Pointer declaration

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C++
C++ language
Declarations

Declares a variable of a pointer or pointer-to-member type.

Syntax

A pointer declaration is any simple declaration whose declarator has the form

* attr(optional) cv(optional) declarator (1)
nested-name-specifier * attr(optional) cv(optional) declarator (2)
1) Pointer declarator: the declaration S* D; declares D as a pointer to the type determined by decl-specifier-seq S.
2) Pointer to member declarator: the declaration S C::* D; declares D as a pointer to non-static member of C of type determined by decl-specifier-seq S.
declarator - any declarator other than a reference declarator (there are no pointers to references). It can be another pointer declarator (pointer to pointers are allowed)
attr(C++11) - optional list of attributes
cv - const/volatile qualification which apply to the pointer that is being declared (not to the pointed-to type, whose qualifications are part of decl-specifier-seq)
nested-name-specifier - a sequence of names and scope resolution operators ::

There are no pointers to references and there are no pointers to bit fields. Typically, mentions of "pointers" without elaboration do not include pointers to (non-static) members.

Pointers

Every value of pointer type is one of the following:

  • a pointer to an object or function (in which case the pointer is said to point to the object or function), or
  • a pointer past the end of an object, or
  • the null pointer value for that type, or
  • an invalid pointer value.

A pointer that points to an object represents the address of the first byte in memory occupied by the object. A pointer past the end of an object represents the address of the first byte in memory after the end of the storage occupied by the object.

Note that two pointers that represent the same address may nonetheless have different values. 

struct C {
   int x, y;
} c;

int* px = &c.x;   // value of px is "pointer to c.x"
int* pxe= px + 1; // value of pxe is "pointer past the end of c.x"
int* py = &c.y;   // value of py is "pointer to c.y"

assert(pxe == py); // == tests if two pointers represent the same address
                   // may or may not fire

*pxe = 1; // undefined behavior even if the assertion does not fire


Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior. Any other use of an invalid pointer value has implementation-defined behavior.

Pointers to objects

A pointer to object can be initialized with the return value of the address-of operator applied to any expression of object type, including another pointer type: 

int n;
int* np = &n; // pointer to int
int* const* npp = &np; // non-const pointer to const pointer to non-const int

int a[2];
int (*ap)[2] = &a; // pointer to array of int

struct S { int n; };
S s = {1};
int* sp = &s.n; // pointer to the int that is a member of s

Pointers may appear as operands to the built-in indirection operator (unary operator*), which returns the lvalue expression identifying the pointed-to object: 

int n;
int* p = &n;     // pointer to n
int& r = *p;     // reference is bound to the lvalue expression that identifies n
r = 7;           // stores the int 7 in n
std::cout << *p; // lvalue-to-rvalue implicit conversion reads the value from n

Pointers to class objects may also appear as the left-hand operands of the member access operators operator-> and operator->*.

Because of the array-to-pointer implicit conversion, pointer to the first element of an array can be initialized with an expression of array type: 

int a[2];
int* p1 = a; // pointer to the first element a[0] (an int) of the array a

int b[6][3][8];
int (*p2)[3][8] = b; // pointer to the first element b[0] of the array b,
                     // which is an array of 3 arrays of 8 ints

Because of the derived-to-base implicit conversion for pointers, pointer to a base class can be initialized with the address of a derived class: 

struct Base {};
struct Derived : Base {};

Derived d;
Base* p = &d;

If Derived is polymorphic, such pointer may be used to make virtual function calls.

Certain addition, subtraction, increment, and decrement operators are defined for pointers to elements of arrays: such pointers satisfy the RandomAccessIterator requirements and allow the C++ library algorithms to work with raw arrays.

Comparison operators are defined for pointers to objects in some situations: two pointers that represent the same address compare equal, two null pointer values compare equal, pointers to elements of the same array compare the same as the array indexes of those elements, and pointers to non-static data members with the same member access compare in order of declaration of those members.

Many implementations also provide strict total ordering of pointers of random origin, e.g. if they are implemented as addresses within continuous virtual address space. Those implementations that do not (e.g. where not all bits of the pointer are part of a memory address and have to be ignored for comparison, or an additional calculation is required or otherwise pointer and integer is not a 1 to 1 relationship), provide a specialization of std::less for pointers that has that guarantee. This makes it possible to use all pointers of random origin as keys in standard associative containers such as std::set or std::map.

Pointers to void

Pointer to object of any type can be implicitly converted to pointer to void (optionally cv-qualified); the pointer value is unchanged. The reverse conversion, which requires static_cast or explicit cast, yields the original pointer value: 

int n = 1;
int* p1 = &n;
void* pv = p1;
int* p2 = static_cast<int*>(pv);
std::cout << *p2 << '\n'; // prints 1

If the original pointer is pointing to a base class subobject within an object of some polymorphic type, dynamic_cast may be used to obtain a void* that is pointing at the complete object of the most derived type.

Pointers to void are used to pass objects of unknown type, which is common in C interfaces: std::malloc returns void*, std::qsort expects a user-provided callback that accepts two const void* arguments. pthread_create expects a user-provided callback that accepts and returns void*. In all cases, it is the caller's responsibility to cast the pointer to the correct type before use.


Pointers to functions

A pointer to function can be initialized with an address of a non-member function or a static member function. Because of the function-to-pointer implicit conversion, the address-of operator is optional: 

void f(int);
void (*p1)(int) = &f;
void (*p2)(int) = f; // same as &f

Unlike functions or references to functions, pointers to functions are objects and thus can be stored in arrays, copied, assigned, etc.

A pointer to function can be used as the left-hand operand of the function call operator, this invokes the pointed-to function: 

int f(int n)
{
    std::cout << n << '\n';
    return n * n;
}

int main()
{
    int (*p)(int) = f;
    int x = p(7);
}

Dereferencing a function pointer yields the lvalue identifying the pointed-to function: 

int f();
int (*p)() = f;  // pointer p is pointing to f
int (&r)() = *p; // the lvalue that identifies f is bound to a reference
r();             // function f invoked through lvalue reference
(*p)();          // function f invoked through the function lvalue
p();             // function f invoked directly through the pointer

A pointer to function may be initialized from an overload set which may include functions, function template specializations, and function templates, if only one overload matches the type of the pointer (see address of an overloaded function for more detail): 

template<typename T> T f(T n) { return n; }
double f(double n) { return n; }

int main()
{
    int (*p)(int) = f; // instantiates and selects f<int>
}

Equality comparison operators are defined for pointers to functions (they compare equal if pointing to the same function).

Pointers to members

Pointers to data members

A pointer to non-static member object m which is a member of class C can be initialized with the expression &C::m exactly. Expressions such as &(C::m) or &m inside C's member function do not form pointers to members.

Such pointer may be used as the right-hand operand of the pointer-to-member access operators operator.* and operator->*: 

struct C { int m; };

int main()
{
    int C::* p = &C::m;          // pointer to data member m of class C
    C c = {7};
    std::cout << c.*p << '\n';   // prints 7
    C* cp = &c;
    cp->m = 10;
    std::cout << cp->*p << '\n'; // prints 10
}

Pointer to data member of an accessible unambiguous non-virtual base class can be implicitly converted to pointer to the same data member of a derived class: 

struct Base { int m; };
struct Derived : Base {};

int main()
{
    int Base::* bp = &Base::m;
    int Derived::* dp = bp;
    Derived d;
    d.m = 1;
    std::cout << d.*dp << ' ' << d.*bp << '\n'; // prints 1 1
}

Conversion in the opposite direction, from a pointer to data member of a derived class to a pointer to data member of an unambiguous non-virtual base class, is allowed with static_cast and explicit cast, even if the base class does not have that member (but the most-derived class does, when the pointer is used for access): 

struct Base {};
struct Derived : Base { int m; };

int main()
{
    int Derived::* dp = &Derived::m;
    int Base::* bp = static_cast<int Base::*>(dp);

    Derived d;
    d.m = 7;
    std::cout << d.*bp << '\n'; // okay: prints 7

    Base b;
    std::cout << b.*bp << '\n'; // undefined behavior
}

The pointed-to type of a pointer-to-member may be a pointer-to-member itself: pointers to members can be multilevel, and can be cv-qualifed differently at every level. Mixed multi-level combinations of pointers and pointers-to-members are also allowed: 

struct A
{
    int m;
    // const pointer to non-const member
    int A::* const p;
};

int main()
{
    // non-const pointer to data member which is a const pointer to non-const member
    int A::* const A::* p1 = &A::p;

    const A a = {1, &A::m};
    std::cout << a.*(a.*p1) << '\n'; // prints 1

    // regular non-const pointer to a const pointer-to-member
    int A::* const* p2 = &a.p;
    std::cout << a.**p2 << '\n'; // prints 1
}

Pointers to member functions

A pointer to non-static member function f which is a member of class C can be initialized with the expression &C::f exactly. Expressions such as &(C::f) or &f inside C's member function do not form pointers to member functions.

Such pointer may be used as the right-hand operand of the pointer-to-member access operators operator.* and operator->*. The resulting expression can be used only as the left-hand operand of a function-call operator: 

struct C
{
    void f(int n) { std::cout << n << '\n'; }
};

int main()
{
    void (C::* p)(int) = &C::f; // pointer to member function f of class C
    C c;
    (c.*p)(1);                  // prints 1
    C* cp = &c;
    (cp->*p)(2);                // prints 2
}


Pointer to member function of a base class can be implicitly converted to pointer to the same member function of a derived class: 

struct Base
{
    void f(int n) { std::cout << n << '\n'; }
};
struct Derived : Base {};

int main()
{
    void (Base::* bp)(int) = &Base::f;
    void (Derived::* dp)(int) = bp;
    Derived d;
    (d.*dp)(1);
    (d.*bp)(2);
}

Conversion in the opposite direction, from a pointer to member function of a derived class to a pointer to member function of an unambiguous non-virtual base class, is allowed with static_cast and explicit cast, even if the base class does not have that member function (but the most-derived class does, when the pointer is used for access): 

struct Base {};
struct Derived : Base
{
    void f(int n) { std::cout << n << '\n'; }
};

int main()
{
    void (Derived::* dp)(int) = &Derived::f;
    void (Base::* bp)(int) = static_cast<void (Base::*)(int)>(dp);

    Derived d;
    (d.*bp)(1); // okay: prints 1

    Base b;
    (b.*bp)(2); // undefined behavior
}

Pointers to member functions may be used as callbacks or as function objects, often after applying std::mem_fn or std::bind:

Run this code
#include <iostream>
#include <string>
#include <algorithm>
#include <functional>

int main()
{
    std::vector<std::string> v = {"a", "ab", "abc"};
    std::vector<std::size_t> l;
    transform(v.begin(), v.end(), std::back_inserter(l),
              std::mem_fn(&std::string::size));
    for(std::size_t n : l)
        std::cout << n << ' ';
}

Output: 

1 2 3

Null pointers

Pointers of every type have a special value known as null pointer value of that type. A pointer whose value is null does not point to an object or a function (dereferencing a null pointer is undefined behavior), and compares equal to all pointers of the same type whose value is also null.

To initialize a pointer to null or to assign the null value to an existing pointer, the null pointer literal nullptr, the null pointer constant NULL, or the implicit conversion from the integer value 0 may be used.

Zero- and value-initialization also initialize pointers to their null values.

Null pointers can be used to indicate the absence of an object (e.g. function::target()), or as other error condition indicators (e.g. dynamic_cast). In general, a function that receives a pointer argument almost always needs to check if the value is null and handle that case differently (for example, the delete expression does nothing when a null pointer is passed).

Constness

  • If cv appears before * in the pointer declaration, it is part of decl-specifier-seq and applies to the the pointed-to object.
  • If cv appears after * in the pointer declaration, it is part of declarator and applies to the pointer that's being declared.
Syntax meaning
const T* pointer to constant object
T const* pointer to constant object
T* const constant pointer to object
const T* const constant pointer to constant object 
// pc is a non-const pointer to const int
// cpc is a const pointer to const int
// ppc is a non-const pointer to non-const pointer to const int
const int ci = 10, *pc = &ci, *const cpc = pc, **ppc;
// p is a non-const pointer to non-const int
// cp is a const pointer to non-const int
int i, *p, *const cp = &i;

i = ci;    // okay: value of const int copied into non-const int
*cp = ci;  // okay: non-const int (pointed-to by const pointer) can be changed
pc++;      // okay: non-const pointer (to const int) can be changed
pc = cpc;  // okay: non-const pointer (to const int) can be changed
pc = p;    // okay: non-const pointer (to const int) can be changed
ppc = &pc; // okay: address of pointer to const int is pointer to pointer to const int

ci = 1;    // error: const int cannot be changed
ci++;      // error: const int cannot be changed
*pc = 2;   // error: pointed-to const int cannot be changed
cp = &ci;  // error: const pointer (to non-const int) cannot be changed
cpc++;     // error: const pointer (to const int) cannot be changed
p = pc;    // error: pointer to non-const int cannot point to const int
ppc = &p;  // error: pointer to pointer to const int cannot point to
           // pointer to non-const int

In general, implicit conversion from one multi-level pointer to another follows the rules described in qualification conversions and in pointer comparison operators.

See also

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