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### has_unary_plus

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

Inherits: If (i) `rhs` of type `Rhs` can be used in expression `+rhs`, and (ii) `Ret=dont_care` or the result of expression `+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 prefix `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);
    Rhs rhs;
    f(+rhs); // is valid if has_unary_plus<Rhs, Ret>::value==true

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

Header: `#include <boost/type_traits/has_unary_plus.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_unary_plus<Rhs, Ret>::value_type` is the type `bool`.

> `has_unary_plus<Rhs, Ret>::value` is a `bool` integral constant expression.

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

> `has_unary_plus<long>` inherits from `true_type`.

> `has_unary_plus<int, int>` inherits from `true_type`.

> `has_unary_plus<int, long>` inherits from `true_type`.

> `has_unary_plus<double, double>` inherits from `true_type`.

> `has_unary_plus<double, int>` inherits from `true_type`.

> `has_unary_plus<const int>` inherits from `true_type`.

> `has_unary_plus<int, std::string>` inherits from `false_type`.

See also: Operator Type Traits

Known issues:

- This trait cannot detect whether prefix `operator+` is public or not: if `operator+` is defined as a private member of `Rhs` then instantiating `has_unary_plus<Rhs>` will produce a compiler error. For this reason `has_unary_plus` cannot be used to determine whether a type has a public `operator+` or not.

      struct A { private: void operator+(); };
      boost::has_unary_plus<A>::value; // error: A::operator+() 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&);
      struct B { operator A(); };
      boost::has_unary_plus<A>::value; // this is fine
      boost::has_unary_plus<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_unary_plus.hpp>
      #include <iostream>

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

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

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

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

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

Known issues:

For modern compilers (those that support arbitrary SFINAE-expressions and decltype/declval) this trait offers near perfect detection. In this situation the macro `BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION` will be defined after including `<boost/type_traits/has_unary_plus.hpp>`. Please note however, that detection is based on function signature only, in the case that the operator is a function template then has_unary_plus cannot perform introspection of the template function body to ensure that the type meets all of the conceptual requirements of the actual code.

For older compilers (`BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION` not defined) then there are a number of issues:

- This trait cannot detect whether unary `operator`+ is public or not: if `operator`+ is defined as a private class member of type T then instantiating `has_unary_plus<T>` will produce a compiler error. For this reason `has_unary_plus` cannot be used to determine whether a type has a public `operator`+ or not.
- `volatile` qualifier is not properly handled and would lead to undefined behavior
- Capturless lambdas are not supported and will lead to a compiler error if has_unary_plus is instantiated on the lambda type.
- Scoped enum types are not supported and will lead to a compiler error if has_unary_plus is instantiated on such a type.