Question

I need to read input from standard input until a space or a TAB is pressed.

Eventually, I need to hold the input in a std::string object.

Answer 1

Since a tab counts as whitespace, you are asking to extract the first token from the stream. This is very easy in C++, as token extraction is already the default behaviour of the >> operator:

#include <iostream>
#include <string>

int main()
{
    std::string token;

    if (!(std::cin >> token)) { /* error */ return 1; }

    // now you have the first token in "token".
}

(The error can only occur in arcane circumstances, e.g. when the input file descriptor is already closed when the program starts.)