Need C++ help with overloading definition of a value

dwhitney67 · 02-27-2009, 09:22 AM

The title is probably misleading, but here is what I am trying to do... and of course it doesn't compile. Any idea why I cannot get a TokenString to auto-convert to an std::string or const char* that can be used for string assignment?

Code:

#include <string>
#include <cstdlib>

class TokenString
{
  public:
    TokenString(const std::string& str) : m_str(str) {}

    operator std::string() const  { return m_str; }
    operator int() const          { return atoi(m_str.c_str()); }
    operator unsigned int() const { return atoi(m_str.c_str()); }
    operator double() const       { return strtod(m_str.c_str(), 0); }
    operator const char*() const  { return m_str.c_str(); }

  private:
    std::string m_str;
};

int main()
{
  TokenString tk("foo");

  // This is ok
  std::string str1 = tk;

  // This is ok
  std::string str2;
  str2 = "foo";

  // This is NOT ok
  std::string str3;
  str3 = tk;           <--- Will not compile!
}

tuxdev · 02-27-2009, 09:33 AM

It's an ambiguous overload. std::string has assignment operator overloads for const std::string &, const char *, and char, and can't decide which combination of conversion operators and assignment operators to use.

I suggest you do what some boost code does: create a template as() member function which is specialized for each kind of conversion. Or for that matter, use std::string rather than a special type and boost::lexical_cast to convert to whatever.

dwhitney67 · 02-27-2009, 09:40 AM

Thanks for the suggestion, however I do not have Boost available on my MontaVista system.

The only way I have found to "resolve" the problem is to wrap the token string into an std::string.

Code:

...

  // Now this is ok
  std::string str3;
  str3 = std::string(tk);