explicit_operator_bool

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## explicit_operator_bool

### Authors

- Andrey Semashev

### Overview

Header `<boost/core/explicit_operator_bool.hpp>` provides `BOOST_EXPLICIT_OPERATOR_BOOL()`, `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()` and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` compatibility helper macros that expand to an explicit conversion operator to `bool`. For compilers not supporting explicit conversion operators introduced in C++11 the macros expand to a conversion operator that implements the safe bool idiom. In case if the compiler is not able to handle safe bool idiom well the macros expand to a regular conversion operator to `bool`.

### Examples

Both macros are intended to be placed within a user's class definition. The generated conversion operators will be implemented in terms of `operator!()` that should be defined by user in this class. In case of `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` the generated conversion operator will be declared `constexpr` which requires the corresponding `operator!()` to also be `constexpr`.

    template< typename T >
    class my_ptr
    {
        T* m_p;

    public:
        BOOST_EXPLICIT_OPERATOR_BOOL()

        bool operator!() const
        {
            return !m_p;
        }
    };

Now `my_ptr` can be used in conditional expressions, similarly to a regular pointer:

    my_ptr< int > p;
    if (p)
        std::cout << "true" << std::endl;

### History

##### boost 1.56

- Added new macros `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT` and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL` to define `noexcept` and `constexpr` operators.
- The header moved to Boost.Core.

##### boost 1.55

- The macro was extracted from Boost.Log.