# cv (const and volatile) type qualifiers

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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 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.

| This section is incomplete Reason: should discuss more about the differences between cv-qualified objects and cv-qualified expressions

### 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 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 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`

### 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.

### Example

Run this code

    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
    }

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

### See also

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