# `const_cast` conversion

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Converts between types with different cv-qualification.

### Syntax

`const_cast<` target-type `>(` expression `)` |  |

Returns a value of type target-type.

### Explanation

Only the following conversions can be done with const_cast:

1) For two similar object pointer or pointer to data member types `T1` and `T2`, a prvalue of type `T1` can be converted to `T2` if `T1` and `T2` differ only in cv-qualification (formally, if, considering the qualification-decompositions of both types, each `P1_i` is the same as `P2_i` for all i).

- If expression is a null pointer value, the result is also a null pointer value.
- If expression is a null member pointer value, the result is also a null member pointer value.
- If expression points to an object, the result points to the same object.
- If expression points past an object, the result points past the same object.
- If expression points to a data member, the result points to the same data member.

Even if expression is a prvalue, temporary materialization is not performed. | (since C++17)

2) For two object types `T1` and `T2`, if a pointer to `T1` can be explicitly converted to the type “pointer to `T2`” using const_cast<T2*>, then the following conversions can also be made:

- An lvalue of type `T1` can be explicitly converted to an lvalue of type `T2` using const_cast<T2&>.

- A glvalue of type `T1` can be explicitly converted to an xvalue of type `T2` using const_cast<T2&&>.
- If `T1` is a class or array type, a prvalue of type `T1` can be explicitly converted to an xvalue of type `T2` using const_cast<T2&&>.

(since C++11)

The result reference refers to the original object. | (until C++17)
If expression is a glvalue, the result reference refers to the original object. Otherwise, the result reference refers to the materialized temporary. | (since C++17)

As with all cast expressions, the result is:

- an lvalue if target-type is an lvalue reference type or an rvalue reference to function type(since C++11);

- an xvalue if target-type is an rvalue reference to object type;

(since C++11)

- a prvalue otherwise.

### Casting away constness

For two different types `T1` and `T2`, a conversion from `T1` to `T2` casts away constness if there exists a qualification-decomposition of `T2` of the form “cv2_0 P2_0 cv2_1 P2_1 ... cv2_n−1 P2_n−1 cv2_n U2”, and there is no qualification conversions that converts `T1` to “cv2_0 P1_0 cv2_1 P1_1 ... cv2_n−1 P1_n−1 cv2_n U1” (same cv-components, different P-components and U-components).

If a cast from a prvalue of type `T1*` to the type `T2*` casts away constness, casting from an expression of type `T1` to a reference to `T2` will also cast away constness.

Only const_cast may be used to cast away constness.

“Casting away constness” implies “casting away volatility”, as qualification conversions cannot cast away volatility as well.

### 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
    {
        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
        [[maybe_unused]]
        int* pj = const_cast<int*>(&j);
        // *pj = 4;      // undefined behavior

        [[maybe_unused]]
        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
    }

Output:

    i = 4
    type::i = 4

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 1965 | C++11 | const_cast could not bind rvalue references to array prvalues | allowed to bind such references
CWG 2879 | C++17 | pointer pvalue operands were materialized | they are not materialized

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 7.6.1.11 Const cast [expr.const.cast]

- C++20 standard (ISO/IEC 14882:2020):

  - 7.6.1.10 Const cast [expr.const.cast]

- C++17 standard (ISO/IEC 14882:2017):

  - 8.2.11 Const cast [expr.const.cast]

- C++14 standard (ISO/IEC 14882:2014):

  - 5.2.11 Const cast [expr.const.cast]

- C++11 standard (ISO/IEC 14882:2011):

  - 5.2.11 Const cast [expr.const.cast]

- C++98 standard (ISO/IEC 14882:1998):

  - 5.2.11 Const cast [expr.const.cast]

- C++03 standard (ISO/IEC 14882:2003):

  - 5.2.11 Const cast [expr.const.cast]

### See also

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

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