# const_cast conversion

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Converts between types with different cv-qualification.

### Syntax

`const_cast <` new_type `> (` expression `)` |  |

Returns a value of type `new_type`.

### Explanation

Only the following conversions can be done with const_cast. In particular, only const_cast may be used to cast away (remove) constness or volatility.

1) Two possibly multilevel pointers to the same type may be converted between each other, regardless of cv-qualifiers at each level.
2) lvalue of any type `T` may be converted to a lvalue or rvalue reference to the same type `T`, more or less cv-qualified. Likewise, an rvalue may be converted to a more or less cv-qualified rvalue reference. The result of a reference `const_cast` refers to the original object if expression is a glvalue and to the materialized temporary otherwise (since C++17).
3) Same rules apply to possibly multilevel pointers to data members and possibly multilevel pointers to arrays of known and unknown bound (arrays to cv-qualified elements are considered to be cv-qualified themselves) (since C++17)
4) null pointer value may be converted to the null pointer value of new_type

As with all cast expressions, the result is:

- an lvalue if new_type is an lvalue reference type or an rvalue reference to function type;
- an xvalue if new_type is an rvalue reference to object type;
- a prvalue otherwise.

### Notes

Pointers to functions and pointers to member functions are not subject to const_cast

`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.

### Keywords

`const_cast`

### Example

Run this code

    #include <iostream>

    struct type {
        type() :i(3) {}
        void m1(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 i;
    };

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

        type t; // note, if this is const type t;, then t.m1(4); is UB
        t.m1(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::*mfp)(int) const = &type::m1; // pointer to member function
    //  const_cast<void(type::*)(int)>(mfp); // compiler error: const_cast does not
                                             // work on function pointers
    }

Output:

    i = 4
    type::i = 4

### See also

- static_cast
- dynamic_cast
- reinterpret_cast
- explicit cast
- implicit conversions

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