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72 lines
2.5 KiB
C++
72 lines
2.5 KiB
C++
/*
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Copyright Barrett Adair 2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http ://boost.org/LICENSE_1_0.txt)
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*/
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#include <type_traits>
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#include <callable_traits/callable_traits.hpp>
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// NOTE: Due to non-compliance in MSVC, can_invoke_constexpr
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// always return std::false_type on that compiler, which causes
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// the static asserts below to fail.
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namespace ct = callable_traits;
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using T1 = std::integral_constant<int, 3>;
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using T2 = std::integral_constant<int, 7>;
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//'subtract' is a constexpr function object that
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// subtracts std::integral_constant types.
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struct subtract {
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// To compile failing cases of can_invoke_constexpr, the function object
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// must have a SFINAE-safe signature. In this case, 'subtract' is made
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// SFINAE-safe with an explicit, trailing return type.
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template<typename T1, typename T2>
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constexpr auto operator()(T1, T2) const -> decltype(T1::value - T2::value) {
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return T1::value - T2::value;
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}
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};
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// can_invoke_constexpr returns std::true_type in the first case, because
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// INVOKE(subtract{}, T1{}, T2{}) is a valid expression, AND 'subtract{}' is
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// a constexpr function object.
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static_assert(ct::can_invoke_constexpr(subtract{}, T1{}, T2{}), "");
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static_assert(!ct::can_invoke_constexpr(subtract{}, 3, 7), "");
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static_assert(!ct::can_invoke_constexpr(subtract{}, T1{}), "");
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//this is a function object, but is NOT constexpr
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struct add {
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template<typename T1, typename T2>
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auto operator()(T1, T2) const -> decltype(T1::value + T2::value) {
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return T1::value + T2::value;
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}
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};
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// Even though INVOKE(add{}, T1{}, T2{}) is valid, the respective
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// can_invoke_constexpr call returns std::false_type because 'add{}'
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// is not a constexpr function object.
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static_assert(!ct::can_invoke_constexpr(add{}, T1{}, T2{}), "");
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static_assert(!ct::can_invoke_constexpr(add{}, 3, 7), "");
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/*
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// This last section serves to demonstrate that can_invoke_constexpr
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// can only be used with trivially default constructible types. Even
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// though 'S' is a constexpr function object, it is incompatible with
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// can_invoke_constexpr because it has no trivial default constructor.
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// The same restrictions also apply to the supplied arguments.
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// Error message: "Cannot perform constexpr checks with this type,
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// because it is not trivially default constructible. "
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struct S {
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S() = delete;
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constexpr int operator()() { return 0; }
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};
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using S_result = decltype(ct::can_invoke_constexpr(std::declval<S>()));
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static_assert(S_result::value, "");
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*/
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int main() {}
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