How to parse a string to an int in C++?

Question

What\'s the C++ way of parsing a string (given as char *) into an int? Robust and clear error handling is a plus (instead of returning zero).

Answer 1

I like Dan Moulding's answer, I'll just add a bit of C++ style to it:

#include 
#include 
#include 
#include 

int to_int(const std::string &s, int base = 0)
{
    char *end;
    errno = 0;
    long result = std::strtol(s.c_str(), &end, base);
    if (errno == ERANGE || result > INT_MAX || result < INT_MIN)
        throw std::out_of_range("toint: string is out of range");
    if (s.length() == 0 || *end != '\0')
        throw std::invalid_argument("toint: invalid string");
    return result;
}

It works for both std::string and const char* through the implicit conversion. It's also useful for base conversion, e.g. all to_int("0x7b") and to_int("0173") and to_int("01111011", 2) and to_int("0000007B", 16) and to_int("11120", 3) and to_int("3L", 34); would return 123.

Unlike std::stoi it works in pre-C++11. Also unlike std::stoi, boost::lexical_cast and stringstream it throws exceptions for weird strings like "123hohoho".

NB: This function tolerates leading spaces but not trailing spaces, i.e. to_int(" 123") returns 123 while to_int("123 ") throws exception. Make sure this is acceptable for your use case or adjust the code.

Such function could be part of STL...