Operator < detection

check_hash_less.cpp

#include <iostream>
#include <string>
#include <boost/type_traits/has_less.hpp>

#define HAS_LESS(type) (boost::has_less<type>::value \
                        ? #type ": has less" \
                        : #type ": doesn't have less")

#define CHECK(type) (std::cout << HAS_LESS(type) << "\n")

struct A
{
};

bool operator <(A, A)
{
    return false;
}

struct B
{
    bool operator <(B)
    {
        return false;
    }
};

struct C
{
};

int main()
{
    CHECK(void);
    CHECK(int);
    CHECK(std::string);
    CHECK(A);
    CHECK(B);
    CHECK(C);

    return 0;
}

output-gcc-4.7.2.txt

void: doesn't have less
int: has less
std::string: has less
A: has less
B: has less
C: doesn't have less