C++复合赋值运算符（无师自通）

number = number + 1;

赋值运算符右侧的表达式给 number 加 1，然后将结果赋值给 number，替换先前存储的值。实际上，这个声明给 number 增加了 1。同样的方式，以下语句从 number 中减去 5：

number = number – 5;

如果之前从未看过这种类型的语句，则它可能会导致一些初学者理解上的困惑，因为相同的变量名称出现在赋值运算符的两边。表 1 显示了以这种方式编写的语句的其他示例：

由于这些类型的运算在编程中很常见，因此 C++ 提供了一组专门为这些任务而设计的 运算符。表 2 显示了复合赋值运算符，也称为复合运算符。

表 2 中的用法示例说明，复合赋值运算符不需要程序员键入变量名称两次。

下面的程序使用了多变量赋值语句和复合赋值运算符：

//This program tracks the inventory of two widget stores.
// It illustrates the use of multiple and combined assignment.
#include <iostream>
using namespace std;

int main()
{
    int beglnv, // Beginning inventory for both stores
    sold,   // Number of widgets sold
    store1, // Store 1's inventory
    store2; // Store 2's inventory
    // Get the beginning inventory for the two stores
    cout << "One week ago, 2 new widget stores opened/n";
    cout << "at the same time with the same beginning/n";
    cout << "inventory. What was the beginning inventory? ";
    cin >> beglnv;
    // Set each store1s inventory
    store1 = store2 = beglnv;
    // Get the number of widgets sold at each store
    cout << "How many widgets has store 1 sold? ";
    cin >> sold;
    store1 -= sold; // Adjust store 1's inventory
    cout << " How many widgets has store 2 sold? ";
    cin >> sold;
    store2 -= sold; // Adjust store 2's inventory
    //Display each store1s current inventory
    cout << " /nThe current inventory of each store: /n";
    cout << "Store 1: " << store1 << endl;
    cout << "Store 2: " << store2 << endl;
    return 0;
}

程序输出结果：

One week ago, 2 new widget stores opened at the same time with the same beginning inventory. What was the beginning inventory? 100
How many widgets has store 1 sold? 25
How many widgets has store 2 sold? 15
The current inventory of each store: Store 1: 75
Store 2: 85

可以使用复合赋值运算符来表达更精细的语句，示例如下：

result * = a + 5;

在该语句中，和 result 相乘的是 a+5 的和。请注意，复合赋值运算符的优先级低于常规算术运算符的优先级。上述语句和以下语句是等效的：

result = result *(a + 5);

表 3 显示了使用复合赋值运算符的其他示例。