将交换函数放在文件末尾即可使其工作

Question

我有一个交换两个数字的程序：

#include <iostream>
using namespace std;

void swap(int a, int b) {
    int temp;
    temp = a;
    a = b;
    b = temp;
}

int main() {
    int a, b;

    cout << "Enter the first number: ";
    cin >> a;
    cout << "Enter the second number: ";
    cin >> b;

    cout << "Before swapping, the first number was " << a << ", and the second was " << b << "." << endl;

    swap(a, b);

    cout << "After swapping, the first number is " << a << ", and the second is " << b << "." << endl;

    return 0;
}

当我运行它时，我得到以下输出：

Enter the first number: 1
Enter the second number: 3
Before swapping, the first number was 1, and the second was 3.
After swapping, the first number is 1, and the second is 3.

这是预期的，因为

swap

函数修改了函数内的变量

a

和

b

，并且这应该不会影响它们在

main

函数中的值。

但是，如果我将

swap

函数放在

main

函数之后，没有函数原型，则实际上会发生交换：

#include <iostream>
using namespace std;

int main() {
    int a, b;

    cout << "Enter the first number: ";
    cin >> a;
    cout << "Enter the second number: ";
    cin >> b;

    cout << "Before swapping, the first number was " << a << ", and the second was " << b << "." << endl;

    swap(a, b);

    cout << "After swapping, the first number is " << a << ", and the second is " << b << "." << endl;

    return 0;
}

void swap(int a, int b) {
    int temp;
    temp = a;
    a = b;
    b = temp;
}

上面产生以下输出：

Enter the first number: 1
Enter the second number: 3
Before swapping, the first number was 1, and the second was 3.
After swapping, the first number is 3, and the second is 1.

为什么会出现这种情况？有谁可以解释一下吗？

Answer 1

如果我将交换函数放在主函数之后，没有函数原型，则交换实际上会发生：

是的，那是因为正在使用的不是 your

swap

函数，而是

std::swap

。

您可以通过删除

using namespace std;

或将函数命名为

MySwap

并在

main

之前为其提供原型来进行测试。