使用迭代器将部分文件流读入字符串 [英] Reading a partial file stream into a string using iterators

问题描述

这是我到目前为止所尝试的但没有成功：

This is what I have tried so far but with no success:

std::string ReadPartial( std::ifstream& _file, int _size )
{
    std::istreambuf_iterator<char> first( _file );
    std::istreambuf_iterator<char> last( _file );
    std::advance( last, _size );
    return std::string( first, last ); 
}

我知道如何读取整个文件。

I know how to read the whole file.

std::string Read( std::ifstream& _file )
{
    std::istreambuf_iterator<char> first( _file );
    std::istreambuf_iterator<char> last();
    return std::string( first, last ); 
}

但这不是我想做的。我收到一个空字符串。如果我在调试器中查看第一个也是最后一个，即使在std :: advance之后它们指向相同的东西。

But this is not what i want to do. I'm getting an empty string. If I look at first and last in a debugger they point to the same thing even after the std::advance.

推荐答案

是你有什么特别的理由要使用迭代器吗？你可以一次读取字节：

Is there some particular reason you want to use iterators? You could just read the bytes in one go:

std::string s(_size, '\0');
_file.read(&s[0], _size);

如果你真的想用迭代器读取，你可以这样做：

If you really want to read using iterators, you could do this:

std::string ReadPartial( std::ifstream& _file, int _size )
{
    std::istreambuf_iterator<char> first( _file );
    std::istreambuf_iterator<char> last;
    std::string s;
    s.reserve(_size);
    while (_size-- && first != last) s += *first++;
    return s;
}

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