getline in if statement

问题

From what I've read, getline() used in a Boolean context returns an implicitly conversion to void*. I haven't found anywhere on the web any real reference to this statement. Everywhere it says that implicit conversion doesn't exist and that in a Boolean context pointers should be of the same kind (and if ptr == 0 than 0 is converted to type of the pointer ptr).

Also in the standard says in a Boolean context it is converted to an unspecified-Boolean-type. What does that even mean?

回答1:

In short:

It means that you can use getline() in a if statement and if it works you enter the if statement block.

In Long:

getline() used in a Boolean context returns an implicitly conversion to void*.

The above is not technically correct (but that is the result). getline() actually returns a reference to the stream it was used on. When the stream is used in a Boolean context this is converted into a unspecified type (C++03) that can be used in a Boolean context. In C++11, this was updated and it is converted to bool.

• If the getline() succeeded it returns a stream in a good state. When this is converted to a bool like type it returns a non-null pointer (C++03) which when used in a Boolean context is equivalent to true.
• If the getline() fails it returns a stream in a bad state. When this is converted to a bool like type it returns a null pointer (C++03) which when used in a Boolean context is equivalent to false.

I haven't found anywhere on the web any real reference to this statement.

• 21.4.8.9 Inserters and extractors [string.io]
  • Defines: std::istream& getline(std::istream&, std::string&)
• 27.7.2.1 Class template basic_istream [istream]
  • Defines: std::istream& getline(char_type* s, streamsize n);
• 27.5.5.1 Overview [ios.overview]
  • Defines how a stream is converted in a boolean context

Everywhere it says that implicit conversion doesn't exist and that in a Boolean context pointers should be of the same kind (and if ptr == 0 than 0 is converted to type of the pointer ptr).

A null void* in a Boolean context is equivalent to false, any other void* is equivalent to true. (though the type is actually unspecified but you can think of it as a void* (just to make it easy to think about).

Also in the standard says in a Boolean context it is converted to an unspecified-Boolean-type. What does that even mean?

It means you can use it any conditional statements:

if (getline())
{
     // If getline worked processes data
}

while(getline())
{
    // getline. If it works then processes then try again.
}

回答2:

It doesn't say it explicitly anywhere on the web because it involves putting three different facts together.

1. getline() returns istream&
2. istream is convertable to void*
3. This conversion is used when an istream is used in boolean context (such as an if or while statement).

来源：https://stackoverflow.com/questions/8204113/getline-in-if-statement