type_traits metaprogramming library learning

type_traitsis part of the metaprogramming library,This library is mainly used to determine the data type,比如,判断类型是否为空,是否为空指针,是否为整型,Whether it is a float or not whether it is an array,Is it an enumeration type,Is it a union,是否为函数,是否为指针,Is it an lvalue reference,Is it an rvalue reference,等等,There are many types of judgments,This article introduces a few basic ones.

Basic type classification
is_void (C++11)	检查类型是否为 void (类模板)
is_null_pointer (C++14)	检查类型是否为 std::nullptr_t (类模板)
is_integral (C++11)	Check if the type is an integer (类模板)
is_floating_point (C++11)	Check if the type is a floating point type (类模板)
is_array (C++11)	Check if the type is an array type (类模板)
is_enum (C++11)	Check if the type is an enumeration type (类模板)
is_union (C++11)	Check if the type is a union type (类模板)
is_class (C++11)	Check if the type is not a union class type (类模板)
is_function (C++11)	Check if it is a function type (类模板)
is_pointer (C++11)	Check if the type is a pointer type (类模板)
is_lvalue_reference (C++11)	检查类型是否为左值引用 (类模板)
is_rvalue_reference (C++11)	检查类型是否为右值引用 (类模板)
is_member_object_pointer (C++11)	Checks if the type is a pointer to a non-static member object (类模板)
is_member_function_pointer (C++11)	Checks if the type is a pointer to a non-static member function (类模板)

1. is_void    检查类型是否为 void

#include <iostream>
#include <type_traits>

using namespace std;

int main()
{
    cout << "std::is_void<void>::value==========" << std::is_void<void>::value << endl;
    cout << "std::is_void<int>::value===========" << std::is_void<int>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

2. is_null_pointer    检查类型是否为 std::nullptr_t

#include <iostream>
#include <type_traits>

using namespace std;

int main()
{
    cout << "std::is_null_pointer<decltype(nullptr)>::value = " << is_null_pointer<decltype(nullptr)>::value << endl;
    cout << "std::is_null_pointer<int*>::value = " << is_null_pointer<int*>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

3. is_integral    Check if the type is an integer

#include <iostream>
#include <type_traits>

using namespace std;

class A {};

enum E: int {};

template <class T>
T fn(T i)
{
    static_assert (std::is_integral<T>::value, "Integral required.");
    return i;
}

int main()
{
    cout << "std::is_integral<A>::value======= " << std::is_integral<A>::value << endl;
    cout << "std::is_integral<E>::value======= " << std::is_integral<E>::value << endl;
    cout << "std::is_integral<float>::value=== " << std::is_integral<float>::value << endl;
    cout << "std::is_integral<int>::value===== " << std::is_integral<int>::value << endl;
    cout << "std::is_integral<bool>::value==== " << std::is_integral<bool>::value << endl;
    cout << "fn(123)========================== " << fn(123) << endl;
    //cout << "fn(123.5)======================== " << fn(123.5) << endl; //编译失败

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

4. is_floating_point Check if the type is a floating point type

#include <iostream>
#include <type_traits>

using namespace std;

class A {};

int main()
{
    cout << "std::is_floating_point<A>::value====== " << std::is_floating_point<A>::value << endl;
    cout << "std::is_floating_point<float>::value== " << std::is_floating_point<float>::value << endl;
    cout << "std::is_floating_point<float>::value== " << std::is_floating_point<float&>::value << endl;
    cout << "std::is_floating_point<double>::value= " << std::is_floating_point<double>::value << endl;
    cout << "std::is_floating_point<double>::value= " << std::is_floating_point<double&>::value << endl;
    cout << "std::is_floating_point<int>::value==== " << std::is_floating_point<int>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

5. is_array    Check if the type is an array type

#include <iostream>
#include <type_traits>

using namespace std;

class A {};

int main()
{
    cout << "std::is_array<A>::value============ " << std::is_array<A>::value << endl;
    cout << "std::is_array<A[]>::value========== " << std::is_array<A[]>::value << endl;
    cout << "std::is_array<A[3]>::value========= " << std::is_array<A[3]>::value << endl;
    cout << "std::is_array<float>::value======== " << std::is_array<float>::value << endl;
    cout << "std::is_array<int>::value========== " << std::is_array<int>::value << endl;
    cout << "std::is_array<int[]>::value======== " << std::is_array<int[]>::value << endl;
    cout << "std::is_array<int[3]>::value======= " << std::is_array<int[3]>::value << endl;
    cout << "std::is_array<array<int,3>>::value= " << std::is_array<array<int,3>>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

6. is_enum    Check if the type is an enumeration type

#include <iostream>
#include <type_traits>

using namespace std;

class A {};

enum E {};

enum class Ec: int {};

int main()
{
    cout << "std::is_enum<A>::value====== " << std::is_enum<A>::value << endl;
    cout << "std::is_enum<E>::value====== " << std::is_enum<E>::value << endl;
    cout << "std::is_enum<Ec>::value===== " << std::is_enum<Ec>::value << endl;
    cout << "std::is_enum<int>::value==== " << std::is_enum<int>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

7. is_union    Check if the type is a union type

#include <iostream>
#include <type_traits>

using namespace std;

class A {};

typedef union {
    int a;
    float b;
} B;

class C {
    B d;
};

int main()
{
    cout << "std::is_union<A>::value======== " << std::is_union<A>::value << endl;
    cout << "std::is_union<B>::value======== " << std::is_union<B>::value << endl;
    cout << "std::is_union<C>::value======== " << std::is_union<C>::value << endl;
    cout << "std::is_union<int>::value====== " << std::is_union<int>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

 运行结果：

8. is_class    Check if the type is not a union class type

#include <iostream>
#include <type_traits>

using namespace std;

struct A { };

class B { } ;

enum class C { };


int main()
{
    cout << "std::is_class<A>::value========== " << std::is_class<A>::value << endl;
    cout << "std::is_class<B>::value========== " << std::is_class<B>::value << endl;
    cout << "std::is_class<C>::value========== " << std::is_class<C>::value << endl;
    cout << "std::is_class<int>::value======== " << std::is_class<int>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

9. is_function    Check if it is a function type

#include <iostream>
#include <type_traits>

using namespace std;

struct A {
    int fun() const &;
};

template<typename>
struct PM_traits { };

template<class T, class U>
struct PM_traits<U T::*> {
    using member_type = U;
};

int f();

int main()
{
    cout << "std::is_function<A>::value================= " << std::is_function<A>::value << endl;
    cout << "std::is_function<int(int)>::value========== " << std::is_function<int(int)>::value << endl;
    cout << "std::is_function<decltype(f)>::value======= " << std::is_function<decltype(f)>::value << endl;
    cout << "std::is_function<int>::value=============== " << std::is_function<int>::value << endl;
    using T = PM_traits<decltype(&A::fun)>::member_type; //T为int() const&
    cout << "std::is_function<T>::value================= " << std::is_function<T>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

10. is_pointer    Check if the type is a pointer type

#include <iostream>
#include <type_traits>

using namespace std;

struct A {
    int fun() const &;
};

int main()
{
    cout << "std::is_pointer<A>::value ================= " << is_pointer<A>::value << endl;
    cout << "std::is_pointer<A *>::value =============== " << is_pointer<A *>::value << endl;
    cout << "std::is_pointer<A &>::value =============== " << is_pointer<A &>::value << endl;
    cout << "std::is_pointer<int>::value =============== " << is_pointer<int>::value << endl;
    cout << "std::is_pointer<int *>::value ============= " << is_pointer<int *>::value << endl;
    cout << "std::is_pointer<int **>::value ============ " << is_pointer<int **>::value << endl;
    cout << "std::is_pointer<int[10]>::value =========== " << is_pointer<int[10]>::value << endl;
    cout << "std::is_pointer<nullptr_t>::value ========= " << is_pointer<nullptr_t>::value << endl;
    cout << "std::is_pointer<decltype(nullptr)>::value = " << is_pointer<decltype(nullptr)>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

11. is_lvalue_reference    Checks if the type is an lvalue reference

#include <iostream>
#include <type_traits>

using namespace std;

class A { };

int main()
{   
    cout << "std::is_lvalue_reference<A>::value====== " << std::is_lvalue_reference<A>::value << endl;
    cout << "std::is_lvalue_reference<A&>::value===== " << std::is_lvalue_reference<A&>::value << endl;
    cout << "std::is_lvalue_reference<A&&>::value==== " << std::is_lvalue_reference<A&&>::value << endl;
    cout << "std::is_lvalue_reference<int>::value==== " << std::is_lvalue_reference<int>::value << endl;
    cout << "std::is_lvalue_reference<int&>::value=== " << std::is_lvalue_reference<int&>::value << endl;
    cout << "std::is_lvalue_reference<int&&>::value== " << std::is_lvalue_reference<int&&>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

12. is_rvalue_reference    Checks if the type is an rvalue reference

#include <iostream>
#include <type_traits>

using namespace std;

class A { };

int main()
{
    cout << "std::is_rvalue_reference<A>::value====== " << std::is_rvalue_reference<A>::value << endl;
    cout << "std::is_rvalue_reference<A&>::value===== " << std::is_rvalue_reference<A&>::value << endl;
    cout << "std::is_rvalue_reference<A&&>::value==== " << std::is_rvalue_reference<A&&>::value << endl;
    cout << "std::is_rvalue_reference<int>::value==== " << std::is_rvalue_reference<int>::value << endl;
    cout << "std::is_rvalue_reference<int&>::value=== " << std::is_rvalue_reference<int&>::value << endl;
    cout << "std::is_rvalue_reference<int&&>::value== " << std::is_rvalue_reference<int&&>::value << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

13. is_member_object_pointer    Checks if the type is a pointer to a non-static member object

#include <iostream>
#include <type_traits>

using namespace std;

int main()
{
    class cls {};
    std::cout << (std::is_member_object_pointer<int(cls::*)>::value
                     ? "T is member object pointer"
                     : "T is not a member object pointer") << endl;
    std::cout << (std::is_member_object_pointer<int(cls::*)()>::value
                     ? "T is member object pointer"
                     : "T is not a member object pointer") << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

14. is_member_function_pointer    Checks if the type is a pointer to a non-static member function

#include <iostream>
#include <type_traits>

using namespace std;

class A {
public:
    void member() { }
};

int main()
{
    // 若 A::member is a data member not a function,fails at compile time
    static_assert(std::is_member_function_pointer<decltype(&A::member)>::value,
                  "A::member is not a member function.");

    cout << "Hello World!" << endl;
    return 0;
}

参考：

标准库头文件 <type_traits> - cppreference.com