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const_cast

drop CV是危险的，但是C++还是给programmer提供了这种权利，这是遵循C++ philosophy的（C++\Philosophy.md）。

cppreference const_cast

only const_cast may be used to cast away (remove) constness or volatility.

NOTE: C++中drop CV的唯一的方法

1)

2)

NOTE: 需要注意的是，必须是相同类型。

Example: this

#include <iostream>

struct type
{
    int i;

    type() : i(3)
    {
    }

    void f(int v) const
    {
        // this->i = v;                 // compile error: this is a pointer to const
        const_cast<type*>(this)->i = v; // OK as long as the type object isn't const
    }
};

int main()
{
    type t; // if this was const type t, then t.f(4) would be undefined behavior
    t.f(4);
    std::cout << "type::i = " << t.i << '\n';
}
// g++ test.cpp

Example: reference

#include <iostream>

int main()
{
    int i = 3;                 // i is not declared const
    const int& rci = i;
    const_cast<int&>(rci) = 4; // OK: modifies i
    std::cout << "i = " << i << '\n';
}
// g++ test.cpp

Example: different type

#include "stddef.h"

int main()
{
    const void* ptr = NULL;
    const_cast<char*>(ptr); // compiler error
}
// g++ test.cpp

上述程序编译报错如下：

test.cpp: 在函数‘int main()’中:
test.cpp:6:23: 错误：从类型‘const void*’到类型‘char*’中的 const_cast 无效
  const_cast<char*>(ptr); // compiler errorc

Example: cppreference const_cast

#include <iostream>

struct type
{
    int i;

    type()
            : i(3)
    {
    }

    void f(int v) const
    {
        // this->i = v;                 // compile error: this is a pointer to const
        const_cast<type*>(this)->i = v; // OK as long as the type object isn't const
    }
};

int main()
{
    int i = 3;                 // i is not declared const
    const int& rci = i;
    const_cast<int&>(rci) = 4; // OK: modifies i
    std::cout << "i = " << i << '\n';

    type t; // if this was const type t, then t.f(4) would be undefined behavior
    t.f(4);
    std::cout << "type::i = " << t.i << '\n';

    const int j = 3; // j is declared const
    int* pj = const_cast<int*>(&j);
    // *pj = 4;      // undefined behavior

    void (type::*pmf)(int) const = &type::f; // pointer to member function
    // const_cast<void(type::*)(int)>(pmf);   // compile error: const_cast does
    // not work on function pointers
}
// g++ test.cpp

3)

NOTE

const_cast makes it possible to form a reference or pointer to non-const type that is actually referring to a const object or a reference or pointer to non-volatile type that is actually referring to a volatile object. Modifying a const object through a non-const access path and referring to a volatile object through a non-volatile glvalue results in undefined behavior.

NOTE: 虽然危险，但是给予了programmer这样的权利

https://stackoverflow.com/questions/19554841/how-to-use-const-cast

modify a const variable

https://www.geeksforgeeks.org/how-to-modify-a-const-variable-in-c/

https://stackoverflow.com/questions/2006161/changing-the-value-of-const-variable-in-c

Cast away CV

阅读static_cast conversion、dynamic_cast conversion，可以看到两者都有这样的描述：

except when such conversions would cast away constness or volatility.

如果cast away CV，compiler是会报错的：

static_cast from 'const clone_inherit<concrete, cloneable>' to 'concrete &' casts away qualifiers