为什么while(std :: ifstream>> s)工作? [英] Why does while(std::ifstream >> s) work?
问题描述
我在我的C ++编程中使用了很多语句:
std :: string s;
std :: ifstream in(my_input.txt);
if(!in){
std :: cerr<< 文件未打开<< std :: endl;
exit(1);
}
while(in>> s){
//使用s
执行操作}
我想知道的是,为什么会这样?
我看过<$ c的返回值$ c> operator>> ,它是一个 istream
对象,而不是布尔值。如果一个istream对象以某种方式如何解释为一个bool值,可以放在如果
语句和 while
循环?
基类 std :: basic_ios
$ c> operator bool()方法返回一个表示流有效性的布尔值。例如,如果读取到达文件的结尾而没有抓取任何字符,则将在流中设置 std :: ios_base :: failbit
。然后将调用运算符bool()
,返回!fail()
,此时提取将停止,因为条件是false。
条件表达式代表显式布尔转换,因此:
while(in>> s)
while(static_cast< bool>(in>> s))
这相当于
while((> ; s).operator bool())
这相当于
while(!(in>> s).fail())
I've used statements such as this quite a bit in my C++ programming:
std::string s; std::ifstream in("my_input.txt"); if(!in) { std::cerr << "File not opened" << std::endl; exit(1); } while(in >> s) { // Do something with s }
What I want to know is, why does this work?
I looked at the return value of
operator>>
, and it's anistream
object, not a boolean. How does an istream object somehow get interpreted as a bool value that can be put inside ofif
statements andwhile
loops?解决方案The base class
std::basic_ios
provides anoperator bool()
method that returns a boolean representing the validity of the stream. For example, if a read reached the end of file without grabbing any characters, thenstd::ios_base::failbit
will be set in the stream. Thenoperator bool()
will be invoked, returning!fail()
, at which time extraction will stop because the condition is false.A conditional expression represents an explicit boolean conversion, so this:
while (in >> s)
is equivalent to this
while (static_cast<bool>(in >> s))
which is equivalent to this
while ((in >> s).operator bool())
which is equivalent to
while (!(in >> s).fail())
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