【C++】函數和指針

最近在看C++ primer plus，感覺函數與指針這一章難點比較多，記寫筆記，加強理解.

From C++ Primer Plus: Chapter 7 Function:C++ Programming Modules 

1. 如何聲明函數指針？

和函數原型類似: 需要聲明指針指向函數的返回值和參數列表

double pam(int); //參數為int 類型，返回值為double 類型的函數
double (*pf);(int)  //指向參數為int類型,返回值為double 類型的指針
pf = pam;   //函數名代表了函數的地址

double x = pam(4); //函數名調用
double x = (*pf)(4); //指針調用
double x = pf(4); //C++也允許將指針名當作函數名使用

2. C++ 11 自動類型推斷

const double * f1(const double *, int);

const double * (*p1)(const double *, int); //p1 poitns to f1
auto p2 = f1; //C++11 automatic type deduction,p2 points to f1 as well

3. 將指針名當作函數名使用

//前面函數為double *類型，cout第一部分返回double指針，第二部分返回double指針指向的值
cout<<(*p1)(av,3)<<":"<<*(*p1)(av,3)<<endl;
//和上面的cout一樣只不過是使用函數指針名來調用函數
cout<<p2(av,3)<<":"<<*p2(av,3)<<endl;

4.  函數指針數組

const double *(*pa[3]) (const double *,int) = {f1,f2,f3}; //創建函數指針數組
//通過指針調用函數，得到返回的指針
const double *px = pa[0](av,3); //call by pointer as if it were a function name
const double *py = (*pa[0])(av,3); //正常調用

//得到函數返回指針指向的值
double x = *pa[0](av,3);
double x = *(*pa[0])(av,3);

5. 指向指針數組的指針

指針數組和數組指針的區別

*pd[3] //an array of 3 pointers
(*pd)[3] //a pointer to an array of three elements

 指向數組的指針

1 auto pc = &pa;   //&pa是整個數組的地址, pa是數組第一個元素首地址
2  
3 const double * (*(*pd)[3])(const double *,  int ) = &pa; //和第一個等價
4 
5 **&pa = *pa = pa[0]

 代碼：

 1 //arfupt.cpp -- an array of function pointers
 2 #include<iostream>
 3 //various notations,same signatures
 4 const double *f1(const double ar[],int n);
 5 const double *f2(const double [],int);
 6 const double *f3(const double *,int);
 7 
 8 int main()
 9 {
10     using namespace std;
11     double av[3] = {1112.3,1542.6,2227.9};
12 
13     //pointer to a function
14 
15     const double *(*p1)(const double *,int) = f1;
16     auto p2 = f2;//C++ 11 utomatic  type deduction
17     //pre-C++11 can use the following code instead
18     //const double *(*p2)(const double *,int) = f2;
19     cout<<"Using pointers to functions:\n";
20     cout<<"Address Value\n";
21     cout<<(*p1)(av,3)<<":"<<*(*p1)(av,3)<<endl;
22     cout<<p2(av,3)<<":"<<*p2(av,3)<<endl;
23 
24     //pa an array of pointers
25     //auto doesn't work with list initialization
26     const double *(*pa[3])(const double *,int) = {f1,f2,f3};
27     //pb a pointer to first element of pa
28     auto pb = pa;
29     // pre-C++11 can use the following code instead
30     // const double *(**pb)(const double *, int) = pa;
31     cout<<"\nUsing an array of pointers to functions:\n";
32     cout<<"Address Value\n";
33     for(int i = 0;i < 3; i++)
34         cout<<pa[i](av,3)<<":"<<*pa[i](av,3)<<endl;
35     cout<<"\nUsing a pointer to a pointer to a function:\n";
36     cout<<"Address Value\n";
37     for(int i = 0;i < 3; i++)
38         cout<<pb[i](av,3)<<":"<<*pb[i](av,3)<<endl;
39 
40     //what about a pointer to an array of function pointers
41     cout<<"\nUsing pointers to an array of pointers:\n";
42     cout<<"Address Value\n";
43     //easy way to declare pc
44     auto pc = &pa;
45     // pre-C++11 can use the following code instead
46     // const double *(*(*pc)[3])(const double *, int) = &pa;
47     cout<<(*pc)[0](av,3)<<":"<<*(*pc)[0](av,3)<<endl;
48     //hard way to declare pd
49     const double *(*(*pd)[3])(const double *,int) = &pa;
50     //store return value in pdb
51     const double *pdb = (*pd)[1](av,3);
52     cout<<pdb<<":"<<*pdb<<endl;
53     //alternative notation
54     cout<<(*(pd)[2])(av,3)<<":"<<*(*(*pd)[2])(av,3)<<endl;
55 }
56 
57 const double * f1(const double * ar, int n)
58 {
59 return ar;
60 }
61 const double * f2(const double ar[], int n)
62 {
63 return ar+1;
64 }
65 const double * f3(const double ar[], int n)
66 {
67 return ar+2;
68 }