has_equal

has_equal_to

template <class Lhs, class Rhs=Lhs, class Ret=dont_care>
struct has_equal_to : public true_type-or-false_type {};

Inherits: If (i) lhs of type Lhs and rhs of type Rhs can be used in expression lhs==rhs, and (ii) Ret=dont_care or the result of expression lhs==rhs is convertible to Ret then inherits from true_type, otherwise inherits from false_type.

The default behaviour (Ret=dont_care) is to not check for the return value of binary operator==. If Ret is different from the default dont_care type, the return value is checked to be convertible to Ret. Convertible to Ret means that the return value of the operator can be used as argument to a function expecting Ret:

void f(Ret);
Lhs lhs;
Rhs rhs;
f(lhs==rhs); // is valid if has_equal_to<Lhs, Rhs, Ret>::value==true

If Ret=void, the return type is checked to be exactly void.

Header: #include <boost/type_traits/has_equal_to.hpp> or #include <boost/type_traits/has_operator.hpp> or #include <boost/type_traits.hpp>

Compiler Compatibility: Requires working SFINAE (i.e. BOOST_NO_SFINAE is not set). Only a minority of rather old compilers do not support this.

Examples:

has_equal_to<Lhs, Rhs, Ret>::value_type is the type bool.

has_equal_to<Lhs, Rhs, Ret>::value is a bool integral constant expression.

has_equal_to<int>::value is a bool integral constant expression that evaluates to true.

has_equal_to<long> inherits from true_type.

has_equal_to<int, int, bool> inherits from true_type.

has_equal_to<int, double, bool> inherits from true_type.

has_equal_to<const int> inherits from true_type.

has_equal_to<int*, int> inherits from false_type.

has_equal_to<int*, double*> inherits from false_type.

has_equal_to<int, int, std::string> inherits from false_type.

See also: Operator Type Traits

Known issues:

This trait cannot detect whether binary operator== is public or not: if operator== is defined as a private member of Lhs then instantiating has_equal_to<Lhs> will produce a compiler error. For this reason has_equal_to cannot be used to determine whether a type has a public operator== or not.
```
struct A { private: void operator==(const A&); };
boost::has_equal_to<A>::value; // error: A::operator==(const A&) is private
```

There is an issue if the operator exists only for type A and B is convertible to A. In this case, the compiler will report an ambiguous overload.

struct A { };
void operator==(const A&, const A&);
struct B { operator A(); };
boost::has_equal_to<A>::value; // this is fine
boost::has_equal_to<B>::value; // error: ambiguous overload

There is an issue when applying this trait to template classes. If operator== is defined but does not bind for a given template type, it is still detected by the trait which returns true instead of false. Example:

#include <boost/type_traits/has_equal_to.hpp>
#include <iostream>

template <class T>
struct contains { T data; };

template <class T>
bool operator==(const contains<T> &lhs, const contains<T> &rhs) {
	return f(lhs.data, rhs.data);
}

class bad { };
class good { };
bool f(const good&, const good&) { }

int main() {
	std::cout<<std::boolalpha;
	// works fine for contains<good>
	std::cout<<boost::has_equal_to< contains< good > >::value<<'\n'; // true
	contains<good> g;
	g==g; // ok
	// does not work for contains<bad>
	std::cout<<boost::has_equal_to< contains< bad > >::value<<'\n'; // true, should be false
	contains<bad> b;
	b==b; // compile time error
	return 0;
}

volatile qualifier is not properly handled and would lead to undefined behavior

Boost C++ Libraries

has_equal_to