doc/is_detected.qbk

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Copyright 2018 Glen Joseph Fernandes
<glenjofe -at- gmail.com>

Distributed under the Boost Software License,
Version 1.0. (See accompanying file LICENSE_1_0.txt
or copy at http://www.boost.org/LICENSE_1_0.txt).
]

[section:is_detected is_detected]

    template<template<class...> class Op, class... Args>
    using is_detected = __below;

    template<template<class...> class Op, class... Args>
    constexpr bool is_detected_v = is_detected<Op, Args...>::value;

__alias If `Op<Args...>` is a valid template-id, aliases __true_type,
otherwise aliases __false_type.

__std_paper [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf N4502]

__compat Requires C++11 variadic templates and C++11 template aliases.

__header `#include <boost/type_traits/is_detected.hpp>`

__examples

Suppose we wish to "reset" a value of type T, if the type has a `clear()` member function then we should call
it, otherwise we should assign a default constructed value:

    template<class T>
    using clear_t = decltype(boost::declval<T&>().clear());

    template<class T>
    void clear(T& value)
    {
        if constexpr (boost::is_detected_v<clear_t, T>) {
            value.clear();
        } else {
            value = T();
        }
    }

See also:  __is_detected_convertible, __is_detected_exact.

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