const_cast conversion

 

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Operators

Assignment operators: a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b

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Arithmetic operators: +a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b

Logical operators: a||b, a&&b, !a

Comparison operators: a==b, a!=b, a<b, a>b, a<=b, a>=b, a<=>b(C++20)

Member access operators: a[b], *a, &a, a->b, a.b, a->*b, a.*b

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Conversions

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const_cast

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User-defined conversion

 

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, a prvalue of class type or an xvalue of any type 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 a reference type; an rvalue otherwise.	(until C++11)
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.	(since C++11)

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

See also

• static_cast
• dynamic_cast
• reinterpret_cast
• explicit cast
• implicit conversions