Counting function arguments at compile time

问题

I'm trying to count the number of arguments to a function at compile time (I'm wrapping sprintf up in some templates for compile time checks and type safety). I need to check that the number of arguments matches the number of formatting placeholders at compile time. A first pass at this is pretty simple:

template <typename... Args>
constexpr u32
CountArgs(Args&&... args)
{
    return sizeof...(args);
}

constexpr u32
CountFormatSpecifiers(c8* format);

template <typename... Args>
c8*
String_FormatImpl(c8* format, Args&&... args);

#define String_Format(format, ...) \
    String_FormatImpl(format, __VA_ARGS__); \
    static_assert(CountFormatSpecifiers(format) == CountArgs(__VA_ARGS__));

But this breaks down for certain types of arguments. Namely, when passing a reference.

int x = 0;
int& xRef = x;
String_Format("%", xRef);

The compiler complains about CountArgs(__VA_ARGS__) because xRef is not a constant expression. I don't need the value, just the ability to count it. I could wrap it in sizeof or something similar, but that's tough when all I have is __VA_ARGS__ to work with.

Example: https://godbolt.org/z/Diwffy

回答1:

You can change your macro to something like this

#define String_Format(format, ...) \
    String_FormatImpl<CountFormatSpecifiers(format)>(format, __VA_ARGS__);

template <std::size_t I, typename... Args>
void String_FormatImpl(const char* format, Args&&...) {
    static_assert(I == sizeof...(Args));
    ...
}

回答2:

Maybe using decltype() and std::integral_constant ?

I mean... you can declare (only declare: no needs of define it) the following function (EDIT: modified, following a Davis Herring's suggestion (thanks!), to accept const references; this permits to works also with non copyable types)

template <typename ... Args>
std::integral_constant<std::size_t, sizeof...(Args)> CArgs (Args const & ...);

and use it, by example

#define bar(num, ...) \
    static_assert(num == decltype(CArgs(__VA_ARGS__))::value);

This way you don't use the __VA_ARGS__ values in a static_assert() but the type returned by a function that accept __VA_ARGS__.

And the type returned (std::integral_constant<std::size_t, sizeof...(Args)>) contain the number (accessible through ::value), as compile-time constant, of the arguments.

The following is a full compiling example

#include <type_traits>

template <typename ... Args>
std::integral_constant<std::size_t, sizeof...(Args)> CArgs (Args const & ...);

#define bar(num, ...) \
    static_assert(num == decltype(CArgs(__VA_ARGS__))::value);

int main()
{
   int x = 0;
   int& xRef = x;

   //..VV  number of the following arguments    
   bar(3u, x, xRef, 42);
}

来源：https://stackoverflow.com/questions/58468574/counting-function-arguments-at-compile-time

标签

c++

templates

c++17

variadic-templates

variadic-macros