Comparison operators

cppreference Comparison operators

NOTE:

• comparison operators的return type是bool
• comparison operators 不支持chain

Examples

Example 1: 错误使用chain ==

在 How do I use arrays in C++?的answer的“Indexing operator"章节中给出了一个例子：

#include <iostream>

int main()
{
    int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
    int i = 3;
    bool equal = &x[i] == &*(x + i) == (x + i);
    std::cout << std::boolalpha;
    std::cout << equal << std::endl;
}
// g++ --std=c++11 test.cpp

NOTE: 上诉程序编译是会报错的：

test.cpp: 在函数‘int main()’中:
test.cpp:7:43: 错误：ISO C++ 不允许比较指针和整数的值 [-fpermissive]
bool equal = &x[i] == &*(x + i) == (x + i);

问题原因在于对==的错误使用，&x[i] == &*(x + i) == (x + i)等价于:

bool equal = &x[i] == &*(x + i);
equal = equal == (x + i);

显然，equal == (x + i);就触发了“比较指针和整数的值”，则导致上述编译报错。修改后的版本如下：

#include <iostream

int main()
{
    int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
    int i = 3;
    bool equal = &x[i] == &*(x + i) && &*(x + i) == (x + i);
    std::cout << std::boolalpha;
    std::cout << equal << std::endl;
}
// g++ --std=c++11 test.cpp

输出:

true