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 think these three links sum it up:

• http://tinodidriksen.com/2010/02/07/cpp-convert-int-to-string-speed/
• http://tinodidriksen.com/2010/02/16/cpp-convert-string-to-int-speed/
• http://www.fastformat.org/performance.html

stringstream and lexical_cast solutions are about the same as lexical cast is using stringstream.

Some specializations of lexical cast use different approach see http://www.boost.org/doc/libs/release/boost/lexical_cast.hpp for details. Integers and floats are now specialized for integer to string conversion.

One can specialize lexical_cast for his/her own needs and make it fast. This would be the ultimate solution satisfying all parties, clean and simple.

Articles already mentioned show comparison between different methods of converting integers <-> strings. Following approaches make sense: old c-way, spirit.karma, fastformat, simple naive loop.

Lexical_cast is ok in some cases e.g. for int to string conversion.

Converting string to int using lexical cast is not a good idea as it is 10-40 times slower than atoi depending on the platform/compiler used.

Boost.Spirit.Karma seems to be the fastest library for converting integer to string.

ex.: generate(ptr_char, int_, integer_number);

and basic simple loop from the article mentioned above is a fastest way to convert string to int, obviously not the safest one, strtol() seems like a safer solution

int naive_char_2_int(const char *p) {
    int x = 0;
    bool neg = false;
    if (*p == '-') {
        neg = true;
        ++p;
    }
    while (*p >= '0' && *p <= '9') {
        x = (x*10) + (*p - '0');
        ++p;
    }
    if (neg) {
        x = -x;
    }
    return x;
}