Boost C++ Libraries

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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>

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

Limitation:

Known issues:


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