Converts between types with different cv-qualification.

[edit]Syntax

Returns a value of typetarget-type.

[edit]Explanation

Only the following conversions can be done withconst_cast:

As with all cast expressions, the result is:

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

• a prvalue otherwise.

[edit]Casting away constness

For two different typesT1 andT2, a conversion fromT1 toT2casts away constness if there exists aqualification-decomposition ofT2 of the form “cv2_0 P2_0 cv2_1 P2_1 ... cv2_n−1 P2_n−1 cv2_n U2”, and there is noqualification conversions that convertsT1 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 typeT1* to the typeT2* casts away constness, casting from an expression of typeT1 to a reference toT2 will also cast away constness.

Onlyconst_cast may be used to cast away constness.

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

[edit]Notes

Pointers to functions and pointers to member functions are not subject toconst_cast.

const_cast makes it possible to form a reference or pointer to non-const type that is actually referring to aconst object or a reference or pointer to non-volatile type that is actually referring to avolatile object. Modifying a const object through a non-const access path and referring to a volatile object through a non-volatileglvalue results in undefined behavior.

[edit]Keywords

const_cast

[edit]Example

#include <iostream> struct type{int i;     type(): i(3){} void f(int v)const{// this->i = v;                 // compile error: this is a pointer to constconst_cast<type*>(this)->i= v;// OK as long as the type object isn't const}}; int main(){int i=3;// i is not declared constconstint& rci= i;const_cast<int&>(rci)=4;// OK: modifies istd::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'; constint 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}

i = 4type::i = 4

[edit]Defect reports

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

[edit]References

[edit]See also

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

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