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C++ 函数参数
2018-03-24 17:09 更新
学习C++ - C++函数参数
函数参数和值传递
C++通常通过值传递参数。
例如，
double volume = cube(side); 这边是一个变量，在运行中，值为5。
cube()的函数头是这样的:
double cube(double x) 调用此函数时，将创建一个新的类型double变量x，并将其初始化为5。
多个参数
一个函数可以有多个参数。
在函数调用中，你只需用逗号分隔参数：
#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";
return 0;
}
void n_chars(char c, int n) // displays c n times
{
while (n-- > 0) // continue until n reaches 0
cout << c;
}
上面的代码生成以下结果。
函数和数组
以下代码说明如何使用指针，就像它是一个数组名称一样。
该程序将数组初始化为某些值，并使用sum_arr()函数来计算和。
#include <iostream>
using namespace std;
const int SIZE = 8;
int sum_arr(int arr[], int n); // prototype
int main()
{
int cookies[SIZE] = {1,2,4,8,16,32,64,128};
int sum = sum_arr(cookies, SIZE);
cout << "Total cookies eaten: " << sum << "\n";
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;
}
上面的代码生成以下结果。
数组名称与数组指针
以下代码显示了Cookie和arr具有相同的值。
它显示了指针如何使sum_arr函数更加通用。
该程序使用std:: qualifier而不是using指令来提供对cout和endl的访问。
#include <iostream>
const int ArSize = 8;
int sum_arr(int arr[], int n);
int main()
{
int cookies[ArSize] = {1,2,4,8,16,32,64,128};
std::cout << cookies << " = array address, ";
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";
return 0;
}
int sum_arr(int arr[], int n){
int total = 0;
std::cout << arr << " = arr, ";
std::cout << sizeof arr << " = sizeof arr\n";
for (int i = 0; i < n; i++)
total = total + arr[i];
return total;
}
上面的代码生成以下结果。
使用数组范围的函数
以下代码使用两个指针来指定范围。
#include <iostream>
using namespace std;
const int SIZE = 8;
int sum_arr(const int * begin, const int * end);
int main()
{
int cookies[SIZE] = {1,2,4,8,16,32,64,128};
int sum = sum_arr(cookies, cookies + SIZE);
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";
return 0;
}
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;
}
上面的代码生成以下结果。
函数和C风格字符串
以下代码计算给定字符在字符串中出现的次数。
因为程序不需要处理负值，它使用unsigned int作为计数类型。
#include <iostream>
using namespace std;
unsigned int c_in_str(const char * str, char ch);
int main()
{
char mmm[15] = "minimum"; // string in an array
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;
return 0;
}
// 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;
}
上面的代码生成以下结果。
返回C风格字符串的函数
以下代码定义了一个返回指针的名为buildstr()的函数。
此函数有两个参数：一个字符和一个数字。
使用new，该函数创建一个长度等于数字的字符串，然后将每个元素初始化为字符。
然后它返回一个指向新字符串的指针。
#include <iostream>
using namespace std;
char * buildstr(char c, int n);
int main()
{
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
return 0;
}
// builds string of 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;
}
上面的代码生成以下结果。
传递和返回结构
#include <iostream>
using namespace std;
struct my_time
{
int hours;
int mins;
};
const int MINUTES = 60;
my_time sum(my_time t1, my_time t2);
void show_time(my_time t);
int main()
{
my_time day1 = {5, 45}; // 5 hrs, 45 min
my_time day2 = {4, 55}; // 4 hrs, 55 min
my_time trip = sum(day1, day2);
show_time(trip);
return 0;
}
my_time sum(my_time t1, my_time t2)
{
my_time total;
total.mins = (t1.mins + t2.mins) % MINUTES;
total.hours = t1.hours + t2.hours + (t1.mins + t2.mins) / MINUTES;
return total;
}
void show_time(my_time t)
{
cout << t.hours << " hours, " << t.mins << " minutes\n";
}
上面的代码生成以下结果。
函数和字符串类对象
以下代码提供了一个简短的示例，它声明一个字符串对象数组，并将该数组传递给显示内容的函数。
#include <iostream>
#include <string>
using namespace std;
const int SIZE = 5;
void display(const string sa[], int n);
int main()
{
string list[SIZE]; // an array holding 5 string object
cout << "Enter your " << SIZE << " favorite astronomical sights:\n";
for (int i = 0; i < SIZE; i++){
cout << i + 1 << ": ";
getline(cin,list[i]);
}
cout << "Your list:\n";
display(list, SIZE);
return 0;
}
void display(const string sa[], int n){
for (int i = 0; i < n; i++)
cout << i + 1 << ": " << sa[i] << endl;
}
上面的代码生成以下结果。
函数和数组对象
#include <iostream>
#include <array>
#include <string>
const int Seasons = 4;
const std::array<std::string, Seasons> Snames =
{"Baseball", "Football", "Basketball", "Hockey"};
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);
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;
for (int i = 0; i < Seasons; i++){
std::cout << Snames[i] << ": $" << da[i] << "\n";
total += da[i];
}
std::cout << "Total: $" << total << "\n";
}
上面的代码生成以下结果。
C++内联函数
内联函数可以加速程序。
以下代码说明了使用inline square()函数的内联技术。
#include <iostream>
using namespace std;
inline double square(double x) { return x * x; }
int main(){
double a, b;
double c = 13.0;
a = square(5.0);
b = square(4.5 + 7.5); // can pass expressions
cout << "a = " << a << ", b = " << b << "\n";
cout << "c = " << c;
cout << ", c squared = " << square(c++) << "\n";
cout << "Now c = " << c << "\n";
// cin.get();
return 0;
}
上面的代码生成以下结果。
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