为什么我需要包含iostream和fstream头来打开一个文件 [英] Why do I need to include both the iostream and fstream headers to open a file

问题描述

#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ofstream myfile;
  myfile.open ("test.txt");
  return 0;
}

fstream是从iostream派生的，为什么我们应该在上面的代码中包含？

fstream is derived from iostream, why should we include both in the code above?

我删除了fstream，但是，withstream有一个错误。我的问题是ofstream是从ostream派生的，为什么需要fstream来编译？

I removed fstream, however, there is an error with ofstream. My question is ofstream is derived from ostream, why fstream is needed to make it compile?

推荐答案

$ c> fstream ，因为这是 ofstream 类的定义。

You need to include fstream because that's where the definition of the ofstream class is.

你有一种向后的感觉：因为 ofstream 源于 ostream ，所以 fstream 头包括 iostream 头，所以你可以省略 iostream ，它仍然会编译。但是你不能省略 fstream ，因为你没有 ofstream 的定义。

You've kind of got this backwards: since ofstream derives from ostream, the fstream header includes the iostream header, so you could leave out iostream and it would still compile. But you can't leave out fstream because then you don't have a definition for ofstream.

这样想。如果我把它放在 ah ：

Think about it this way. If I put this in a.h:

class A {
  public:
    A();
    foo();
};

然后我创建一个从 A bh ：

#include <a.h>

class B : public A {
  public:
    B();
    bar();
};

然后我想写这个程序：

int main()
{
  B b;
  b.bar();

  return 0;
}

我需要包含哪个文件？ b.h 。我如何只包括 ah 并期望有 B ？的定义？

Which file would I have to include? b.h obviously. How could I include only a.h and expect to have a definition for B?

请记住，在C和C ++中， include 是字面值。它字面上粘贴包含 include 语句的包含文件的内容。这不是一个更高层次的给我在这个班上的一切的声明。

Remember that in C and C++, include is literal. It literally pastes the contents of the included file where the include statement was. It's not like a higher-level statement of "give me everything in this family of classes".

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