Difference between revisions of "cpp/language/cv"

Revision as of 06:14, 2 June 2015

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.
• mutable - applies to non-static class members of non-reference non-const type and specifies 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). mutable members of const classes are modifiable (Note: the C++ language grammar treats mutable as a storage-class-specifier, but it does not affect storage class)

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.) on the volatile object is treated as a visible side-effect for the purposes of optimization (that is, within a single thread of execution, volatile accesses cannot be reordered or optimized out. 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 refer to a volatile object through a non-volatile glvalue (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.

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

• 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

Example

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 X {
        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 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
}

# 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

See Also