C ++：如何在条件表达式中将istream转换为bool [英] C++: how is istream is converted to bool inside a conditional expression

问题描述

istream operator >> 用于读取数据，函数返回对istream的引用。

例如，

  istream& operator>> （bool& val）; 
  

但是当istream在条件语句中使用时如何转换为bool。 p>

例如，

  ifstream ifs //打开文件
 istream& is =（istream&）ifs; 
 
 char c; 
 
 if（is>> c）//如何将istream计算为bool 
 {
 //字符读取
} 
  

任何人都可以解释在条件表达式中如何将它转换为bool？

解决方案

从 cppreference ：

  explicit std :: basic_ios :: operator bool 
  

如果流没有发生错误并且可以进行I / O操作，则返回true。具体来说，返回！fail（）。

一个布尔上下文，它将调用 std :: istream 的成员函数运算符。

The istream operator>> is used to read the data and the function returns reference to istream.

For example,

istream& operator>> (bool& val);

But how does the istream is converted into a bool when it is used inside the conditional statement.

For example,

ifstream ifs(.....);  // open the file
istream &is = (istream&)ifs;

char c;

if(is >> c)   // how the istream is been evaluated into as bool
{
    // character read
}

Can anyone explain how it is being converted into a bool inside a conditional expression?

解决方案

From cppreference:

   explicit std::basic_ios::operator bool() const;

Returns true if the stream has no errors occurred and is ready of I/O operations. Specifically, returns !fail().

So since an if statement is a boolean context, it will invoke std::istream's member function operator.

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