const_cast conversion

< 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 or volatility from any pointer or reference, using the resulting pointer or reference to write to an object that was declared const or to access an object that was declared volatile invokes undefined behavior.

[edit] Keywords


[edit] Example

#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
    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


i = 4
type::i = 4

[edit] See also