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# const_cast conversion

From cppreference.com
< cpp‎ | language

Converts between types with different cv-qualification.

### [edit] Syntax

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

Returns a value of type `new_type`.

### [edit] 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.
3) Same rules apply to possibly multilevel pointers to data members.
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.

### [edit] Notes

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

Even though const_cast may remove constness from any pointer or reference, using the resulting pointer or reference to write to an object that was declared const invokes undefined behavior.

### [edit] Keywords

`const_cast`

### [edit] 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
        }
        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;
        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

### [edit] See also

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

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