在C ++中将隐式比较器与具有结构作为键的映射一起使用

Question

I want to use a map of the standard libc++. So defining operator< for the key type is one solution to instante a map. I did it but it does not work.

I checked that operator< works (compilation and execution).

png_pixel p1, p2;
p1.red = 5; p2.red = 20; // init
if(p1 < p2)
  puts("true");
else
  puts("false");

That is the definition of operator<:

#include <tuple> // C++11

bool
operator<(const png_pixel& a, const png_pixel& b)
{
  return std::tie(a.red, a.green, a.blue, a.alpha) < std::tie(b.red, b.green, b.blue, b.alpha);
}

png_pixel is a struct defined like this:

// My source code code
typedef png::rgba_pixel png_pixel;

// libpng++ rgba_pixel.hpp http://www.nongnu.org/pngpp/
namespace png
{
    template< typename T >
    struct basic_rgba_pixel
    {
        typedef pixel_traits< basic_rgba_pixel< T > > traits;

        basic_rgba_pixel()
            : red(0), green(0), blue(0), alpha(0)
        {
        }

        /**
         * \brief Constructs rgba_pixel object from \a red, \a green,
         * \a blue and \a alpha components passed as parameters.
         * Alpha defaults to full opacity.
         */
        basic_rgba_pixel(T red, T green, T blue,
                         T alpha = traits::get_alpha_filler())
            : red(red), green(green), blue(blue), alpha(alpha)
        {
        }

        T red;
        T green;
        T blue;
        T alpha;
    };

    typedef basic_rgba_pixel< byte > rgba_pixel;

    // other things
}

But when I try to use png_pixel as a key in a C++ map, GCC and Clang are not happy.

# GCC 4.9
error: no match for ‘operator<’
       (operand types are ‘const png::basic_rgba_pixel<unsigned char>’
       and ‘const png::basic_rgba_pixel<unsigned char>’)

# Clang 3.5
error: invalid operands to binary expression
      ('const png::basic_rgba_pixel<unsigned char>'
      and 'const png::basic_rgba_pixel<unsigned char>')

If needed, the full source code is available in branch map-converter of https://gitlab.com/RyDroid/Libre2DCarRacingsSimulator libpng++ is packaged at least in Debian as libpng++-dev.

Regards.

Answer 1

Your operator< isn't in the template definition context of std::less , the default comparator of std::map , and in any event is likely hidden by the gazillion operator< s in std for ordinary unqualified lookup, so if it is to be used, it must be found by ADL. But it's not in the png namespace, the only associated namespace of png::basic_rgba_pixel<unsigned char> , so ADL doesn't find it either.

Write a comparator function object and pass that to map . You can also try putting it into the png namespace, but that feels more icky.