C++ alternative for parsing input with sscanf

Question

Assuming my program expects arguments of the form [ 0.562 , 1.4e-2 ] (i.e. pairs of floats), how should I parse this input in C++ without regular expressions? I know there are many corner cases to consider when it comes to user input, but let's assume the given input closely matches the above format (apart from further whitespace).

In C, I could do something like sscanf(string, "[%g , %g]", &f1, &f2); to extract the two floating point values, which is very compact.

In C++, this is what I've come up with so far:

std::string s = "[ 0.562 , 1.4e-2 ]"; // example input

float f1 = 0.0f, f2 = 0.0f;

size_t leftBound = s.find('[', 0) + 1;
size_t count = s.find(']', leftBound) - leftBound;

std::istringstream ss(s.substr(leftBound, count));
string garbage;

ss >> f1 >> garbage >> f2;

if(!ss)
  std::cout << "Error while parsing" << std::endl;

How could I improve this code? In particular, I'm concerned with the garbage string, but I don't know how else to skip the , between the two values.

Answer 1

The obvious approach is to create a simple manipulator and use that. For example, a manipulator using a statically provided char to determine if the next non-whitespace character is that character and, if so, extracts it could look like this:

#include <iostream>
#include <sstream>

template <char C>
std::istream& expect(std::istream& in)
{
    if ((in >> std::ws).peek() == C) {
        in.ignore();
    }
    else {
        in.setstate(std::ios_base::failbit);
    }
    return in;
}

You can then use the thus build manipulator to extract characters:

int main(int ac, char *av[])
{
    std::string s(ac == 1? "[ 0.562 , 1.4e-2 ]": av[1]);
    float f1 = 0.0f, f2 = 0.0f;

    std::istringstream in(s);
    if (in >> expect<'['> >> f1 >> expect<','> >> f2 >> expect<']'>) {
        std::cout << "read f1=" << f1 << " f2=" << f2 << '\n';
    }
    else {
        std::cout << "ERROR: failed to read '" << s << "'\n";
    }
}

Answer 2

I you can afford to use boost, you could use Spirit.

See

• From a string Live On Coliru (in c++03):

• Update And here's the approach if you were actually trying to read from a stream (it's actually somewhat simpler, and integrates really well with your other stream reading activities):
  Live On Coliru too (c++03)

Allthough this seems more verbose, Spirit is also a lot more powerful and type-safe than sscanf. And it operates on streams.

Also note that inf, -inf, nan will be handled as expected.

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/qi_match.hpp>
#include <sstream>

namespace qi = boost::spirit::qi;

int main()
{
    std::istringstream ss("[ 0.562 , 1.4e-2 ]"); // example input
    ss.unsetf(std::ios::skipws); // we might **want** to handle whitespace in our grammar, not needed now

    float f1 = 0.0f, f2 = 0.0f;

    if (ss >> qi::phrase_match('[' >> qi::double_ >> ',' >> qi::double_ >> ']', qi::space, f1, f2))
    {
        std::cout << "Parsed: " << f1 << " and " << f2 << "\n"; // default formatting...
    } else
    {
        std::cout << "Error while parsing" << std::endl;
    }
}