标识符“ostream”是未定义的错误 [英] identifier "ostream" is undefined error

问题描述

我需要实现一个数字类，它支持运算符<<输出。
i有一个错误：identifierostreamis undefined从某种原因，尽管我包括，并尝试

这里的头文件：

Number.h

  #ifndef NUMBER_H 
 #define NUMBER_H 
 #include< iostream> 
 class Number {
 public：
 //输出方法（对于所有类型继承自数字）：
 virtual void show（）= 0; 
 
 //输出运算符：
 friend ostream&运算符<< （ostream& os，const Number& f）; 
 
 
}; 
 
 #endif 
  

为什么编译器不能在friend函数中识别ostream？

解决方案

您需要完全限定名称 ostream 该类所在的命名空间：

  std :: ostream 
 // ^^^^^ 
 
 
 
 
因此，您的运算符声明应为：
  friend std :: ostream&运算符<< （std :: ostream& os，const Number& f）; 
 // ^^^^^ ^^^^^ 
  
在出现未限定名称 ostream 之前使用声明： 
 
 p $ p> 使用std :: ostream; 
  

这将允许您编写 ostream 名称没有完全限定，如在您当前版本的程序。

i need to implement a number class that support operator << for output. i have an error: "identifier "ostream" is undefined" from some reason eventhough i included and try also

here the header file:

Number.h

#ifndef NUMBER_H
#define NUMBER_H
#include <iostream>
class Number{
public:
//an output method (for all type inheritance from number):
virtual void show()=0;

//an output operator:
friend ostream& operator << (ostream &os, const Number &f);


};

#endif

why the compiler isnt recognize ostream in the friend function?

解决方案

You need to fully qualify the name ostream with the name of the namespace that class lives in:

    std::ostream
//  ^^^^^

So your operator declaration should become:

friend std::ostream& operator << (std::ostream &os, const Number &f);
//     ^^^^^                      ^^^^^

Alternatively, you could have a using declaration before the unqualified name ostream appears:

using std::ostream;

This would allow you to write the ostream name without full qualification, as in your current version of the program.

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