type

type_traits是元编程库的一部分，这个库主要用来判断数据类型，比如，判断类型是否为空，是否为空指针，是否为整型，是否为浮点型是否为数组，是否为枚举类型，是否为联合体，是否为函数，是否为指针，是否为左值引用，是否为右值引用，等等，判断的类型非常多，本篇介绍几个基本的。

1. is_void    检查类型是否为 void

#include 
#include 

using namespace std;

int main()
{
    cout << "std::is_void::value==========" << std::is_void::value << endl;
    cout << "std::is_void::value===========" << std::is_void::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 
#include 

using namespace std;

int main()
{
    cout << "std::is_null_pointer::value = " << is_null_pointer::value << endl;
    cout << "std::is_null_pointer::value = " << is_null_pointer::value << endl;

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

运行结果：

3. is_integral    检查类型是否为整型

#include 
#include 

using namespace std;

class A {};

enum E: int {};

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

int main()
{
    cout << "std::is_integral::value======= " << std::is_integral::value << endl;
    cout << "std::is_integral::value======= " << std::is_integral::value << endl;
    cout << "std::is_integral::value=== " << std::is_integral::value << endl;
    cout << "std::is_integral::value===== " << std::is_integral::value << endl;
    cout << "std::is_integral::value==== " << std::is_integral::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 检查类型是否是浮点类型

#include 
#include 

using namespace std;

class A {};

int main()
{
    cout << "std::is_floating_point::value====== " << std::is_floating_point::value << endl;
    cout << "std::is_floating_point::value== " << std::is_floating_point::value << endl;
    cout << "std::is_floating_point::value== " << std::is_floating_point::value << endl;
    cout << "std::is_floating_point::value= " << std::is_floating_point::value << endl;
    cout << "std::is_floating_point::value= " << std::is_floating_point::value << endl;
    cout << "std::is_floating_point::value==== " << std::is_floating_point::value << endl;

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

运行结果：

5. is_array    检查类型是否是数组类型

#include 
#include 

using namespace std;

class A {};

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

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

运行结果：

6. is_enum    检查类型是否是枚举类型

#include 
#include 

using namespace std;

class A {};

enum E {};

enum class Ec: int {};

int main()
{
    cout << "std::is_enum::value====== " << std::is_enum::value << endl;
    cout << "std::is_enum::value====== " << std::is_enum::value << endl;
    cout << "std::is_enum::value===== " << std::is_enum::value << endl;
    cout << "std::is_enum::value==== " << std::is_enum::value << endl;

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

运行结果：

7. is_union    检查类型是否为联合体类型

#include 
#include 

using namespace std;

class A {};

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

class C {
    B d;
};

int main()
{
    cout << "std::is_union::value======== " << std::is_union::value << endl;
    cout << "std::is_union::value======== " << std::is_union::value << endl;
    cout << "std::is_union::value======== " << std::is_union::value << endl;
    cout << "std::is_union::value====== " << std::is_union::value << endl;

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

 运行结果：

8. is_class    检查类型是否非联合类类型

#include 
#include 

using namespace std;

struct A { };

class B { } ;

enum class C { };


int main()
{
    cout << "std::is_class::value========== " << std::is_class::value << endl;
    cout << "std::is_class::value========== " << std::is_class::value << endl;
    cout << "std::is_class::value========== " << std::is_class::value << endl;
    cout << "std::is_class::value======== " << std::is_class::value << endl;

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

运行结果：

9. is_function    检查是否为函数类型

#include 
#include 

using namespace std;

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

template
struct PM_traits { };

template
struct PM_traits {
    using member_type = U;
};

int f();

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

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

#include 
#include 

using namespace std;

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

int main()
{
    cout << "std::is_pointer::value ================= " << is_pointer::value << endl;
    cout << "std::is_pointer::value =============== " << is_pointer::value << endl;
    cout << "std::is_pointer::value =============== " << is_pointer::value << endl;
    cout << "std::is_pointer::value =============== " << is_pointer::value << endl;
    cout << "std::is_pointer::value ============= " << is_pointer::value << endl;
    cout << "std::is_pointer::value ============ " << is_pointer::value << endl;
    cout << "std::is_pointer::value =========== " << is_pointer::value << endl;
    cout << "std::is_pointer::value ========= " << is_pointer::value << endl;
    cout << "std::is_pointer::value = " << is_pointer::value << endl;

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

#include 
#include 

using namespace std;

class A { };

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

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

#include 
#include 

using namespace std;

class A { };

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

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

#include 
#include 

using namespace std;

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

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

#include 
#include 

using namespace std;

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

int main()
{
    // 若 A::member 是数据成员而非函数，则在编译时失败
    static_assert(std::is_member_function_pointer::value,
                  "A::member is not a member function.");

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