Question

What’s the best approach to parsing std::string to some numeric type in C++, when the target type isn’t known in advance?

I’ve looked at lexical_cast, but that takes the target type as a template parameter. I could write wrapper functions that abuse this by catching bad_lexical_cast and returning false, but that seems ugly.

My input values will typically be int or float and have extremely simple formatting, but something that’s flexible would be great!

Answer 1

You could use either Boost Spirit Numerical Parsers or (ab)use Boost Lexicalcast.

Boost Spirit allows you fine grained control of the format accepted, see e.g.

• RealPolicies

Here is a quick demo, that also shows how you could detect several possible numeric input formats (progressively) and return the type that was matched. Of course that could be overkill, but it should demonstrate how to use Spirit further.

The demo also shows how to advance the input iterator so you can easily continue parsing where the numeric input ended.

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;

enum numeric_types
{
    fmt_none,
    fmt_float,
    fmt_double,
    fmt_uint,
    fmt_int,
    // fmt_hex, etc. 
};

template <typename It>
    bool is_numeric(It& f, It l, numeric_types& detected)
{
    return qi::phrase_parse(f,l,
            qi::uint_   [ qi::_val = fmt_uint   ]
          | qi::int_    [ qi::_val = fmt_int    ]
          | qi::float_  [ qi::_val = fmt_float  ]
          | qi::double_ [ qi::_val = fmt_double ]
           ,qi::space, detected);
}

template <typename It>
    bool is_numeric(It& f, It l)
{
    numeric_types detected = fmt_none;
    return is_numeric(f, l, detected);
}

int main()
{
    const std::string input = "124, -25, 582";
    std::string::const_iterator it = input.begin();

    bool ok = is_numeric(it, input.end());

    if (ok)   
    {
        std::cout << "parse success\n";
        if (it!=input.end()) 
            std::cerr << "trailing unparsed: '" << std::string(it,input.end()) << "'\n";
    }
    else 
        std::cerr << "parse failed: '" << std::string(it,input.end()) << "'\n";

    return ok? 0 : 255;
}