使用std :: cin中的std :: setw限制输入大小

Question

Let's say I have sample code:

std::string s;
std::cin >> std::setw(4) >> s;
std::cout << s;

Now for input abcdef the result will be abc and for abc it will be abc too. The question is how can I check whether the string was split in the middle due to the limit or the result string is the actual one? I need to know whether the input fits or some data was skipped.

Answer 1

Although I know that the stream's width is considered when reading into a char* I wasn't aware that it is also considered when reading into a std::string . Assuming it is, reading would stop under three conditions:

1. The stream is completely read in which case eof() is set.
2. The next character is a space.
3. The number of characters which need to be read are read.

That is, you can check in.eof() and std::isspace(in.peek() . Well, is there is a funny std::ctype<char> facet used by the stream you'd really need to use

std::isspace(in.getloc(),
    std::char_traits<char>::to_char_type(in.peek()));