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。
基础类型分类 |
|
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) |
检查类型是否为整型 (类模板) |
is_floating_point (C++11) |
检查类型是否是浮点类型 (类模板) |
is_array (C++11) |
检查类型是否是数组类型 (类模板) |
is_enum (C++11) |
检查类型是否是枚举类型 (类模板) |
is_union (C++11) |
检查类型是否为联合体类型 (类模板) |
is_class (C++11) |
检查类型是否非联合类类型 (类模板) |
is_function (C++11) |
检查是否为函数类型 (类模板) |
is_pointer (C++11) |
检查类型是否为指针类型 (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) |
检查类型是否为指向非静态成员对象的指针 (类模板) |
is_member_function_pointer (C++11) |
检查类型是否为指向非静态成员函数的指针 (类模板) |
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;
}
运行结果:
10. is_pointer 检查类型是否为指针类型
#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;
}
运行结果:
11. is_lvalue_reference 检查类型是否为左值引用
#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;
}
运行结果:
12. is_rvalue_reference 检查类型是否为右值引用
#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;
}
运行结果:
13. is_member_object_pointer 检查类型是否为指向非静态成员对象的指针
#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;
}
运行结果:
14. is_member_function_pointer 检查类型是否为指向非静态成员函数的指针
#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;
}
参考:
标准库头文件