C++ FAQ / Section 39 / FAQ 39.3

 E-mail the author
[ Subject index | About the author | © ]
Revised Jul 4, 2012

[39.3] Can I templatize the above functions so they work with other types?

Yes — for any types that support iostream-style input/output.

For example, suppose you want to convert an object of class Foo to a std::string, or perhaps the reverse: from a std::string to a Foo. You could write a whole family of conversion functions based on the ones shown in the previous FAQs, or you could write a template function so the compiler does the grunt work.

For example, to convert an arbitrary type T to a std::string, provided T supports syntax like std::cout << x, you can use this:

// File: convert.h
#include <iostream>
#include <sstream>
#include <string>
#include <typeinfo>
#include <stdexcept>

class BadConversion : public std::runtime_error {
public:
  BadConversion(std::string const& s)
    : std::runtime_error(s)
    { }
};

template<typename T>
inline std::string stringify(T const& x)
{
  std::ostringstream o;
  if (!(o << x))
    throw BadConversion(std::string("stringify(")
                        + typeid(x).name() + ")");
  return o.str();
}

Here's how to use the stringify() function:

#include "convert.h"

void myCode()
{
  Foo x;
  ...
  std::string s = "this is a Foo: " + stringify(x);
  ...
}

You can also convert from any type that supports iostream input by adding this to file convert.h:

template<typename T>
inline void convert(std::string const& s, T& x,
                    bool failIfLeftoverChars = true)
{
  std::istringstream i(s);
  char c;
  if (!(i >> x) || (failIfLeftoverChars && i.get(c)))
    throw BadConversion(s);
}

Here's how to use the convert() function:

#include "convert.h"

void myCode()
{
  std::string s = ...a string representation of a Foo...;
  ...
  Foo x;
  convert(s, x);
  ...
  ...code that uses x...
}

To simplify your code, particularly for light-weight easy-to-copy types, you probably want to add a return-by-value conversion function to file convert.h:

template<typename T>
inline T convertTo(std::string const& s,
                   bool failIfLeftoverChars = true)
{
  T x;
  convert(s, x, failIfLeftoverChars);
  return x;
}

This simplifies your "usage" code some. You call it by explicitly specifying the template parameter T:

#include "convert.h"

void myCode()
{
  std::string a = ...string representation of an int...;
  std::string b = ...string representation of an int...;
  ...
  if (convertTo<int>(a) < convertTo<int>(b))
    ...;
}