float <-> std::string conversion alternative?

Question

is there any alternative to atof , strtod , lexical_cast , stringstream or sprintf ?

that is:

1. fast
2. C++ way ( std::string instead of char* )
3. safe (no buffer overrun risk)
4. valid (return NaN if conversion couldn't be made)
5. no external library (independent)

I prefer more like this , a simple function, optimized, and to the point

reason :

• atof and strtod is C function and they are not returning NaN upon failure, I prefer working on std::string , so I just asking if anyone already writing some wrapper to std::string that I can use (if you don't mind).
• lexical_cast has boost dependency
• stringstream is slow
• sprintf has buffer overflow risk and its C function

Answer 1

I'd look at Boost Spirit

• http://www.boost.org/doc/libs/1_47_0/libs/spirit/doc/html/spirit/qi/reference/numeric/real.html

At least the benchmarks of the formatters (that is float -> string) consistently turn out as top-of-the-bill* ¹ *

Also the exact input format specification and semantics when parsing can be configured very nicely using a policy class.

---

Here is my absolute min-dependency use of qi::any_real_parser<> and the list of dependendencies it touches:

#include <boost/spirit/include/qi_real.hpp>

namespace qi = boost::spirit::qi;

int main()
{
    const char input[] = "3.1415926";
    const char *f(input);
    const char *l(f+strlen(input));

    qi::any_real_parser<double> x;
    double parsed;
    x.parse(f, l, qi::unused, qi::unused, parsed);

    return 0;
}

• boost/concept
• boost/config
• boost/detail
• boost/exception
• boost/fusion
• boost/iterator
• boost/math
• boost/mpl
• boost/optional
• boost/preprocessor
• boost/proto
• boost/range
• boost/regex
• boost/spirit
• boost/typeof
• boost/type_traits
• boost/utility
• boost/variant

aligned_storage.hpp,assert.hpp,blank_fwd.hpp,blank.hpp,call_traits.hpp,checked_delete.hpp,concept_check.hpp,config.hpp,cstdint.hpp,current_function.hpp,foreach_fwd.hpp,foreach.hpp,get_pointer.hpp,implicit_cast.hpp,iterator.hpp,limits.hpp,math_fwd.hpp,next_prior.hpp,noncopyable.hpp,none.hpp,none_t.hpp,optional.hpp,ref.hpp,static_assert.hpp,swap.hpp,throw_exception.hpp,type.hpp,utility.hpp,variant.hpp,version.hpp

¹ eg http://www.boost.org/doc/libs/1_47_0/libs/spirit/doc/html/spirit/karma/performance_measurements/numeric_performance/double_performance.html

Answer 2

If you want to convert from numerical types to std::string there's a std::to_string function available in the latest standard.

Unfortunately as I've found out recently, in Visual Studio 2010 it is somewhat limited because there are only three overloads available for it; long double, long long, and unsigned long long. This causes issues when trying to use them from within templates.

Answer 3

The fast format library should be able to do the kinds of transformations you're looking for, at least for writing a float out. It does not handle parsing of a float, however.