Converting unary to base 10

unary2base10.cpp

#include <iostream>
#include <string>
#include <sstream>

std::string UnaryToBinary(std::string str) {
    std::string binary;
    binary.reserve(30);
    char flag;
    
    for (std::stringstream ss(str); ss >> str; ) {
        std::size_t size = str.size();
        
        if (size <= 2) flag = (size & 1) + '0';
        else {
            for (std::size_t i = 0; i < size-2; ++i) {
                binary += flag;
            }
        }
    }
    return binary;
}

std::string BinaryToBase10(std::string const& binary) {
    int result = 0;
    for (std::size_t i = 0, size = binary.size(); i < size; ++i) {
        if (binary[i] == '1') {
            result += 1 << (size - i - 1);
        }
    }
    return std::to_string(result);
}

int main() {
    for (std::string line; std::getline(std::cin >> std::ws, line, '#') && line.find('~') == std::string::npos; ) {
        std::cout << BinaryToBase10(UnaryToBinary(line)) << '\n';
    }
}