Member access operators

 

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Accesses a member of an object.

Operator name	Syntax	Over​load​able	Prototype examples (for class T)
			Inside class definition	Outside class definition
array subscript	a[b]	Yes	R& T::operator[](const T2& b);	N/A
indirection (variable pointed to by a)	*a	Yes	R& T::operator*();	R& operator*(T &a);
address of	&a	Yes	R* T::operator&();	R* operator&(T &a);
member of object	a.b	No	N/A	N/A
member of pointer	a->b	Yes	R* T::operator->()	N/A
pointer to member of object	a.*b	No	N/A	N/A
pointer to member of pointer	a->*b	Yes	R* T::operator->*(R)	R* T::operator->*(T, R)
Notes As with most user-defined overloads, return types should match return types provided by the builtin operators so that the user-defined operators can be used in the same manner as the built-ins. However, in a user-defined operator overload, any type can be used as return type (including void). One exception is operator->, which must return a pointer.

Explanation

array subscript operator provides access to the elements in the internal array

indirection, member of pointer and pointer to member of pointer operators provide pointer semantics for any object.

member of pointer and pointer to member of pointer operators return a pointer to the actual object which will be used for member access.

Built-in subscript operator

For every object type T (possibly cv-qualified), the following function signature participates in overload resolution:

T& operator[](T*, std::ptrdiff_t);
T& operator[](std::ptrdiff_t, T*);

The non-pointer operand may be any expression of integral or unscoped enumeration type, it is implicitly converted to std::ptrdiff_t. The expression A[B] is exactly identical to the expression *(A+B), that is, the pointer operand (which may be a result of array-to-pointer conversion, and which must point to an element of some array or one past the end) is adjusted to point at another element of the same array, following the rules of pointer arithmetics, and is then dereferenced.

run this code

#include <iostream>
 
int main()
{
    int a[4] = {1,2,3,4};
    int* p = &a[2];
    std::cout << p[1] << p[-1] << 1[p] << (-1)[p] << '\n';
}

Output:

4242

Built-in indirection operator

For every type T that is either object type (possibly cv-qualified) or function type (not const- or ref-qualified), the following function signature participates in overload resolution:

T& operator*(T*);

The operand of the built-in indirection operator is a pointer to object or function, and the result is the lvalue that the pointer is pointing at. Note that a pointer to incomplete type can be dereferenced, e.g. when initializing a reference.

run this code

#include <iostream>
int f() { return 42; }
int main()
{
    int n = 1;
    int* pn = &n;
 
    int& r = *pn;  // lvalue can be bound to a reference
    int m = *pn;   // indirection + lvalue-to-rvalue conversion
 
    int (*fp)() = &f;
    int (&fr)() = *fp; // function lvalue can be bound to a reference 
}

Built-in address-of operator

The operand of the built-in operator& is either an lvalue expression of any type or the qualified name of a non-static member function/object in some class. This operator does not participate in overload resolution, special rules are used:

If the operand is an lvalue expression of some type T, operator& creates and returns a prvalue of type T*, with the same cv qualification, that is pointing at the object designated by the operand. If the operand has incomplete type, the pointer can be formed, but if that incomplete type happens to be a class that defines its own operator&, the behavior is undefined. For the operands of type with user-defined operator&, std::addressof may be used to obtain the true pointer.

If the operand is the name of an overloaded function, the address may be taken only if the overload can be resolved due to context, that is, the result of operator& is used to initialize an object, in a cast expression, on the left of an assignment, as a function parameter or in a return statement.

If the operand is a qualified name of a non-static member, e.g. &Class::member, the result is a prvalue pointer to member function or pointer to member object of type T in class C. Note that neither &member nor Class::member nor even (&Class::member) may be used to initialize a pointer to member.

run this code

void f(int) {}
void f(double) {}
struct A { int i; };
struct B { void f(); };
int main()
{
    int n = 1;
    int* pn = &n; // pointer
    int A::* mp = &A::i;  // pointer to member object
    void (B::*mpf)() = &B::f; // pointer to member function
 
//    auto pf2 = &f; // error: ambiguous overloaded function type
    void (*pf)(int) = &f; // overload resolution due to initialization
    auto pf2 = static_cast<void(*)(int)>(&f); // overload resolution due to cast
}

Built-in member access operators

The left operand of the built-in operator. and operator-> is an expression of complete class type T (for operator.) or pointer to complete class type T* (for operator->, which is evaluated before the operator can be called. The right operand is the name of a member object or member function of T or of one of T's base classes, e.g. expr.member, optionally qualified, e.g. expr.name::member, optionally prepended by the keyword template, e.g. expr.template member.

The expression A->B is exactly equivalent to (*A).B for builtin types. If a user-defined operator-> is provided, operator-> is called again on the value that it returns, recursively, until the operator-> is reached that returns a plain pointer. After that, builtin semantics are applied to that pointer.

In the expression expr.B,

1) if B has type T&, the result is an lvalue T.

2) if B is a static data member of type T, the result is an lvalue T designating that static data member.

3) if B is a non-static data member of type T, the result is an lvalue if expr is an lvalue, the result is xvalue if expr is an xvalue, and the result is a prvalue otherwise. If B is not declared to be a mutable member, the cv-qualification of the result is the union of cv-qualifications of expr and B. If B is mutable, the cv-qualification of the result is the union of volatile-qualifications.

4) if B is a static member function, the result is an lvalue designating the static memeber function. Essentially, expr is evaluated and discarded in this case.

5) if B is a non-static member function, the result is a special kind of prvalue that can only be used as the left-hand operand in a function-call expression, and for no other purpose.

6) if B is a member enumeration, the result is a prvalue of the.

7) if B is a nested type, the program is ill-formed (won't compile)

8) if expr has non-class scalar type and B is the type name or decltype specifier designating the same type (minus cv-qualifications), then the result is a special kind of prvalue that can only be used as the left-hand operand in a function-call expression, and for no other purpose. The function call is called pseudo destructor call', it takes no arguments, returns void, and performs no action other than initial evaluation of expr.

run this code

#include <iostream>
template<typename T>
struct P { typedef T* ptr; };
template<typename T>
struct A {
    class B {};
    enum E {RED = 1, BLUE = 2};
    int n;
    static int sn;
    int f() { return 10+n; }
    static int fs() { return 4; }
    A(int n) : n(n) {}
 
    // keyword template needed to refer to a dependent template member
    void g() {
        T obj;
        int* p = obj.template ptr<int>;
        p->~T(); // T is int, this calls int's pseudo destructor
    }
};
template<> int A<P<int>>::sn = 2;
 
int main()
{
    A<P<int>> a(1);
    std::cout << a.n << ' '
              << a.sn << ' ' // << A::sn also works
              << a.f() << ' ' 
              << a.fs() << ' ' // A::fs() also works
              << a.RED << ' '  // nested type not allowed
//            << a.B  // nested type not allowed
              ;
}

Output:

1 2 11 4 1

Built-in pointer-to-member access operators

The right operand of both operator.* and operator->* is an expression of type pointer to member in class T. For operator.*, the left operand is an expression of class type T, or of some derived class in which T is unambiguous accessible base. For operator->*, the left operand is a pointer to T or to its base.

For every combination of types B, D, T, where D is either the same as B or a class derived from B, and T is either object or function type, the following function signature participates in overload resolution:

T& operator->*(B*, T D::*);

where both operands may be cv-qualified, in which case the return type's cv-qualification is the union of the cv-qualification of the operands.

The expression E1->*E2 is exactly equivalent to (*E1).*E2 for built-in types.

For the expression expr.*ptr,

1) If the dynamic type of expr does not contain the member to which ptr refers, the behavior is undefined

2) cv-qualification rules are the same as for member access operator, with one additional rule: a pointer to member that refers to a mutable member cannot be used to modify that member in a const object.

4) If expr is rvalue and ptr points to a member function with & ref-qualifier, the program is ill-formed

5) If expr is lvalue and ptr points to a member function with && ref-qualifier, the program is ill-formed

6) The result of expr.*ptr where ptr is a pointer to data member has the same value category as expr.

7) The result of expr.*ptr where ptr is a pointer to member function is a specail kind of prvalue that may only be used as the left-hand argument of a function call expression and for no other purpose.

8) If ptr is a null pointer value, the behavior is undefined

run this code

#include <iostream>
struct S
{
    mutable int mi;
    int f(int n) { return mi+n; }
    S(int n) : mi(n) {}
};
struct D : public S {
        D(int n) : S(n) {}
};
 
int main()
{
    int S::* pmi = &S::mi;
    int (S::*mpf)(int) = &S::f;
 
    const S s(7);
//    s.*pmi = 10; // cannot modify through mutable
    std::cout << s.*pmi << '\n';
 
    D d(7); // base pointers work with derived object
    D* dp = &d;
    std::cout << (d.*mpf)(7) << ' '
              << (dp->*mpf)(8) << '\n';
}

Output:

7
14 15

Standard library

Subscript operator is overloaded by many standard container classes

operator[]	accesses specific bit (public member function of std::bitset)
operator[]	provides indexed access to the managed array (public member function of std::unique_ptr)
operator[]	access specified character (public member function of std::basic_string)
operator[]	access specified element (public member function of std::array)
operator[]	access specified element (public member function of std::deque)
operator[]	access specified element (public member function of std::vector)
operator[]	access specified element (public member function of std::map)
operator[]	access specified element (public member function of std::unordered_map)
operator[]	accesses an element by index (public member function of std::reverse_iterator)
operator[]	obtains rvalue reference to indexed element (public member function of std::move_iterator)
operator[]	get/set valarray element, slice, or mask (public member function of std::valarray)
operator[]	returns specified sub-match (public member function of std::match_results)

The indirection and member access operators are overloaded by many iterators and smart pointer classes

operator* operator->	dereferences pointer to the managed object (public member function of std::unique_ptr)
operator* operator->	dereferences pointer to the managed object (public member function of std::shared_ptr)
operator* operator->	accesses the managed object (public member function of std::auto_ptr)
operator*	returns a reference to this raw_storage_iterator (public member function of std::raw_storage_iterator)
operator* operator->	dereferences the decremented underlying iterator (public member function of std::reverse_iterator)
operator*	no-op (public member function of std::back_insert_iterator)
operator*	no-op (public member function of std::front_insert_iterator)
operator*	no-op (public member function of std::insert_iterator)
operator* operator->	accesses the pointed-to element (public member function of std::move_iterator)
operator* operator->	obtains a copy of the current element accesses a member of the current element (public member function of std::istream_iterator)
operator*	no-op (public member function of std::ostream_iterator)
operator* operator-> (since C++11)	obtains a copy of the current character accesses a member of the current character, if CharT has members (public member function of std::istreambuf_iterator)
operator*	no-op (public member function of std::ostreambuf_iterator)
operator* operator->	accesses the current match accesses a member of the current match (public member function of std::regex_iterator)
operator* operator->	accesses the current result accesses a member of the current result (public member function of std::regex_token_iterator)

No standard library classes overload operator& or operator->*. The best known example of overloaded operator& is the Microsoft COM class CComPtr. operator->* is sometimes overloaded by third-party custom smart pointers.

See also

Operator precedence

Common operators
assignment	increment decrement	arithmetic	logical	comparison	member access	other
a = b a = rvalue 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 a && b a || b	a == b a != b a < b a > b a <= b a >= b	a[b] *a &a a->b a.b a->*b a.*b	a(...) a, b (type) a ? :
Special operators
static_cast converts one type to another compatible type dynamic_cast converts virtual base class to derived class const_cast converts type to compatible type with different cv qualifiers reinterpret_cast converts type to incompatible type new allocates memory delete deallocates memory sizeof queries the size of a type sizeof... queries the size of a parameter pack (since C++11) typeid queries the type information of a type noexcept checks if an expression can throw an exception (since C++11) alignof queries alignment requirements of a type (since C++11)