std::align not supported by g++4.9

Question

While learning about alignment issues etc, I realized that my implementation of g++4.9 (macports OS X) does not have support for std::align. If I try to compile (with -std=c++11) this example code from http://www.cplusplus.com/reference/memory/align/

// align example
#include <iostream>
#include <memory>

int main() {
  char buffer[] = "------------------------";
  void * pt = buffer;
  std::size_t space = sizeof(buffer) - 1;
  while ( std::align(alignof(int), sizeof(char), pt, space) ) {
    char* temp = static_cast<char*>(pt);
    *temp = '*'; ++temp; space -= sizeof(char);
    pt = temp;
  }
  std::cout << buffer << '\n';
  return 0;
}

the compiler spits out the error

error: 'align' is not a member of 'std'

This seems strange as g++ seems to have implemented alignment support since g++4.8, https://gcc.gnu.org/projects/cxx0x.html (N2341)

The code compiles under clang++ without any problems.

Is this a well known issue of g++ that I am not aware of? The online compilers that I tested (ideone and coliru) reject the code also.

Answer 1

As an alternative you could write your own alignment code that matches the behavior of std::align. The next piece of code is written by David Krauss in a post found here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57350

inline void *align( std::size_t alignment, std::size_t size,
                void *&ptr, std::size_t &space ) {
    std::uintptr_t pn = reinterpret_cast< std::uintptr_t >( ptr );
    std::uintptr_t aligned = ( pn + alignment - 1 ) & - alignment;
    std::size_t padding = aligned - pn;
    if ( space < size + padding ) return nullptr;
    space -= padding;
    return ptr = reinterpret_cast< void * >( aligned );
}

Answer 2

Yes, this is a known missing feature for gcc :

• Bug 53814 : std::align missing