Is the `gnu-zero-variadic-macro-arguments` safe to ignore?

问题

Consider the following code (live example):

#define TEST_VA(mX, ...) TEST
#define STRINGIFY_IMPL(mX) #mX
#define STRINGIFY(mX) STRINGIFY_IMPL(mX)

#include <iostream>

int main()
{
    std::cout << STRINGIFY(TEST_VA(1)) << std::endl;
    std::cout << STRINGIFY(TEST_VA()) << std::endl;
    return 0;
}

clang++ 3.4 complains:

main.cpp:9:37: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]
    std::cout << STRINGIFY(TEST_VA(1)) << std::endl;
                                    ^
main.cpp:1:9: note: macro 'TEST_VA' defined here
#define TEST_VA(mX, ...) TEST
        ^
main.cpp:10:33: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]
        std::cout << STRINGIFY(TEST_VA()) << std::endl;
                                       ^
main.cpp:1:9: note: macro 'TEST_VA' defined here
#define TEST_VA(mX, ...) TEST

I use some variadic macros like in the example code in my projects.

Is it safe to pass no arguments for the ... parameter of variadic macros? (Ignoring the warning, basically)

Or could it lead to problems?

Is it explicitly forbidden by the standard?

回答1:

It's technically forbidden by the standard. From C++11 §16.3/4 (emphasis added):

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 parameters in the macro definition. Otherwise, there shall be more arguments in the invocation than there are parameters in the macro definition (excluding the ...). There shall exist a ) preprocessing token that terminates the invocation.

Some compilers may allow this as an extension, but if you want standards-compliant code, you should make sure to always include at least one argument for the ellipsis parameter of a variadic macro.

来源：https://stackoverflow.com/questions/21266380/is-the-gnu-zero-variadic-macro-arguments-safe-to-ignore

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c++

c++11

warnings

c-preprocessor

variadic-macros