cv (const and volatile) type qualifiers (original) (raw)

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.

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[edit] Explanation

Any (possibly incomplete) type other than function type or reference type is a type in a group of the following four distinct but related types:

These four types in the same group have the same representation and alignment requirements.

Array types are considered to have the same cv-qualification as their element types.

[edit] const and volatile objects

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:

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.

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.

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.

[edit] mutable specifier

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).

[edit] 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:

References and pointers to cv-qualified types can be implicitly converted to references and pointers to more cv-qualified types, see qualification conversions for details.

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.

[edit] 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.

[edit] Keywords

const, volatile,mutable

[edit] Example

#include   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

[edit] 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 occurrencesof each cv-qualifier in the same cv-qualifier sequence at most once foreach cv-qualifier
CWG 1799 C++98 mutable could be applied to data members not declaredconst, but the members' types may still be const-qualified cannot apply mutable to datamembers of const-qualified types

[edit] See also