Clean code to printf size_t in C++ (or: Nearest equivalent of C99's %z in C++)

Question

I have some C++ code that prints a size_t:

size_t a;
printf(\"%lu\", a);

I\'d like this to compile without warnings on both 32

Answer 1

C++11

C++11 imports C99 so std::printf should support the C99 %zu format specifier.

C++98

On most platforms, size_t and uintptr_t are equivalent, in which case you can use the PRIuPTR macro defined in <cinttypes>:

size_t a = 42;
printf("If the answer is %" PRIuPTR " then what is the question?\n", a);

If you really want to be safe, cast to uintmax_t and use PRIuMAX:

printf("If the answer is %" PRIuMAX " then what is the question?\n", static_cast<uintmax_t>(a));

Answer 2

The fmt library provides a fast portable (and safe) implementation of printf including the z modifier for size_t:

#include "fmt/printf.h"

size_t a = 42;

int main() {
  fmt::printf("%zu", a);
}

In addition to that it supports Python-like format string syntax and captures type information so that you don't have to provide it manually:

fmt::print("{}", a);

It has been tested with major compilers and provides consistent output across platforms.

Disclaimer: I'm the author of this library.

Answer 3

Most compilers have their own specifier for size_t and ptrdiff_t arguments, Visual C++ for instance use %Iu and %Id respectively, I think that gcc will allow you to use %zu and %zd.

You could create a macro:

#if defined(_MSC_VER) || defined(__MINGW32__) //__MINGW32__ should goes before __GNUC__
  #define JL_SIZE_T_SPECIFIER    "%Iu"
  #define JL_SSIZE_T_SPECIFIER   "%Id"
  #define JL_PTRDIFF_T_SPECIFIER "%Id"
#elif defined(__GNUC__)
  #define JL_SIZE_T_SPECIFIER    "%zu"
  #define JL_SSIZE_T_SPECIFIER   "%zd"
  #define JL_PTRDIFF_T_SPECIFIER "%zd"
#else
  // TODO figure out which to use.
  #if NUMBITS == 32
    #define JL_SIZE_T_SPECIFIER    something_unsigned
    #define JL_SSIZE_T_SPECIFIER   something_signed
    #define JL_PTRDIFF_T_SPECIFIER something_signed
  #else
    #define JL_SIZE_T_SPECIFIER    something_bigger_unsigned
    #define JL_SSIZE_T_SPECIFIER   something_bigger_signed
    #define JL_PTRDIFF_T_SPECIFIER something-bigger_signed
  #endif
#endif

Usage:

size_t a;
printf(JL_SIZE_T_SPECIFIER, a);
printf("The size of a is " JL_SIZE_T_SPECIFIER " bytes", a);