Question

Is there a standard library function to check that a string S is of type T and therefore can be converted to a variable of type T?

I know there is istringstream STL class that can, using operator>>, fill a variable of type T with a value converted from string. However, it will be filled with nonesense if the string content did not have the format of type T.

Answer 1

The best you can do is try and fail, as @Cameron commented:

#include <string>
#include <sstream>
#include <boost/optional.hpp>

template <typename T>
boost::optional<T> convert(std::string const & s)
{
    T x;
    std::istringstream iss(s);
    if (iss >> x >> std::ws && iss.get() == EOF) { return x; }
    return boost::none;
}

Or, without boost:

template <typename T>
bool convert(std::string const & s, T & x)
{
    std::istringstream iss(s);
    return iss >> x >> std::ws && iss.get() == EOF;
}

Usage:

• First version:

  if (auto x = convert<int>(s))
  {
      std::cout << "Read value: " << x.get() << std::endl;
  }
  else
  {
      std::cout << "Invalid string\n";
  }
  

• Second version:

  {
      int x;
      if (convert<int>(s, x))
      {
          std::cout << "Read value: " << x << std::endl;
      }
      else
      {
          std::cout << "Invalid string\n";
      }
  }
  

Note that boost::lexical_cast is basically a cleverer version of this, which claims to be very efficient (potentially moreso than using iostreams unconditionally as we did here).