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's answer, esp because of the avoidance of exceptions. For embedded systems development and other low level system development, there may not be a proper Exception framework available.

Added a check for white-space after a valid string...these three lines

    while (isspace(*end)) {
        end++;
    }

Added a check for parsing errors too.

    if ((errno != 0) || (s == end)) {
        return INCONVERTIBLE;
    }

Here is the complete function..

#include 
#include 
#include 
#include 

enum STR2INT_ERROR { SUCCESS, OVERFLOW, UNDERFLOW, INCONVERTIBLE };

STR2INT_ERROR str2long (long &l, char const *s, int base = 0)
{
    char *end = (char *)s;
    errno = 0;

    l = strtol(s, &end, base);

    if ((errno == ERANGE) && (l == LONG_MAX)) {
        return OVERFLOW;
    }
    if ((errno == ERANGE) && (l == LONG_MIN)) {
        return UNDERFLOW;
    }
    if ((errno != 0) || (s == end)) {
        return INCONVERTIBLE;
    }    
    while (isspace((unsigned char)*end)) {
        end++;
    }

    if (*s == '\0' || *end != '\0') {
        return INCONVERTIBLE;
    }

    return SUCCESS;
}