[英]without declaration the function first, I can swap the value of the variables?
#include <iostream>
using namespace std;
void swap(int, int);
int main()
{
int a=10;
int b=20;
swap (a, b);
cout << "a: " << a << endl;
cout << "b: " << b << endl;
return 0;
}
void swap(int x, int y)
{
int t;
t = x;
x = y;
y = t;
}
those code above can't swap the value of a and b. 上面的代码不能交换a和b的值。 but my question is , when I forgot to type the third line "void swap(int, int); " , the values of a and b swaped !!
但我的问题是,当我忘记键入第三行“void swap(int,int);”时,a和b的值会变换!! why?
为什么?
It's because you have 这是因为你有
using namespace std;
At the beginning of your source code. 在您的源代码的开头。
This is a a bad programming practice , whose consequences you just experienced, first hand. 这是一个糟糕的编程习惯 ,它只是你刚刚经历的后果。 You told the compiler that you want to invoke
std::swap
, without having any clue that you actually did that. 你告诉编译器你要调用
std::swap
,而不知道你实际上是这么做的。
It's ironical, because you version of swap() won't work right, but std::swap
does; 这很讽刺,因为你的swap()版本不能正常工作,但
std::swap
会这样做; so you were operating under the mistaken impression that your code was working, when it didn't. 因此,您错误地认为您的代码正在运行,而不是。
Never use "using namespace std;" 永远不要使用“using namespace std;” with your code.
用你的代码。 Simply forget that this part of the C++ language ever existed.
简单地忘记这部分C ++语言曾经存在过。
#include <iostream>
using namespace std;
int main()
{
int a = 10;
int b = 20;
cout << "a: " << a << endl;
cout << "b: " << b << endl;
system("pause");
swap(a, b);
cout << "a: " << a << endl;
cout << "b: " << b << endl;
system("pause");
return 0;
}
void swap is unnecessary void swap是不必要的
如果你把函数定义放在main之上,那么你不需要原型,否则你确实需要它,如果你没有原型,编译器会给你一个错误
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