std :: cin如何同时返回布尔值和自身? [英] How can std::cin return a bool and itself at the same time?

问题描述

我正在阅读一本有关C ++的书，其中说如果使用>>运算符，它将返回该运算符左侧的对象，因此在此示例中

I'm reading a book on C++ that says that if I use the >> operator it returns the object at the left side of the operator so in this example

std::cin >> value1;

代码返回 std :: cin .

但是如果我这样做

while(std::cin >> value1)

我的代码将一直处于循环状态，直到出现 std :: cin 错误，这必须表示操作员返回了 bool ，当std :: cin 不会失败，并且当 std :: cin 失败时为false.

My code will be in the loop until there is a std::cin error so that must mean that the operator returns a bool that is true when std::cin does not fail and false when std::cin fails.

是哪个?

推荐答案

[...]，因此必须表示操作员返回布尔值[...]

[...] so that must mean that the operator returns a bool [...]

不是，它返回 std :: cin (通过引用). while(std :: cin>>值); 起作用的原因是因为 std :: istream (这是 std :: cin的类型)具有 转换操作符 .

Nope, it returns std::cin (by reference). The reason why while(std::cin >> value); works is because std::istream (which is the type of std::cin) has a conversion operator.

转换运算符基本上允许将一个隐式类(如果未标记为 explicit )转换为给定类型.在这种情况下， std :: istream 定义其 operator bool 来返回是否发生错误(通常为 failbit )；

A conversion operator basically permits a class to be implicitly (if it is not marked explicit) to be converted to a given type. In this case, std::istream defines its operator bool to return whether an error (typically failbit) occurred;

_{[std :: ios_base :: operator bool()]}

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

---

explicit operator bool() const;

请注意，即使运算符是 explicit (不允许像 if(std :: cin); 这样的隐式转换)，使用限定符也会被忽略在需要 bool 的上下文中，例如 if ， while 循环和 for 循环.但这是例外，不是规则.

Note than even though the operator is explicit (which shouldn't allow implicit conversions like if (std::cin);), the qualifier is ignored when used in a context that requires a bool, like if, while loops and for loops. Those are exceptions though, not rules.

这里是一个例子:

if (std::cin >> value);  //OK, a 'std::istream' can be converted into a 'bool', which 
                         //therefore happens implicitly, without the need to cast it: 

if (static_cast<bool>(std::cin >> value)); //Unnecessary

bool b = std::cin >> value;  //Error!! 'operator bool' is marked explicit (see above), so 
                             //we have to call it explicitly:

bool b = static_cast<bool>(std::cin >> value); //OK, 'operator bool' is called explicitly

这篇关于std :: cin如何同时返回布尔值和自身?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！