cv (const and volatile) type qualifiers

Appear in any type specifier, including decl-specifier-seq of declaration grammar, to specify constness or volatility of the object being declared or of the type being named.

const - defines that the type is constant.
volatile - defines that the type is volatile.

Explanation

For any type T (including incomplete types), other than function type or reference type, there are three more distinct types in the C++ type system: const-qualified T, volatile-qualified T, and const-volatile-qualified T.

Note: array types are considered to have the same cv-qualification as their element types.

When an object is first created, the cv-qualifiers used (which could be part of decl-specifier-seq or part of a declarator in a declaration, or part of type-id in a new-expression) determine the constness or volatility of the object, as follows:

const object - an object whose type is const-qualified, or a non-mutable subobject of a const object. Such object cannot be modified: attempt to do so directly is a compile-time error, and attempt to do so indirectly (e.g., by modifying the const object through a reference or pointer to non-const type) results in undefined behavior.
volatile object - an object whose type is volatile-qualified, or a subobject of a volatile object, or a mutable subobject of a const-volatile object. Every access (read or write operation, member function call, etc.) made through a glvalue expression of volatile-qualified type is treated as a visible side-effect for the purposes of optimization (that is, within a single thread of execution, volatile accesses cannot be optimized out or reordered with another visible side effect that is sequenced-before or sequenced-after the volatile access. This makes volatile objects suitable for communication with a signal handler, but not with another thread of execution, see std::memory_order). Any attempt to access a volatile object through a glvalue of non-volatile type (e.g. through a reference or pointer to non-volatile type) results in undefined behavior.
const volatile object - an object whose type is const-volatile-qualified, a non-mutable subobject of a const volatile object, a const subobject of a volatile object, or a non-mutable volatile subobject of a const object. Behaves as both a const object and as a volatile object.

Each cv-qualifier (const and volatile) can appear at most once in any cv-qualifier sequence. For example, const const and volatile const volatile are not valid cv-qualifier sequences.

mutable specifier

mutable - permits modification of the class member declared mutable even if the containing object is declared const.

May appear in the declaration of a non-static class members of non-reference non-const type:

class X
{
    mutable const int* p; // OK
    mutable int* const q; // ill-formed
    mutable int&       r; // ill-formed
};

Mutable is used to specify that the member does not affect the externally visible state of the class (as often used for mutexes, memo caches, lazy evaluation, and access instrumentation).

class ThreadsafeCounter
{
    mutable std::mutex m; // The "M&M rule": mutable and mutex go together
    int data = 0;
public:
    int get() const
    {
        std::lock_guard<std::mutex> lk(m);
        return data;
    }
 
    void inc()
    {
        std::lock_guard<std::mutex> lk(m);
        ++data;
    }
};

Conversions

There is partial ordering of cv-qualifiers by the order of increasing restrictions. The type can be said more or less cv-qualified than:

unqualified < const
unqualified < volatile
unqualified < const volatile
const < const volatile
volatile < const volatile

References and pointers to cv-qualified types may be implicitly converted to references and pointers to more cv-qualified types. In particular, the following conversions are allowed:

reference/pointer to unqualified type can be converted to reference/pointer to const
reference/pointer to unqualified type can be converted to reference/pointer to volatile
reference/pointer to unqualified type can be converted to reference/pointer to const volatile
reference/pointer to const type can be converted to reference/pointer to const volatile
reference/pointer to volatile type can be converted to reference/pointer to const volatile

Note: additional restrictions are imposed on multi-level pointers.

To convert a reference or a pointer to a cv-qualified type to a reference or pointer to a less cv-qualified type, const_cast must be used.

Keywords

const, volatile, mutable

Notes

The const qualifier used on a declaration of a non-local non-volatile non-template (since C++14)non-inline (since C++17) variable that is not declared extern gives it internal linkage. This is different from C where const file scope variables have external linkage.

The C++ language grammar treats mutable as a storage-class-specifier, rather than a type qualifier, but it does not affect storage class or linkage.

Some uses of volatile are deprecated:

lvalue of volatile type as operand of built-in increment/decrement operators;
lvalue of volatile type as left operand of built-in direct assignment, unless the direct assignment expression appears in an unevaluated context or is a discarded-value expression;
volatile object type as function parameter type or return type;
volatile qualifier in structured binding declaration.

(since C++20)

Example

Run this code

#include <cstdlib>
 
int main()
{
    int n1 = 0;          // non-const object
    const int n2 = 0;    // const object
    int const n3 = 0;    // const object (same as n2)
    volatile int n4 = 0; // volatile object
 
    const struct
    {
        int n1;
        mutable int n2;
    } x = {0, 0};        // const object with mutable member
 
    n1 = 1;   // ok, modifiable object
//  n2 = 2;   // error: non-modifiable object
    n4 = 3;   // ok, treated as a side-effect
//  x.n1 = 4; // error: member of a const object is const
    x.n2 = 4; // ok, mutable member of a const object isn't const
 
    const int& r1 = n1; // reference to const bound to non-const object
//  r1 = 2; // error: attempt to modify through reference to const
    const_cast<int&>(r1) = 2; // ok, modifies non-const object n1
 
    const int& r2 = n2; // reference to const bound to const object
//  r2 = 2; // error: attempt to modify through reference to const
//  const_cast<int&>(r2) = 2; // undefined behavior: attempt to modify const object n2
 
    [](...){}(n3, n4, x, r2); // see also: [[maybe_unused]]
 
    std::system("g++ -O3 -Wa,-adhln ./main.cpp"); // may issue asm on POSIX systems
}

Possible output:

# typical machine code produced on an x86_64 platform
# (only the code that contributes to observable side-effects is emitted)
main:
    movl    $0, -4(%rsp) # volatile int n4 = 0;
    movl    $3, -4(%rsp) # n4 = 3;
    xorl    %eax, %eax   # return 0 (implicit)
    ret

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 1428	C++98	the definition of const object was based on declaration	based on object type
CWG 1528	C++98	there was no requirement on the number of occurrences of each cv-qualifier in the same cv-qualifier sequence	at most once for each cv-qualifier
CWG 1799	C++98	mutable could be applied to data members not declared const, but the members' types may still be const-qualified	cannot apply mutable to data members of const-qualified types

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