英文:
forward declaration of std::ostream
问题
以下是您要翻译的内容：
The files are organized as following:
//MyClass.h
#pragma once
namespace std {
class ostream;
};
class MyClass
{
private:
int a;
public:
friend std::ostream& operator<<(std::ostream& o, const MyClass& obj);
};
//MyClass.cpp
#include<iostream>
std::ostream& operator<<(std::ostream& o, const MyClass& obj)
{
o << obj.a << std::endl;
return o;
}
//main.cpp
#include "MyClass.h"
#include<iostream>
int main()
{
MyClass obj;
std::cout << obj;
}
It failed compiling with errors on overloading operator<<:
Error C2371 'std::ostream': redefinition; different basic types
What's the issue here and how to correct it?
英文:
The files are organized as following:
//MyClass.h
#pragma once
namespace std {
class ostream;
};
class MyClass
{
private:
int a;
public:
friend std::ostream& operator<<(std::ostream& o, const MyClass& obj);
};
//MyClass.cpp
#include<iostream>
std::ostream& operator<<(std::ostream& o, const MyClass& obj)
{
o << obj.a << std::endl;
return o;
}
//main.cpp
#include"MyClass.h"
#include<iostream>
int main()
{
MyClass obj;
std::cout << obj;
}
It failed compiling with errors on overloading operator<<:
> Error C2371 'std::ostream': redefinition; different basic types
What's the issue here and how to correct it?
答案1
得分: 4
如其他人所指出，“ostream”不是一个类，而是一个模板实例的别名，基本上是“ostream = basic_ostream
您不应在“std”命名空间内声明任何内容（除非C++标准明确允许）。要进行前向声明“ostream”等内容，请在您的头文件中使用#include <iosfwd>。
或者，只需使用#include <iostream>，而不是前向声明 — 记住的技术细节较少，编译略慢一些 — 您可以根据自己的判断决定哪种更好。
英文:
As other people noted, ostream is not a class but an alias to a template instantiation, basically ostream = basic_ostream<char>. This is an implementation detail of the standard library; it's there because it needs to support also wostream (which is basic_ostream<wchar_t>) and possibly other stream types. But other implementations may do it differently (using e.g. source code generation) or very differently (distributing pre-compiled modules). You should not depend on any particular implementation.
You should not declare anything inside the std namespace (except stuff which the C++ Standard explicitly permits). To forward-declare ostream and such, use #include <iosfwd> in your header file.
Alternatively, just do #include <iostream> instead of forward declaration — less technical details to remember vs slightly slower compilation — do your own judgment on what is better.
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