const_cast conversion

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Revision as of 08:15, 3 January 2014 by Cubbi (Talk | contribs)


Converts between types with different cv-qualification.



const_cast < new_type > ( expression )

Returns a value of type new_type.


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.


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.




#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;
    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

See also