument to the macro despite containing a comma. I would like to implement something similar, with the difference being that the macro is expanded in a nested context. Can I have both problems solved at the same time? I don't think you will get a very useful __LINE__ from Log_Recursive. Viewed 1k times 5 . 144k 12 12 gold badges 130 130 silver badges 207 207 bronze badges. There's a common extension , ## __VA_ARGS__ which "eats" the comma if __VA_ARGS__ is blank. I am trying to write a macro which counts the number of specific elements in a sequence. Variadic recursive preprocessor macros - is it possible? 3. This makes them more useful for recursive, variadic macro situations. Modified 9 years, 5 months ago. Follow edited Oct 5, The definition of the macro is temporarily disabled to avoid infinite recursion. I was thinking something like #define F(a, ) a || Then you can then define a function to tokenize these arguments using the delimiting comma, thereby getting the set of tokenized strings by using the variadic macro: #define MY_VARIADIC_MACRO(X) tokenize_my_arguments(#X) Then there's actually no longer the dependency of having the variadic macro call a variadic function and I can iterate Let’s dig into these four HAS_COMMA macro and leave the others macro to later. See for instance C++ core guidelines: CPL. Macros don't directly expand recursively, but there are workarounds. Googel for empty macro arguments. A group of variadic macros. For reading convenience, here's the macro: I've tried to solve this with the same idea as BOOST_PP_VARIADIC_SIZE has, but it turned out this macro returns 1 for BOOST_PP_VARIADIC_SIZE() (empty arguments), which is not expected (I expected 0). I just made a new consoleApplication project and only changed the main . This property for recursive invocation can not be changed easily, because a lot of existing code would become invalid. Here is a snippet from A variadic macro is a feature of some computer programming languages, especially the C preprocessor, whereby a macro may be declared to accept a varying number of arguments. Improve this question. Ask Question Asked 9 years, 5 months ago. warning: anonymous variadic macros were introduced in C99 For which I don't see any answers. 2: If you must use C, use the common subset of C and C++, and compile the C code as C++. #define UNUSED(x) (void)(x) I'm looking for a way to generalize this to take multiple inputs (of different type): Please note that it works only for C99, because variadic macros were not supported in C89. Is it possible to avoid defining 32 macros, one for each arg count? It may not matter in this particular macro, but it's a good habit to put parentheses around your macros' parameters in your macro definitions. Why you are trying to merge both? – I want to use it together with variadic macros and the macro returning the number of macro arguments shown here. (like your name) when you compile with GCC there is the -pedantic option which should report the extensions used. A mini-preprocessor library to demostrate the recursive capabilites of the preprocessor - pfultz2/Cloak. Follow edited Mar 30, 2009 at 6:44. Share. 179. C variadic macro with two named parameters. Variable arguments in a I'd like to append a stringified macro argument to each element in a variadic macro. It is popularly used to replace a symbolic name with a numerical/variable represented as a Explanation [] #define directiveThe #define directives define the identifier as macro, that is instruct the compiler to replace most successive occurrences of identifier with replacement-list, which will be additionally processed. Are objects inside rvalue referenced object, also rvalue referenced? 0. Not sure, but C++0x allows you to omit encoding prefix for string literal concatenation. Sign in Product Say we want to write a IIF macro, we could write Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog Furthermore, if any nested replacements encounter the name of the macro being replaced, it is not replaced Therefore, recursive macros are not possible, not even just for two levels, let alone indefinitely many. Then, e. So CPP retains the comma when conforming to a specific C standard, and drops it otherwise. These include: How to make a variadic macro (variable number of arguments) which explains the basics, e. This method is completely type independent and supports variable number of I am trying to #stringify each token in a __VA_ARGS__ from a variadic macro. X::do(); Y::do(); As you hoped. About; Products While I agree that functions should be preferred to macros in almost all cases, a for loop's declaration (the first part) is Option C: Variadic Macro? Is there a way to define my operation such that the one-hot computation is reduced and evaluated at compile-time whenever possible? I'm aware that recursive macro definitions are discouraged since the pre-processor makes a single-pass. It is likely that GCC provides alternatives (gcc has non-standard mechanisms to handle 0 variable arguments, so g++ probably does too), but it isn't standard. Is it legal to invoke a variadic macro M with no arguments for its variadic parameter?. If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments (including those arguments consisting of no preprocessing tokens) in an invocation of a function-like macro shall equal the number of To get an overloaded macro, first we need a macro which selects between several implementations. It is called object-like because it looks like an object in code that uses it. I'm hard-pressed to think of what "C-only-isms" one really needs to make it worth not programming in the I have read this article, which illustrates how the __VA_OPT__ function macro can be used to recursively expand a macro. How to convert an enum type variable to a string? 256. Then a variadic macro which generically counts its arguments produces a selector. The GCC option -ftrack-macro-expansion=0 means not to Now with all these macros we can write a recursive WHILE macro. The variable parameter template ensures type safety through recursive expansion, but it may encounter recursive depth limitations. Hot Network Questions I haven't tested it, though. 5, Self-Referential Macros or the C Standard, §6. How to solve this problem? here is my macro definition #ifndef UNREFERENCED I have a variadic function from a third-party C library: int func(int argc, ); argc indicates the number of passed optional arguments. #define MY_CHAIN (alpha) (beta) (gamma) into a comma-separated list which can be passed to a variadic macro, e. So the result of the macro as written would be (a + AddUp(b, c)), which is not very helpful in this case. This part doesn't use a variadic macro. 4. Macro to repeat almost the same code. Variadic recursive preprocessor macros - is it possible? Related. I know I can have recursive lambdas but can't see a way to make it variadic. CPP retains the comma when conforming to a specific C standard. For portability to previous versions of GNU C you should use only named variable argument parameters. h:76:22: warning: ISO C does not permit named variadic macros Variadic macros are C99-only. All arguments you pass to REGIS are being passed to GENERATE_BUILDER, so in the second case that is 4 arguments. The following construct compiles in VisualStudio 2013. g. E. But if you are using GCC, you can use slightly modified macro: C-preprocessor recursive macro. Adding default arguments to variadic macro. Before we delve into advanced topics, let's quickly recap the basics of C macros. So one straightforward solution would be to provide wrapper macros With the use of __VA_ARGS__, the correct macro can be chosen depending on the number of arguments. Folded expressions provide a concise and efficient non-recursive scheme that avoids depth constraints and remains type-safe. Overloading Pre-processor macros for {zero, one} arguments. Variadic recursive preprocessor macros - is it possible? 6. 5. Please feel free to mark this question as a duplicate The C-style variable-length parameter works with C interfaces, but is not type-safe. pp_enum. One can say that PP_NARG() should return 1 for the same reason PP_NARG(,) should return 2. The text inside the (), the argument list, is scanned for parentheses and commas. 1. Hot Network Questions What is `acpi_pad` and how do I stop it taking up so much CPU? "PP_NARG() fails to return 0" isn't necessarily a problem. Follow edited Aug 4, 2022 at 7:11. So, for a cross-language macro you can write: C - Variadic macro which expands into set of macro calls on each argument. If the identifier is already defined as any type of macro, the program is ill-formed For those who are thinking about using X-macros, consider using the following format: (() ()) instead of: ( , , ). At the moment they are only conforming if you program in C99. Hot Network Questions Using the following command . I am also having some additional difficulty, as my previous method of using variadic templates coupled with std::forward, is not working in suppose I got a struct i. Fargs) // recursive variadic function {for (; * format ! = ' \0 '; format ++) The problem with your definition is that some macros require their argument in an explicitly braced broup. You could consider assuming no support for __cplusplus values lower than C++11, but Clang still gives the warning even when wrapped in such a check. Now, in the C code I used the C99 standard variadic macro to fix the problem, but in the C++ code, what is the correct answer? Using the same C99 style results in a different warning. C preprocessor recursion over variadic argument lists has quadratic complexity. It stipulates that there shall be at least one argument corresponding to the ellipsis (ISO/IEC 14882:2011 §16. There are two types of macros in C language: 1. AJMansfield AJMansfield. I want You could also cook up a more general solution using traditional recursive techniques, Using Paul Fultz II's solution in the post C-preprocessor recursive macro, I'd like to expand an unlimited number of parenthesized macro arguments, e. However, the compiler in my build system (beyond my control) does not understand the ,## syntax and so does not swallow the comma. It may not be used as a macro name, macro argument name, or within a different type of warning: variadic macros are incompatible with C++98 [-Wc++98-compat-pedantic] I don't know if this detection is even possible without C++11 variadic macro support. When expanded in a nested macro, the the compiler treats the whole arg list as one argument. variadic format strings in C macros? Hot Network Questions Children's novel about dolls with black eyes and black watch faces to mind control children Variadic macros where you specify the parameter name is a GNU C feature that has been supported for a long time. C variadic macro call another variadic macro. , both PPCAT_NX(s, 1) and PPCAT(s, 1) produce the identifier s1, unless s is defined as a macro, in which case PPCAT(s, 1) produces <macro value of s>1. Recursive macro expansion is not possible in C/C++. For CV_VA_NUM_ARGS(a,b,c,d,e,f), the 11th element is 6 and so on. # define PI 3. Here is an example: Have you ever wanted to write a “for each” loop over all the args of a variadic macro? Or have you ever wanted to overload a macro on the number of arguments? We can emulate recursion by taking advantage of argument prescanning; when the preprocessor encounters a function-like macro, it first expands its arguments, substitutes the Many C++ compilers now support recursive macros via __VA_OPT__. The relevant standard quote is [cpp. It is possible to cause macros to be expanded multiple times by using other macros to expand them. One way is to use the token-paste ##__VAR_ARGS__, but pasted arguments are not macro expanded, which means that macros cannot be nested. Thus you can try design your macro to prepend L only to the first string literal and use it as follows: VSTR("Beginning of a format string, with a string '" VPFTSTR "' included. answered Mar 30, 2009 at 6:00. That is, a type-safe sustitute of the unsafe, error-prone, and very limmited variadic functions (And variadic macros with it) that C++ inherited from C. The idea is to use these tokens as entries in an enum and also push them (stringified) to a std::vector<std::string>. cpp, so you can just paste this and try it out. (At least no easy way. If there's any way to index it; A good example of a recursive macro that could be relied upon to do this (as well as its limitations in max # of args, compiler compatibility, standards compliance, etc). Below is a definition of non-variadic macro that concatenates 2 arguments into a quoted string: #define TO_STRING(x) #x #define CONCAT_STRINGIFY(x,y) TO_STRING(x##y) Invoking this macro like this: CONCAT_STRINGIFY(AAA,BBB) PS: I know how I could write 5 macros (one for each number of params) that would do the job, so actually the question is: Is there an "easy" way to write a variadic macro that does the job? On the other hand, I will add more stuff to the structs, so having it all in one place would save lots of boilerplate. Here is some example code: #include <stdio. c-preprocessor; variadic-macros; or ask your own question. The macro substitution process is recursive and multiple-pass, due to what the standards call "rescanning and further replacement. void logF(const char *pFormat, ) { va_list args; /* Choose a Apart from the fact your macro is an int and your constexpr unsigned is an unsigned, there are important differences and macros only have one advantage. Note that the define contains a block and that c is local to this block. Hi Daniel, I found this recursive implementation useful in some mocking/unit testing work — thanks! Wrapped it and some extensions in a ruby generator script for an arbitrary number of arguments here if Apparently, MS VC++ behaves differently than expected with the recursive expansion of __VA_ARGS__ that includes multiple comma-separated arguments. 3. ) This precludes recursive macro expansion, which in most cases would produce infinite recursion if allowed. The downside to this is that you then cannot call your macro with only one argument; for example, debug_tcprintf ("hello world") will not work. How to make a lambda be recursive and at the same time variadic. In this article I'll show you how to use a variadic template in C++. First of all beware that variadic macros are not part of the current C++. Since we don't have partial specialization for function templates yet, (hopefully it will be supported soon) one way to do it is to use a class Conditional Compilation and Debugging/Logging is the domain where macros are really handy and legitimate. Skip to main content. C++11 did introduce the concept of macros with a variable number of arguments, using __VA_ARGS__ , but I don't know too much about it. In short, is it necessary to always create three methods for variadic methods that need to recurse? As I understand, it's necessary to handle when the parameter pack is unpacked, whether there's none, one, or more values inside it. Like Variable Length Array or Variadic Macros, for example. I only need this for at most 4 arguments currently. A macro can be declared to accept a variable number of arguments much as a function can. I'm hoping somebody might know of a workaround, or perhaps can explain to me why it behaves this way. 4,129 3 3 gold badges 31 31 silver badges 53 53 bronze badges. Community Bot C-preprocessor recursive macro. But in C++11 you could use variadic templates. asked Jan 16, 2018 at 15:29. 9k 14 14 gold badges 58 58 silver badges 91 91 bronze badges. C++11 templated function with rvalue param call. m. We use a WHILE_INDIRECT macro to refer back to itself recursively. Defining Multiple Variables Evaluating this macro does nothing, which ends the recursion. C/C++ expand multiple macros into one variadic macro. This prevents the macro from being painted blue, since it will expand on a different scan(and using a different disabling context). 2014-11-25. So for CV_VA_NUM_ARGS(a,b,c), the 11th element is 3. How to define recursive variadic macros? 1. C99 mandates that the only place the identifier __VA_ARGS__ can appear is in the replacement list of a variadic macro. Variadic templates allow you to have a template with a variable number of arguments, also called a parameter pack. It's calling func<>(); but the definition of your attempted base case func() is not a template function, ie. If you are not using the gcc compiler, you may need to omit the ## operator. I've been wondering if there is a nice alternative in ANSI C. It is possible to work around this limitation up to an arbitrary fixed number of Ok, I had the same problem, My aim was to print all the values of an array of N bytes using macros. A macro is defined by the preprocessor and is simply substituted into the code every time it occurs. Macro recursive expansion to a sequence Addendum: The way token-pasting is used in this example is actually a gcc extension to the standard C pre-processor. h to facilitate auto-generation of string representations of members I believe that recursive macros are not formally possible. 3 in Linux. Navigation Menu Toggle navigation. Warning: Writing this was mostly a learning exercise. variadic format strings in C macros? Hot Network Questions Old Sci-Fi movie about a sister searching for her astronaut brother, lost in space Is the term "AUROC curve" actually correct or meaningful? Can Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Macros can't be recursive, since macro expansion won't expand the name of a macro in the process of being expanded. e. 0; double area = PI * SQUARE(radius); I would like to write a macro to generate boilerplate code for interface methods in the . . You can still delegate to these macros from a single macro used elsewhere in your source, though. When expanding a macro, its name is said to be painted blue, and further occurences of this symbol are no longer expanded. for the sequence (A)(B)(A) I want to get the count 2 for A. 10. . va_list args; va_start(args, fmt); // your code here va_end(args); So CPP retains the comma when conforming to a specific C standard, and drops it otherwise. It may not be used as a macro name, macro argument name, or within a different type of macro. The macro returns MAP_END if the test item matches, or the next parameter if the item is anything else: rearranging the order of the macros so they work with my current compiler, g++ (Technically this code is c++, not c, but it's basically the same thing here. Inside the replacement of macro invocation, that macro is not expanded. I want to use it this way: int fn(int a, int b, int c) {UNREFERENCED_PARAMETERS(a,b,c); return 0;} Is it possible to do in c++? I thing that here is problem with expansion when just 1 parameter remain. gcc -c -Wall -Wextra -pedantic -ansi -std=c99 -fstack-protector-all -fstack-check -O3 root. The C standard mandates that the only place the identifier __VA_ARGS__ can appear is in the replacement list of a variadic macro. Option two has a drawback because there are some limitations to calling variadic macros. To get around this you must delay argument evaluation until after __VA_ARGS__ is substituted, by wrapping the macro call in EXPAND, forcing the macro call to be evaluated as a string, substituting __VA_ARGS__ to do so. There are endless variations on the theme It is not possible to pass the arguments from a variadic function to a variadic macro. vhnxgn tkcvq gewxstb nxzsp waldwon gzjxu oytxyccj sxatgn hjey yyzo