reading formatted input using C++ streams

Question

When using stdio.h, I can easily read certain kinds of formatted input like this:

FILE* fin = fopen(...);
fscanf(fin, "x = %d, y = %d", &x, &y);

The great thing about this is that I don't really have to worry about how many spaces there are between the character 'x' and the following '=', and other minor details.

In C++ it appears to me as though,

ifstream fin(...);
string s;
fin >> s;

may result in s being "x" or "x=", or even "x=12" depending on the spacing of the input.

Is there a convenient way to get behavior similar to scanf/fscanf using iostream/fstream?

Answer 1

This is actually surprisingly easy, given a prerequisite. I have these three functions that I stick in a header somewhere. These allow you to stream in character literals, and string literals. I've never quite understood why these aren't standard.

#include <iostream>

//These are handy bits that go in a header somewhere
template<class e, class t, int N>
std::basic_istream<e,t>& operator>>(std::basic_istream<e,t>& in, const e(&sliteral)[N]) {
        e buffer[N-1] = {}; //get buffer
        in >> buffer[0]; //skips whitespace
        if (N>2)
                in.read(buffer+1, N-2); //read the rest
        if (strncmp(buffer, sliteral, N-1)) //if it failed
                in.setstate(std::ios::failbit); //set the state
        return in;
}
template<class e, class t>
std::basic_istream<e,t>& operator>>(std::basic_istream<e,t>& in, const e& cliteral) {
        e buffer(0);  //get buffer
        in >> buffer; //read data
        if (buffer != cliteral) //if it failed
                in.setstate(std::ios::failbit); //set the state
        return in;
}
//redirect mutable char arrays to their normal function
template<class e, class t, int N>
std::basic_istream<e,t>& operator>>(std::basic_istream<e,t>& in, e(&carray)[N]) {
        return std::operator>>(in, carray);
}

Given those, the rest is easy:

in>>'x'>>'='>>data.first>>','>>'y'>>'='>>data.second;

Proof here

For more complex situations, you probably want to use std::regex or boost::regex, or maybe a real lexer/parser.