brainfuck in 20 lines

#include <iostream>
#include <map>
#include <fstream>
int main(int argc, char **argv) {    
    std::map<int,unsigned char> cells;
    std::ifstream is(argv[1], std::ifstream::binary);
    auto  loop = [&](int s, char x, char y, int cnt=0) -> bool {
        while(cnt >= 0 && is.seekg(s, is.cur))
           [&](int d){cnt += (d == x) - (d == y);}(is.get());
    };
    for(int p=0;!is.eof();) {
        auto c = is.get();
        p += (c == '>') - (c == '<');
        cells[p] += (c == '+') - (c == '-');
        c=='.' && std::cout << cells[p];
        c==',' && (cells[p] = std::cin.get());
        c=='[' && !cells[p] && loop(0, '[', ']');
        c==']' && cells[p] && loop(-2, ']', '[');
    }
}