std::is_function

 

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类型特征
类型类别 is_void (C++11) is_null_pointer (C++14)   is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11) is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11)   is_fundamental (C++11) is_arithmetic (C++11) is_reference (C++11) is_lvalue_reference (C++11) is_rvalue_reference (C++11) is_member_pointer (C++11) is_member_object_pointer (C++11) is_member_function_pointer (C++11) 类型属性 is_const (C++11) is_volatile (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11) is_aggregate (C++17) is_implicit_lifetime (C++23) is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(直到 C++20*) is_pod (C++11)(在 C++20 中已弃用) is_signed (C++11) is_unsigned (C++11) is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_representations (C++17) 类型特征常量 integral_constantbool_constanttrue_typefalse_type (C++11)(C++17)(C++11)(C++11) 元函数 conjunction (C++17) disjunction (C++17) negation (C++17)	支持的操作 is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11) is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11) is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11) is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11) has_virtual_destructor (C++11) is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) 关系和属性查询 is_same (C++11) is_convertibleis_nothrow_convertible (C++11)(C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20)   is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) reference_converts_from_temporary (C++23) is_base_of (C++11) is_virtual_base_of (C++26) alignment_of (C++11) rank (C++11) extent (C++11) is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) 类型修改 remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11) remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11) 类型转换 aligned_storage (C++11)(在 C++23 中已弃用) aligned_union (C++11)(在 C++23 中已弃用) decay (C++11) remove_cvref (C++20) enable_if (C++11) void_t (C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) result_ofinvoke_result (C++11)(直到 C++20*)(C++17) type_identity (C++20)
编译时有理数运算
编译时整数序列
integer_sequence (C++14)

 

定义在头文件 <type_traits> 中
template< class T > struct is_function;		(自 C++11 起)

std::is_function 是一个 UnaryTypeTrait.

检查 T 是否为函数类型。诸如 std::function、lambda 表达式、具有重载 operator() 的类和指向函数的指针不计为函数类型。提供成员常量 value，如果 T 为函数类型，则等于 true。否则，value 等于 false。

如果程序为 std::is_function 或 std::is_function_v 添加了特化，则行为未定义。

模板参数

T

-

要检查的类型

辅助变量模板

template< class T > constexpr bool is_function_v = is_function<T>::value;		(自 C++17 起)

继承自 std::integral_constant

成员常量

value [静态]

true 如果 T 是函数类型，则为 false，否则为 (公有静态成员常量)

成员函数

operator bool	将对象转换为 bool，返回 value (公有成员函数)
operator() (C++14)	返回 value (公有成员函数)

成员类型

类型	定义
value_type	bool
type	std::integral_constant<bool, value>

备注

std::is_function 可以以更简单的方式实现。新版本的 libc++、libstdc++ 和 MS STL 使用类似于以下实现。

template<class T>
struct is_function : std::integral_constant<
    bool,
    !std::is_const<const T>::value && !std::is_reference<T>::value
> {};

下面显示的实现用于教学目的，因为它展示了各种函数类型。

可能的实现

// primary template template<class> struct is_function : std::false_type {};   // specialization for regular functions template<class Ret, class... Args> struct is_function<Ret(Args...)> : std::true_type {};   // specialization for variadic functions such as std::printf template<class Ret, class... Args> struct is_function<Ret(Args......)> : std::true_type {};   // specialization for function types that have cv-qualifiers template<class Ret, class... Args> struct is_function<Ret(Args...) const> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile> : std::true_type {};   // specialization for function types that have ref-qualifiers template<class Ret, class... Args> struct is_function<Ret(Args...) &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile &&> : std::true_type {};   // specializations for noexcept versions of all the above (C++17 and later) template<class Ret, class... Args> struct is_function<Ret(Args...) noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile && noexcept> : std::true_type {};

示例

运行此代码

#include <functional>
#include <type_traits>
 
int f();
static_assert(std::is_function_v<decltype(f)>);
 
static_assert(std::is_function_v<int(int)>);
static_assert(!std::is_function_v<int>);
static_assert(!std::is_function_v<decltype([]{})>);
static_assert(!std::is_function_v<std::function<void()>>);
 
struct O { void operator()() {} };
static_assert(std::is_function_v<O()>);
 
struct A
{
    static int foo();
    int fun() const&;
};
static_assert(!std::is_function_v<A>);
static_assert(std::is_function_v<decltype(A::foo)>);
static_assert(!std::is_function_v<decltype(&A::fun)>);
 
template<typename>
struct PM_traits {};
template<class T, class U>
struct PM_traits<U T::*> { using member_type = U; };
 
int main()
{
    using T = PM_traits<decltype(&A::fun)>::member_type; // T is int() const&
    static_assert(std::is_function_v<T>);
}

另请参阅

is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)	检查类型是否可以调用（就像通过 std::invoke 调用）并具有给定的参数类型 (类模板)
is_object (C++11)	检查类型是否为对象类型 (类模板)
is_class (C++11)	检查类型是否为非联合类类型 (类模板)