When expanding A(), the traditional preprocessor forwards all of the arguments packaged in __VA_ARGS__ to the first argument of TWO_STRINGS, which leaves the variadic argument of TWO_STRINGS empty. The order of operations for ## and # is unspecified, although all compilers seem to evaluate the # operator before ## in this case. It's found at the top of the table of contents on this page. The algorithm used by the traditional preprocessor for doing the rescan isn't conforming, as shown in this example based on actual code: Although this example may seem a bit contrived, we've seen it in real-world code. In the call to FUNC2(1, ) the variadic argument is empty, but not missing (notice the comma in the argument list). Prior to calling va_arg, ap must be initialized by a call to either va_start or va_copy, with no intervening call to va_end. Starting in Visual Studio 2019 version 16.5, preprocessor support for the C++20 standard is feature-complete. A separate issue kept the empty string literal token from being generated. The arguments (called variable arguments) can be accessed only with __VA_ARGS__ identifier, which is then replaced with arguments, supplied with the identifier to be replaced. For the comma to be removed, the variadic argument must be missing (not just empty) and it must be marked with a ## operator. Prior to calling va_arg, ap must be initialized by a call to either va_start or va_copy, with no intervening call to va_end. Each invocation of the va_arg macro modifies ap to point to the next variable argument. The new preprocessor matches the behavior of most other compilers and emits a diagnostic. We've also added new diagnostics to warn on errors in macro definitions. Check the value of the predefined macro _MSVC_TRADITIONAL to tell if the traditional preprocessor is in use. This extension matches the behavior of other major cross-platform C++ compilers. Starting in Visual Studio 2019 version 16.5, new preprocessor support for the C++20 standard is feature-complete. In MSVC, the new preprocessor extends this C++20 behavior to lower language standard modes (/std:c++14, /std:c++17). typedef struct { unsigned int gp_offset; unsigned int fp_offset; void *overflow_arg_area; void *reg_save_area; } va_list[1]; va_list is defined . You cannot use __VA_ARGS__ and this extension in the same macro. This set of tokens replaces the identifier __VA_ARGS__ in the macro body wherever it appears. I think that is sprintf_P (). In the traditional preprocessor, if a macro forwards one of its arguments to another dependent macro then the argument doesn't get "unpacked" when it's inserted. To review, open the file in an editor that reveals hidden Unicode characters. It lets you use the MSVC compiler with libraries that are currently blocked by the traditional behaviors. There are two versions of the macros: The macros defined in STDARG.H conform to the ISO C99 standard; the macros defined in VARARGS.H are deprecated but are retained for backward . We could define eprintf like this, instead: #define eprintf (format, .) It allows unusual behavior such as the following preprocessor comment trick, which doesn't work under the conforming preprocessor: The standards-conforming fix is to declare int myVal inside the appropriate #ifdef/#endif directives: The traditional preprocessor incorrectly combines a string prefix to the result of the stringizing operator (#) operator: In this case, the L prefix is unnecessary because the adjacent string literals are combined after macro expansion anyway. Then you can define a va_list and use va_start to set it: va_list args; va_start (args, fmt); // your code here va_end (args); Now you can use args to access the arguments; calling va_arg (args, TYPE) will return the next variadic argument, so you can just keep calling that until you've read all the arguments. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. You declare required arguments as ordinary parameters to the function and access the arguments through the parameter names. You signed in with another tab or window. To see the documentation for your preferred version of Visual Studio, use the Version selector control. Defined in header <stdarg.h> T va_arg( va_list ap, T ); The va_arg macro expands to an expression of type T that corresponds to the next parameter from the va_list ap . Visual Studio 2015 uses the traditional preprocessor, which doesn't conform with Standard C++ or C99. It finds the function-like macro, Boost blocking bug: Logical operators in preprocessor constant expressions aren't fully implemented in the new preprocessor before version 16.5. In previous versions of Visual Studio, the new preprocessor is mostly complete, although some preprocessor directive logic still falls back to the traditional behavior. The number of arguments should match the number of parameters in the macro definition, unless the parameter list in the definition ends with an ellipsis . See the CPP manual for more information. In this case, the identifier __VA_ARGS__ may appear in the replacement list. If you're following along closely, then you may be wondering what happened to the result of #__VA_ARGS__ in the traditional preprocessor expansion: if the variadic parameter is empty it should result in an empty string literal "". This macro is set unconditionally by versions of the compiler that support it, independent of which preprocessor is invoked. The new preprocessor more closely follows the behavior of other popular cross-platform compilers. The C Standard specifies that at least one argument must be passed to the ellipsis to ensure the macro doesn't resolve to an expression with a trailing comma. Without the ## it would complain about the trailing comma being a syntax error. va_start should be invoked with an instance to a valid va_list object ap before any calls to va_arg . You can enable it by using the /experimental:preprocessor compiler switch. Consider the following example: In the following example, in the call to FUNC2(1) the variadic argument is missing in the macro being invoked. Here's a partial list of incomplete features in Visual Studio versions before 16.5: More info about Internet Explorer and Microsoft Edge, The preprocessor finds the function-like macro identifier, The preprocessor moves on to the following tokens. Do you use the F () macro to put your format string in PROGMEM? If so you will need call the version of sprintf () that expects the format to be in PROGMEM. Without it we would be flying blind." These changes are available by using the /Zc:preprocessor compiler switch. #define MACRO_VA_ARGS (.) The three . This page was last modified on 24 April 2020, at 21:25. It means you can call it with any number of arguments. The C denition of va_list type is given in gure below. Defined in header <cstdarg> void va_start( std::va_list ap, parm_n ); The va_start macro enables access to the variable arguments following the named argument parm_n. If the type of the next argument in ap (after promotions) is not compatible with T, the behavior is undefined, unless: If va_arg is called when there are no more arguments in ap, the behavior is undefined. How do you call the PRINTF () macro? A tag already exists with the provided branch name. In the macro expansion, these values that are then referenced by the special identifier __VA_ARGS__. It's 0 for the conforming preprocessor. using this extension. New preprocessor support for __VA_OPT__ is available starting in Visual Studio 2019 version 16.5. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Adjacent string literals are combined after macro expansion: Add the prefix after #str is stringized with additional macro expansion. The initial work on the new preprocessor has been focused on making all macro expansions conform to the standard. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Function-like macro invocation: An identifier followed by a comma-separated list of arguments in parentheses. After a macro is replaced, the resulting tokens are rescanned for additional macro identifiers to replace. Are you sure you want to create this branch? is similar to the same construct used in a variadic function in C That means you can use the macro like this DEBUG ("foo", "bar", "baz"); Or with any number of arguments. These changes are available by using the /Zc:preprocessor compiler switch. 18 It's a non-portable syntax introduced by gcc to specifically deal with this corner case of passing no arguments at all. In the upcoming C++20 standard, this issue has been addressed by adding __VA_OPT__. To see what's going on, we can break down the expansion starting with DO_THING: To modify the macro to behave the same way under both the new preprocessor and the traditional preprocessor, add another layer of indirection: Starting in Visual Studio 2019 version 16.5, the new preprocessor is feature-complete for C++20. GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. The traditional preprocessor silently fails to combine the tokens. An experimental version of the new preprocessor is available starting in Visual Studio 2017 version 15.8 and later by using the /experimental:preprocessor compiler switch. We're updating the Microsoft C++ preprocessor to improve standards conformance, fix longstanding bugs, and change some behaviors that are officially undefined. Here is an example: https://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html The new preprocessor supports C++20 variadic macro argument elision: This code isn't conforming before the C++20 standard. The backward-compatible fix is to change the definition: The same issue is also found in convenience macros that "stringize" the argument to a wide string literal: Use string concatenation of L"" and #str to add prefix. Its value is 1 for the traditional preprocessor. fprintf (stderr, format, __VA_ARGS__) Here are some of the more common breaking changes we found: The traditional preprocessor is based on character buffers rather than preprocessor tokens. We tested the updated preprocessor on real world projects. That causes the result of #first to be "1, 2" rather than just "1". That causes the result of #first to be "1, 2" rather than just "1". __VA_ARGS__ is replaced by all of the arguments that match the ellipsis, including commas between them. The va_arg macro expands to an expression of type T that corresponds to the next parameter from the va_list ap. When the token-pasting operator (##) doesn't result in a single valid preprocessing token, the behavior is undefined. Usually this optimization goes unnoticed, but it can lead to unusual behavior: When expanding A(), the traditional preprocessor forwards all of the arguments packaged in __VA_ARGS__ to the first argument of TWO_STRINGS, which leaves the variadic argument of TWO_STRINGS empty. On some. This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository. The default preprocessor behavior remains the same as in previous versions. You can detect which preprocessor is in use at compile time. A tag already exists with the provided branch name. https://en.cppreference.com/mwiki/index.php?title=c/variadic/va_arg&oldid=118364, makes a copy of the variadic function arguments, ends traversal of the variadic function arguments, holds the information needed by va_start, va_arg, va_end, and va_copy, enables access to variadic function arguments, one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types; or. RESULT (__VA_ARGS__) MACRO_VA_ARGS () MACRO_VA_ARGS (one) MACRO_VA_ARGS (two,three) MACRO_VA_ARGS (foo, bar, baz) Github respository about-preprocessor, path: /macros/__VA_ARGS__/basics.c It is possible to avoid the use of GCC's ,##__VA_ARGS__ extension if you are willing to accept some hardcoded upper limit on the number of arguments you can pass to your variadic macro, as described in Richard Hansen's answer to this question.If you do not want to have any such limit, however, to the best of my knowledge it is not possible using only C99-specified preprocessor features; you . Defined in header <cstdarg> T va_arg( std::va_list ap, T ); The va_arg macro expands to an expression of type T that corresponds to the next parameter from the va_list ap . Prior to calling va_arg, ap must be initialized by a call to either va_start or va_copy, with no intervening call to va_end. General description The va_arg(), va_end(), and va_start() macros access the arguments to a function when it takes a fixed number of required arguments and a variable number of optional arguments. The va_arg, va_copy, va_end, and va_start macros provide a portable way to access the arguments to a function when the function takes a variable number of arguments. More information about using the new preprocessor in Visual Studio 2017 and Visual Studio 2019 is available. 59 It's a variadic macro. "Rollbar allows us to go from alerting to impact analysis and resolution in a matter of minutes. The __VA_ARGS__ refers back again to the variable arguments in the macro itself. Use the concatenation operator ## to combine the tokens. The va_list type is an array containing a single element of one structure containing the necessary information to implement the va_arg macro. The traditional MSVC preprocessor always removes commas before empty __VA_ARGS__ replacements. You can have named arguments as well as variable arguments in a variadic macro. This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. An experimental version of the new preprocessor is available in earlier versions starting in Visual Studio 2017 version 15.8. This page has been accessed 61,215 times. reworking launch.py: add references to renamed file, Learn more about bidirectional Unicode characters.
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