Skip to content

Pointer

Pointer and the stored-program machine

在工程Hardware的Computer-architecture\Function and data model章节中，提出了“Function and data model”，可以据此来对pointer进行分类:

两大类pointer

pointer 说明
pointer to object 参见:
- 在C++\Language-reference\Basic-concept\Data-model\Object\Object.md中，将次描述为“Pointer can alias to an existing object”
- C++\Language-reference\Basic-concept\index.md
pointer to function 参见:
- C++\Language-reference\Basic-concept\index.md

wikipedia Pointer (computer programming)

cppreference Pointer declaration

Syntax

NOTE: 不同于上面的基于“Function and data model”来对pointer进行分类，cppreference中是基于OOP来对pointer进行分类的:

  • pointer
  • pointer-to-member
pointer pointer-to-member
syntax decl-specifier-seq * attr(optional) cv(optional) declarator decl-specifier-seq nested-name-specifier * attr(optional) cv(optional) declarator
example S* D;
declares D as a pointer to the type determined by decl-specifier-seq S.		 S C::* D;
declares D as a pointer to non-static member of C of type determined by decl-specifier-seq S.

参见:

nested-name-specifier - a sequence of names and scope resolution operators ::

attr(C++11) - optional list of

declarator - declarator

Initialization

NOTE: pointer是object，因此需要对它进行initialization

address-of operator

Pointers to objects

#include <iostream>

int main()
{
    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 = { 1 };
    int* sp = &s.n; // pointer to the int that is a member of s
}
// g++ --std=c++11 test.cpp

Pointers to void

NOTE: 这部分内容放到了下面的Pointer to void章节

Pointers to functions

Because of the function-to-pointer implicit conversion, the address-of operator is optional:

void f(int)
{

}

int main()
{

    void (*p1)(int) = &f;
    void (*p2)(int) = f; // same as &f
}
// g++ test.cpp

Pointers to members

Pointers to data members

Pointers to member functions

参见: C++\Language-reference\Functions\First-class-function\Member-function-as-first-class-function 章节

Null pointers

NOTE:

1、参见 Null-pointer 章节

Constness

Syntax meaning 说明
const T* ptr pointer to constant object
T const* ptr pointer to constant object
T* const ptr constant pointer to object 按照这篇文章中的看法:
A reference can be thought of as a constant pointer
const T* const ptr constant pointer to constant object
T const* const ptr constant pointer to constant object

NOTE: 原文，并没有添加variable name，不添加variable name是不易理解的，上面是我添加了variable name的描述。

在下面文章中，对这个问题进行了深入分析:

1) What is the difference between char * const and const char *?

2) Constant pointer vs Pointer to constant [duplicate]

3) What is the difference between const int*, const int * const, and int const *?

其实，使用 Clockwise/Spiral Rule， 我们可以非常快速地理解其语义，参见C-and-C++\Declaration\The-Clockwise-Spiral-Rule.md。

stackoverflow What is the difference between const int*, const int * const, and int const *?

Q:

I always mess up how to use const int*, const int * const, and int const * correctly. Is there a set of rules defining what you can and cannot do?

I want to know all the do's and all don'ts in terms of assignments, passing to the functions, etc.

A:

You can use the "Clockwise/Spiral Rule" to decipher most C and C++ declarations. – James McNellis

A:

cdecl.org is a great website which auto-translates C declarations for you. – Dave Gallagher

A:

Read it backwards (as driven by Clockwise/Spiral Rule):

  • int* - pointer to int
  • int const * - pointer to const int
  • int * const - const pointer to int
  • int const * const - const pointer to const int

Now the first const can be on either side of the type so:

  • const int * == int const *
  • const int * const == int const * const

If you want to go really crazy you can do things like this:

  • int ** - pointer to pointer to int
  • int ** const - a const pointer to a pointer to an int
  • int * const * - a pointer to a const pointer to an int
  • int const ** - a pointer to a pointer to a const int
  • int * const * const - a const pointer to a const pointer to an int
  • ...

And to make sure we are clear on the meaning of const

const int* foo;
int *const bar; //note, you actually need to set the pointer 
                //here because you can't change it later ;)

foo is a variable pointer to a constant integer. This lets you change what you point to but not the value that you point to. Most often this is seen with C-style strings where you have a pointer to a const char. You may change which string you point to but you can't change the content of these strings. This is important when the string itself is in the data segment of a program and shouldn't be changed.

bar is a constant or fixed pointer to a value that can be changed. This is like a reference without the extra syntactic sugar. Because of this fact, usually you would use a reference where you would use a T* const pointer unless you need to allow NULL pointers.

cppreference Pointer comparison operators