const_cast conversion

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression	try-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
Storage duration specifiers
Initialization
Default initialization Value initialization Zero initialization Copy initialization Direct initialization	Aggregate initialization     List initialization (C++11) Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)       final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Expressions

General
Value categories (lvalue, rvalue, xvalue)
Order of evaluation (sequence points)
Constant expressions
Potentially-evaluated expressions
Primary expressions
Lambda expressions(C++11)
Literals
Integer literals
Floating-point literals
Boolean literals
Character literals including escape sequences
String literals
Null pointer literal(C++11)
User-defined literal(C++11)
Operators
Assignment operators: Increment and decrement:   Arithmetic operators: Logical operators: Comparison operators: Member access operators: Other operators:	a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b ++a, --a, a++, a-- +a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b a||b, a&&b, !a a==b, a!=b, a<b, a>b, a<=b, a>=b, a<=>b (since C++20) a[b], *a, &a, a->b, a.b, a->*b, a.*b a(...), a,b, a?b:c
new-expression
delete-expression
throw-expression
alignof
sizeof
sizeof...(C++11)
typeid
noexcept(C++11)
Fold expressions(C++17)
Alternative representations of operators
Precedence and associativity
Operator overloading
Default comparisons(C++20)
Conversions
Implicit conversions
Usual arithmetic conversions
const_cast
static_cast
reinterpret_cast
dynamic_cast
Explicit conversions: (T)a, T(a), auto(a), auto{a} (since C++23)
User-defined conversion

 

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. 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 an 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 target-type.

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.

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