第7章 函数-C++的编程模块

函数基本知识

函数原型

按值传递函数参数

设计处理数组的函数

使用const指针参数

设计处理文本字符串的函数

设计处理结构的函数

设计处理string对象的函数

调用自身的函数（递归）

指向函数的指针

目录

7.1 复习函数的基本知识

7.1.1 定义函数

7.1.2 函数原型和函数调用

7.2 定义函数

7.2.1 多个参数

7.2.2 另外一个接受两个参数的函数

7.3 函数和数组

7.3.1 函数如何使用指针来处理数组

7.3.2 将数组作为参数意味着什么

7.3.3 更多数组函数示例

7.3.4 使用数组区间的函数

7.3.5 指针和const

7.4 函数和二维数组

7.5 函数和C-风格字符串

7.5.1 将C-风格字符串作为参数的函数

7.5.2 返回C-风格字符串的函数

7.6 函数和结构

7.6.1 传递和返回结构

7.6.2 另一个处理结构的函数示例

7.6.3 传递结构的地址

7.7 函数和string对象

7.8 函数与array对象

7.9 递归

7.9.1 包含一个递归调用的递归

7.9.2 包含多个递归调用的递归

7.10 函数指针

7.10.1 函数指针的基础知识

7.10.2 函数指针示例

7.10.3 深入探讨函数指针

7.10.4 使用typedef进行简化

7.11 总结

---

7.1 复习函数的基本知识

C++自带包含函数的大型库

提供函数定义
提供函数原型
调用函数

// calling.cpp -- defining, prototyping, and calling a function
#include <iostream>

void simple();    // function prototype

int main()
{
    using namespace std;
    cout << "main() will call the simple() function:\n";
    simple();     // function call
	cout << "main() is finished with the simple() function.\n";
    // cin.get();
    return 0;
}

// function definition
void simple()
{
    using namespace std;
    cout << "I'm but a simple function.\n";
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ calling.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
main() will call the simple() function:
I'm but a simple function.
main() is finished with the simple() function.

7.1.1 定义函数

对于有返回值的函数，结果的类型被转化为typeName的类型,
对于返回值的类型，不能是数组，但可以是其他类型

7.1.2 函数原型和函数调用

// calling.cpp -- defining, prototyping, and calling a function
#include <iostream>

void simple();    // function prototype

int main()
{
    using namespace std;
    cout << "main() will call the simple() function:\n";
    simple();     // function call
	cout << "main() is finished with the simple() function.\n";
    // cin.get();
    return 0;
}

// function definition
void simple()
{
    using namespace std;
    cout << "I'm but a simple function.\n";
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ protos.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Cheers! Cheers! Cheers! Cheers! Cheers! 
Give me a number: 5
A 5-foot cube has a volume of 125 cubic feet.
Cheers! Cheers! Cheers! Cheers! Cheers! Cheers! Cheers! Cheers! 

原型的功能：
（1）编译器正确处理函数返回值
（2）编译器检查使用的参数数目是否正确
（3）编译器检查使用的参数类型是否正确，如果不正确，则转换为正确的类型（如果可能）

7.2 定义函数

C++标准使用argument表示实参，使用parameter表示行参

在函数中声明的变量（包括参数）是该函数私有的。在函数被调用时，计算机将为这些变量分配内存；在函数结束时，计算机将释放这些变量使用的内存（C++将分配和释放内存称为创建和毁坏变量），保证了数据的完整性

7.2.1 多个参数

// twoarg.cpp -- a function with 2 arguments
#include <iostream>
using namespace std;
void n_chars(char, int);
int main()
{
    int times;
    char ch;

    cout << "Enter a character: ";
    cin >> ch;
    while (ch != 'q')        // q to quit
    {
        cout << "Enter an integer: ";
        cin >> times;
        n_chars(ch, times); // function with two arguments
        cout << "\nEnter another character or press the"
                " q-key to quit: ";
           cin >> ch;
    }
    cout << "The value of times is " << times << ".\n";
    cout << "Bye\n";
    // cin.get();
    // cin.get();
    return 0;
}

void n_chars(char c, int n) // displays c n times
{
    while (n-- > 0)         // continue until n reaches 0
        cout << c;
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ twoarg.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter a character: W
Enter an integer: 79
WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
Enter another character or press the q-key to quit: q
The value of times is 79.
Bye

使用cin>>ch的原因是
两个cin.get()函数读取所有的输入字符，包括空格和换行符。而cin>>跳过空格和换行符
当用户对程序提示作出响应时，必须在每行的最后按Enter键，生成换行符，而cin>>ch可以跳过这些换行符，

7.2.2 另外一个接受两个参数的函数

// lotto.cpp -- probability of winning
#include <iostream>
// Note: some implementations require double instead of long double
long double probability(unsigned numbers, unsigned picks);
int main()
{
    using namespace std;
    double total, choices;
    cout << "Enter the total number of choices on the game card and\n"
            "the number of picks allowed:\n";
    while ((cin >> total >> choices) && choices <= total)
    {
        cout << "You have one chance in ";
        cout << probability(total, choices);      // compute the odds
        cout << " of winning.\n";
        cout << "Next two numbers (q to quit): ";
    }
    cout << "bye\n";
    // cin.get();
    // cin.get();
    return 0;
}

// the following function calculates the probability of picking picks
// numbers correctly from numbers choices
long double probability(unsigned numbers, unsigned picks)
{
    long double result = 1.0;  // here come some local variables
    long double n;
    unsigned p;

    for (n = numbers, p = picks; p > 0; n--, p--)
        result = result * n / p ; 
    return result;
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ lotto.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter the total number of choices on the game card and
the number of picks allowed:
49 6
You have one chance in 1.39838e+07 of winning.
Next two numbers (q to quit): 51 6
You have one chance in 1.80095e+07 of winning.
Next two numbers (q to quit): q
bye

7.3 函数和数组

int sum_arr(int arr[], int n);
arr是指针，看作可以将如何长度的数组传递给该函数

// arrfun1.cpp -- functions with an array argument
#include <iostream>
const int ArSize = 8;
int sum_arr(int arr[], int n);        // prototype
int main()
{
    using namespace std;
    int cookies[ArSize] = {1,2,4,8,16,32,64,128};
// some systems require preceding int with static to
// enable array initialization

    int sum = sum_arr(cookies, ArSize);
    cout << "Total cookies eaten: " << sum <<  "\n";
    // cin.get();
    return 0;
}

// return the sum of an integer array
int sum_arr(int arr[], int n)
{
    int total = 0;

    for (int i = 0; i < n; i++)
        total = total + arr[i];
    return total; 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ arrfun1.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Total cookies eaten: 255

7.3.1 函数如何使用指针来处理数组

cookies == &cokkies[0]  //array name is address of first element

7.3.2 将数组作为参数意味着什么

// arrfun2.cpp -- functions with an array argument
#include <iostream>
const int ArSize = 8;
int sum_arr(int arr[], int n);
// use std:: instead of using directive
int main()
{
    int cookies[ArSize] = {1,2,4,8,16,32,64,128};
//  some systems require preceding int with static to
//  enable array initialization

    std::cout << cookies << " = array address, ";
//  some systems require a type cast: unsigned (cookies)

    std::cout << sizeof cookies << " = sizeof cookies\n";
    int sum = sum_arr(cookies, ArSize);
    std::cout << "Total cookies eaten: " << sum <<  std::endl;
    sum = sum_arr(cookies, 3);        // a lie
    std::cout << "First three eaters ate " << sum << " cookies.\n";
    sum = sum_arr(cookies + 4, 4);    // another lie
    std::cout << "Last four eaters ate " << sum << " cookies.\n";
    // std::cin.get();
	return 0;
}

// return the sum of an integer array
int sum_arr(int arr[], int n)
{
    int total = 0;
    std::cout << arr << " = arr, ";
// some systems require a type cast: unsigned (arr)

    std::cout << sizeof arr << " = sizeof arr\n";
    for (int i = 0; i < n; i++)
        total = total + arr[i];
    return total; 
}

0x7ffeefbff500 = array address, 32 = sizeof cookies
0x7ffeefbff500 = arr, 8 = sizeof arr
Total cookies eaten: 255
0x7ffeefbff500 = arr, 8 = sizeof arr
First three eaters ate 7 cookies.
0x7ffeefbff510 = arr, 8 = sizeof arr
Last four eaters ate 240 cookies.
Program ended with exit code: 0

sizeof cookies是整个数组的长度，而sizeof arr是指针变量的长度

注意：
void filArray(int arr[], in size)//
void filArray(int arr[size])

7.3.3 更多数组函数示例

// arrfun3.cpp -- array functions and const
#include <iostream>
const int Max = 5;

// function prototypes
int fill_array(double ar[], int limit);
void show_array(const double ar[], int n);  // don't change data
void revalue(double r, double ar[], int n);

int main()
{
    using namespace std;
    double properties[Max];

    int size = fill_array(properties, Max);
    show_array(properties, size);
    if (size > 0)
    {
        cout << "Enter revaluation factor: ";
        double factor;
        while (!(cin >> factor))    // bad input
        {
            cin.clear();
            while (cin.get() != '\n')
                continue;
           cout << "Bad input; Please enter a number: ";
        }
        revalue(factor, properties, size);
        show_array(properties, size);
    }
    cout << "Done.\n";
    // cin.get();
    // cin.get();
    return 0;
}

int fill_array(double ar[], int limit)
{
    using namespace std;
    double temp;
    int i;
    for (i = 0; i < limit; i++)
    {
        cout << "Enter value #" << (i + 1) << ": ";
        cin >> temp;
        if (!cin)    // bad input
        {
            cin.clear();
            while (cin.get() != '\n')
                continue;
           cout << "Bad input; input process terminated.\n";
           break;
        }
        else if (temp < 0)     // signal to terminate
            break;
        ar[i] = temp;
    }
    return i;
}

// the following function can use, but not alter,
// the array whose address is ar
void show_array(const double ar[], int n)
{
    using namespace std;
    for (int i = 0; i < n; i++)
    {
        cout << "Property #" << (i + 1) << ": $";
        cout << ar[i] << endl;
    }
}

// multiplies each element of ar[] by r
void revalue(double r, double ar[], int n)
{
    for (int i = 0; i < n; i++)
        ar[i] *= r;
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ arrfun3.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter value #1: 100000
Enter value #2: 8000
Enter value #3: 222000
Enter value #4: 240000
Enter value #5: 118000
Property #1: $100000
Property #2: $8000
Property #3: $222000
Property #4: $240000
Property #5: $118000
Enter revaluation factor: 0.8
Property #1: $80000
Property #2: $6400
Property #3: $177600
Property #4: $192000
Property #5: $94400
Done.

C++将声明const double ar [] 解释为 const double *ar;

7.3.4 使用数组区间的函数

// arrfun4.cpp -- functions with an array range
#include <iostream>
const int ArSize = 8;
int sum_arr(const int * begin, const int * end);
int main()
{
    using namespace std;
    int cookies[ArSize] = {1,2,4,8,16,32,64,128};
//  some systems require preceding int with static to
//  enable array initialization

    int sum = sum_arr(cookies, cookies + ArSize);
    cout << "Total cookies eaten: " << sum <<  endl;
    sum = sum_arr(cookies, cookies + 3);        // first 3 elements
    cout << "First three eaters ate " << sum << " cookies.\n";
    sum = sum_arr(cookies + 4, cookies + 8);    // last 4 elements
    cout << "Last four eaters ate " << sum << " cookies.\n";
    // cin.get();
    return 0;
}

// return the sum of an integer array
int sum_arr(const int * begin, const int * end)
{
    const int * pt;
    int total = 0;

    for (pt = begin; pt != end; pt++)
        total = total + *pt;
    return total; 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ arrfun4.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Total cookies eaten: 255
First three eaters ate 7 cookies.
Last four eaters ate 240 cookies.

7.3.5 指针和const

C++禁止将const的地址赋给非const指针

const float g_earth = 9.80;
(const) float* pe = &g_earth;

double trouble = 2..0E30;
const double* const stick = &trouble;
stick只能指向trouble, 而stick不能用来修改trouble的值。

7.4 函数和二维数组

7.5 函数和C-风格字符串

7.5.1 将C-风格字符串作为参数的函数

// strgfun.cpp -- functions with a string argument
#include <iostream>
unsigned int c_in_str(const char * str, char ch);
int main()
{
    using namespace std;
    char mmm[15] = "minimum";    // string in an array
// some systems require preceding char with static to
// enable array initialization

    char *wail = "ululate";    // wail points to string

    unsigned int ms = c_in_str(mmm, 'm');
    unsigned int us = c_in_str(wail, 'u');
    cout << ms << " m characters in " << mmm << endl;
    cout << us << " u characters in " << wail << endl;
    // cin.get();
    return 0;
}

// this function counts the number of ch characters
// in the string str
unsigned int c_in_str(const char * str, char ch)
{
    unsigned int count = 0;

    while (*str)        // quit when *str is '\0'
    {
        if (*str == ch)
            count++;
        str++;        // move pointer to next char
    }
    return count; 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ strgfun.cpp 
strgfun.cpp:11:18: warning: conversion from string literal to 'char *' is deprecated [-Wc++11-compat-deprecated-writable-strings]
    char *wail = "ululate";    // wail points to string
                 ^
1 warning generated.

7.5.2 返回C-风格字符串的函数

// strgback.cpp -- a function that returns a pointer to char
#include <iostream>
char * buildstr(char c, int n);     // prototype
int main()
{
    using namespace std;
    int times;
    char ch;

    cout << "Enter a character: ";
    cin >> ch;
    cout << "Enter an integer: ";
    cin >> times;
    char *ps = buildstr(ch, times);
    cout << ps << endl;
    delete [] ps;                   // free memory
    ps = buildstr('+', 20);         // reuse pointer
    cout << ps << "-DONE-" << ps << endl;
    delete [] ps;                   // free memory
    // cin.get();
    // cin.get();
    return 0;
}

// builds string made of n c characters
char * buildstr(char c, int n)
{
    char * pstr = new char[n + 1];
    pstr[n] = '\0';         // terminate string
    while (n-- > 0)
        pstr[n] = c;        // fill rest of string
    return pstr; 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ strgback.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter a character: V
Enter an integer: 46
VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
++++++++++++++++++++-DONE-++++++++++++++++++++

7.6 函数和结构

7.6.1 传递和返回结构

// travel.cpp -- using structures with functions
#include <iostream>
struct travel_time
{
    int hours;
    int mins;
};
const int Mins_per_hr = 60;

travel_time sum(travel_time t1, travel_time t2);
void show_time(travel_time t);

int main()
{
    using namespace std;
    travel_time day1 = {5, 45};    // 5 hrs, 45 min
    travel_time day2 = {4, 55};    // 4 hrs, 55 min

    travel_time trip = sum(day1, day2);
    cout << "Two-day total: ";
    show_time(trip);

    travel_time day3= {4, 32};
    cout << "Three-day total: ";
    show_time(sum(trip, day3));
    // cin.get();

    return 0;
}

travel_time sum(travel_time t1, travel_time t2)
{
    travel_time total;

    total.mins = (t1.mins + t2.mins) % Mins_per_hr;
    total.hours = t1.hours + t2.hours +
                 (t1.mins + t2.mins) / Mins_per_hr;
    return total;
}

void show_time(travel_time t)
{
    using namespace std;
    cout << t.hours << " hours, "
         << t.mins << " minutes\n";
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ travel.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Two-day total: 10 hours, 40 minutes
Three-day total: 15 hours, 12 minutes

7.6.2 另一个处理结构的函数示例

7.6.3 传递结构的地址

// strctptr.cpp -- functions with pointer to structure arguments
#include <iostream>
#include <cmath>

// structure templates
struct polar
{
    double distance;      // distance from origin
    double angle;         // direction from origin
};
struct rect
{
    double x;             // horizontal distance from origin
    double y;             // vertical distance from origin
};

// prototypes
void rect_to_polar(const rect * pxy, polar * pda);
void show_polar (const polar * pda);

int main()
{
    using namespace std;
    rect rplace;
    polar pplace;

    cout << "Enter the x and y values: ";
    while (cin >> rplace.x >> rplace.y)
    {
        rect_to_polar(&rplace, &pplace);    // pass addresses
        show_polar(&pplace);        // pass address
        cout << "Next two numbers (q to quit): ";
    }
    cout << "Done.\n";
    return 0;
}

// show polar coordinates, converting angle to degrees
void show_polar (const polar * pda)
{
    using namespace std;
    const double Rad_to_deg = 57.29577951;

    cout << "distance = " << pda->distance;
    cout << ", angle = " << pda->angle * Rad_to_deg;
    cout << " degrees\n";
}

// convert rectangular to polar coordinates
void rect_to_polar(const rect * pxy, polar * pda)
{
    using namespace std;
    pda->distance =
        sqrt(pxy->x * pxy->x + pxy->y * pxy->y);
    pda->angle = atan2(pxy->y, pxy->x); 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ travel.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Two-day total: 10 hours, 40 minutes
Three-day total: 15 hours, 12 minutes
[wlsh@wlsh-MacbookPro] chapter_7$ g++ strctptr.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter the x and y values: 30 40
distance = 50, angle = 53.1301 degrees
Next two numbers (q to quit): 100 -100
distance = 141.421, angle = -45 degrees
Next two numbers (q to quit): q
Done.

7.7 函数和string对象

// strctptr.cpp -- functions with pointer to structure arguments
#include <iostream>
#include <cmath>

// structure templates
struct polar
{
    double distance;      // distance from origin
    double angle;         // direction from origin
};
struct rect
{
    double x;             // horizontal distance from origin
    double y;             // vertical distance from origin
};

// prototypes
void rect_to_polar(const rect * pxy, polar * pda);
void show_polar (const polar * pda);

int main()
{
    using namespace std;
    rect rplace;
    polar pplace;

    cout << "Enter the x and y values: ";
    while (cin >> rplace.x >> rplace.y)
    {
        rect_to_polar(&rplace, &pplace);    // pass addresses
        show_polar(&pplace);        // pass address
        cout << "Next two numbers (q to quit): ";
    }
    cout << "Done.\n";
    return 0;
}

// show polar coordinates, converting angle to degrees
void show_polar (const polar * pda)
{
    using namespace std;
    const double Rad_to_deg = 57.29577951;

    cout << "distance = " << pda->distance;
    cout << ", angle = " << pda->angle * Rad_to_deg;
    cout << " degrees\n";
}

// convert rectangular to polar coordinates
void rect_to_polar(const rect * pxy, polar * pda)
{
    using namespace std;
    pda->distance =
        sqrt(pxy->x * pxy->x + pxy->y * pxy->y);
    pda->angle = atan2(pxy->y, pxy->x); 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ topfive.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter your 5 favorite astronomical sights:
1: Orion Nebula   
2: M13
3: Saturn   
4: Jupiter
5: Moon  
Your list:
1: Orion Nebula
2: M13
3: Saturn
4: upiter
5: Moon

7.8 函数与array对象

//arrobj.cpp -- functions with array objects
#include <iostream>
#include <array>
#include <string>
const int Seasons = 4;
const std::array<std::string, Seasons> Snames =
    {"Spring", "Summer", "Fall", "Winter"};

void fill(std::array<double, Seasons> * pa);
void show(std::array<double, Seasons> da);
int main()
{
    std::array<double, 4> expenses;
    fill(&expenses);
    show(expenses);
    // std::cin.get();
    // std::cin.get();
    return 0;
}

void fill(std::array<double, Seasons> * pa)
{
    for (int i = 0; i < Seasons; i++)
    {
        std::cout << "Enter " << Snames[i] << " expenses: ";
        std::cin >> (*pa)[i];
    }
}

void show(std::array<double, Seasons> da)
{
    double total = 0.0;
    std::cout << "\nEXPENSES\n";
    for (int i = 0; i < Seasons; i++)
    {
        std::cout << Snames[i] << ": $" << da[i] << '\n';
        total += da[i];
    }
    std::cout << "Total: $" << total << '\n';
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ arrobj.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
Enter Spring expenses: 212
Enter Summer expenses: 256
Enter Fall expenses: 208
Enter Winter expenses: 244

EXPENSES
Spring: $212
Summer: $256
Fall: $208
Winter: $244
Total: $920

7.9 递归

7.9.1 包含一个递归调用的递归

// recur.cpp -- using recursion
#include <iostream>
void countdown(int n);

int main()
{
    countdown(4);           // call the recursive function
    // std::cin.get();
    return 0;
}

void countdown(int n)
{
    using namespace std;
    cout << "Counting down ... " << n << endl;
    if (n > 0)
        countdown(n-1);     // function calls itself
    cout << n << ": Kaboom!\n";
}

// recur.cpp -- using recursion
#include <iostream>
void countdown(int n);

int main()
{
    countdown(4);           // call the recursive function
    // std::cin.get();
    return 0;
}

void countdown(int n)
{
    using namespace std;
    cout << "Counting down ... " << n << endl;
    if (n > 0)
        countdown(n-1);     // function calls itself
    cout << n << ": Kaboom!\n";
}

7.9.2 包含多个递归调用的递归

// ruler.cpp -- using recursion to subdivide a ruler
#include <iostream>
const int Len = 66;
const int Divs = 6;
void subdivide(char ar[], int low, int high, int level);
int main()
{
    char ruler[Len];
    int i;
    for (i = 1; i < Len - 2; i++)
        ruler[i] = ' ';
    ruler[Len - 1] = '\0';
    int max = Len - 2;
    int min = 0;
    ruler[min] = ruler[max] = '|';
    std::cout << ruler << std::endl;
    for (i = 1; i <= Divs; i++)
    {
        subdivide(ruler,min,max, i);
        std::cout << ruler << std::endl;
        for (int j = 1; j < Len - 2; j++)
            ruler[j] = ' ';  // reset to blank ruler
    }
    // std::cin.get();

    return 0;
}

void subdivide(char ar[], int low, int high, int level)
{
    if (level == 0)
        return;
    int mid = (high + low) / 2;
    ar[mid] = '|';
    subdivide(ar, low, mid, level - 1);
    subdivide(ar, mid, high, level - 1); 
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ ruler.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
|                                                               |
|                               |                               |
|               |               |               |               |
|       |       |       |       |       |       |       |       |
|   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

7.10 函数指针

与数据项相似，函数也有地址，函数的地址是存储其及其语言代码的内存的开始地址。

7.10.1 函数指针的基础知识

7.10.2 函数指针示例

// fun_ptr.cpp -- pointers to functions
#include <iostream>
double betsy(int);
double pam(int);

// second argument is pointer to a type double function that
// takes a type int argument
void estimate(int lines, double (*pf)(int));

int main()
{
    using namespace std;
    int code;

    cout << "How many lines of code do you need? ";
    cin >> code;
    cout << "Here's Betsy's estimate:\n";
    estimate(code, betsy);
    cout << "Here's Pam's estimate:\n";
    estimate(code, pam);
    // cin.get();
    // cin.get();
    return 0;
}

double betsy(int lns)
{
    return 0.05 * lns;
}

double pam(int lns)
{
    return 0.03 * lns + 0.0004 * lns * lns;
}

void estimate(int lines, double (*pf)(int))
{
    using namespace std;
    cout << lines << " lines will take ";
    cout << (*pf)(lines) << " hour(s)\n";
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ fun_ptr.cpp 
[wlsh@wlsh-MacbookPro] chapter_7$ ./a.out 
How many lines of code do you need? 30
Here's Betsy's estimate:
30 lines will take 1.5 hour(s)
Here's Pam's estimate:
30 lines will take 1.26 hour(s)

7.10.3 深入探讨函数指针

// 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++0x automatic type deduction
    // pre-C++0x can use the following code instead
    // const double *(*p2)(const double *, int) = f2;
     cout << "Using pointers to functions:\n";
    cout << " Address  Value\n";
    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};
    // but it does work for initializing to a single value
    // pb a pointer to first element of pa
    auto pb = pa;
    // pre-C++0x can use the following code instead
    // const double *(**pb)(const double *, int) = pa;
    cout << "\nUsing an array of pointers to functions:\n";
    cout << " Address  Value\n";
    for (int i = 0; i < 3; i++)
        cout << pa[i](av,3) << ": " << *pa[i](av,3) << endl;
    cout << "\nUsing a pointer to a pointer to a function:\n";
    cout << " Address  Value\n";
    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 << "\nUsing pointers to an array of pointers:\n";
    cout << " Address  Value\n";
    // easy way to declare pc 
    auto pc = &pa; 
     // pre-C++0x 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;
    // cin.get();
    return 0;
}

// some rather dull functions

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;
}

[wlsh@wlsh-MacbookPro] chapter_7$ g++ arfupt.cpp 
arfupt.cpp:15:5: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
    auto p2 = f2;  // C++0x automatic type deduction
    ^
arfupt.cpp:28:5: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
    auto pb = pa;
    ^
arfupt.cpp:44:5: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
    auto pc = &pa; 
    ^
3 warnings generated.

7.10.4 使用typedef进行简化

7.11 总结