remove_cv

去掉变量的cv属性，获取其原始类型信息。

实现方式
模版推导丢弃const和volatile参数。

性能开销
模版推导完成操作，不涉及运行时开销。

调用stl
remove_const
remove_volatile

// Const-volatile modifications.

/// remove_const
template<typename _Tp>
struct remove_const
{ typedef _Tp     type; };

template<typename _Tp>
struct remove_const<_Tp const>
{ typedef _Tp     type; };

/// remove_volatile
template<typename _Tp>
struct remove_volatile
{ typedef _Tp     type; };

template<typename _Tp>
struct remove_volatile<_Tp volatile>
{ typedef _Tp     type; };

/// remove_cv
template<typename _Tp>
struct remove_cv
{
  typedef typename
  remove_const<typename remove_volatile<_Tp>::type>::type     type;
};

代码示例

代码源地址：http://www.cplusplus.com/reference/type_traits/remove_cv/
自己添加了b、c、const char*的输出。
这里b、c都会被去掉const，这是模版自动完成的吗？有些小疑问？

// remove_cv example
#include <iostream>
#include <type_traits>

int main() {
  typedef const volatile char cvchar;
  std::remove_cv<cvchar>::type a;       // char a
  std::remove_cv<char* const>::type b;  // char* b
  std::remove_cv<const char*>::type c;  // const char* c (no changes)

  if (std::is_const<decltype(a)>::value)
    std::cout << "type of a is const" << std::endl;
  else
    std::cout << "type of a is not const" << std::endl;

  if (std::is_volatile<decltype(a)>::value)
    std::cout << "type of a is volatile" << std::endl;
  else
    std::cout << "type of a is not volatile" << std::endl;

  if (std::is_const<decltype(b)>::value)
    std::cout << "type of b is const" << std::endl;
  else
    std::cout << "type of b is not const" << std::endl;

  if (std::is_const<decltype(c)>::value)
    std::cout << "type of c is const" << std::endl;
  else
    std::cout << "type of c is not const" << std::endl;

  if (std::is_const<const char*>::value)
    std::cout << "type of const char* is const" << std::endl;
  else
    std::cout << "type of const char* is not const" << std::endl;

  return 0;
}

Output:
type of a is not const
type of a is not volatile
type of b is not const
type of c is not const
type of const char* is not const

应用场景

Obtains the type T without any top-level const or volatile qualification.

参考：gcc8源码