【C++】函数和指针

最近在看C++ primer plus，感觉函数与指针这一章难点比较多，记写笔记，加强理解.

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

1. 如何声明函数指针？

和函数原型类似: 需要声明指针指向函数的返回值和参数列表

double pam(int); //参数为int 类型，返回值为double 类型的函数
double (*pf);(int)  //指向参数为int类型,返回值为double 类型的指针
pf = pam;   //函数名代表了函数的地址

double x = pam(4); //函数名调用
double x = (*pf)(4); //指针调用
double x = pf(4); //C++也允许将指针名当作函数名使用

2. C++ 11 自动类型推断

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. 将指针名当作函数名使用

//前面函数为double *类型，cout第一部分返回double指针，第二部分返回double指针指向的值
cout<<(*p1)(av,3)<<":"<<*(*p1)(av,3)<<endl;
//和上面的cout一样只不过是使用函数指针名来调用函数
cout<<p2(av,3)<<":"<<*p2(av,3)<<endl;

4.  函数指针数组

const double *(*pa[3]) (const double *,int) = {f1,f2,f3}; //创建函数指针数组
//通过指针调用函数，得到返回的指针
const double *px = pa[0](av,3); //call by pointer as if it were a function name
const double *py = (*pa[0])(av,3); //正常调用

//得到函数返回指针指向的值
double x = *pa[0](av,3);
double x = *(*pa[0])(av,3);

5. 指向指针数组的指针

指针数组和数组指针的区别

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

 指向数组的指针

auto pc = &pa;   //&pa是整个数组的地址, pa是数组第一个元素首地址
 
const double * (*(*pd)[3])(const double *,  int ) = &pa; //和第一个等价

**&pa = *pa = pa[0]

 代码：

//arfupt.cpp -- an array of function pointers
#include<iostream>
//various notations,same signatures
const double *f1(const double ar[],int n);
const double *f2(const double [],int);
const double *f3(const double *,int);

int main()
{
    using namespace std;
    double av[3] = {1112.3,1542.6,2227.9};

    //pointer to a function

    const double *(*p1)(const double *,int) = f1;
    auto p2 = f2;//C++ 11 utomatic  type deduction
    //pre-C++11 can use the following code instead
    //const double *(*p2)(const double *,int) = f2;
    cout<<"Using pointers to functions:
";
    cout<<"Address Value
";
    cout<<(*p1)(av,3)<<":"<<*(*p1)(av,3)<<endl;
    cout<<p2(av,3)<<":"<<*p2(av,3)<<endl;

    //pa an array of pointers
    //auto doesn't work with list initialization
    const double *(*pa[3])(const double *,int) = {f1,f2,f3};
    //pb a pointer to first element of pa
    auto pb = pa;
    // pre-C++11 can use the following code instead
    // const double *(**pb)(const double *, int) = pa;
    cout<<"
Using an array of pointers to functions:
";
    cout<<"Address Value
";
    for(int i = 0;i < 3; i++)
        cout<<pa[i](av,3)<<":"<<*pa[i](av,3)<<endl;
    cout<<"
Using a pointer to a pointer to a function:
";
    cout<<"Address Value
";
    for(int i = 0;i < 3; i++)
        cout<<pb[i](av,3)<<":"<<*pb[i](av,3)<<endl;

    //what about a pointer to an array of function pointers
    cout<<"
Using pointers to an array of pointers:
";
    cout<<"Address Value
";
    //easy way to declare pc
    auto pc = &pa;
    // pre-C++11 can use the following code instead
    // const double *(*(*pc)[3])(const double *, int) = &pa;
    cout<<(*pc)[0](av,3)<<":"<<*(*pc)[0](av,3)<<endl;
    //hard way to declare pd
    const double *(*(*pd)[3])(const double *,int) = &pa;
    //store return value in pdb
    const double *pdb = (*pd)[1](av,3);
    cout<<pdb<<":"<<*pdb<<endl;
    //alternative notation
    cout<<(*(pd)[2])(av,3)<<":"<<*(*(*pd)[2])(av,3)<<endl;
}

const double * f1(const double * ar, int n)
{
return ar;
}
const double * f2(const double ar[], int n)
{
return ar+1;
}
const double * f3(const double ar[], int n)
{
return ar+2;
}