specialized template function results in duplicate symbols

Hi all,

as this is my first post to this board, I hope, I do not repeat a FAQ here. Anyhow, I was searching for my problem in the forum without success. Hence, I'd like to apologize in advance, if this question has been asked too often, already ;)

But here's my problem: I have a helper method for string to any, using a stringstream. Unfortunately, the stringstream stops at white spaces and hence, I would like to specialize the function for strings:

Code:

---

// template to convert a string to any type supported by stringstream
template <class type>
bool string2any( const std::string input, type &output )
{
  // use a stringstream to convert the string to a number
  std::stringstream converter(input);
  converter >> output;

  if( converter.fail() )
    return false;
  return true;
}

// specialized template from above - for both std::strings, to support strings with whitespaces
template <>
bool string2any<std::string>( const std::string input, std::string &output )
{
  output = input;
  return true;
}

---

If I use this template in two cpp files and link them together, I get the following error:

Code:

---

ld: duplicate symbol bool string2any<std::basic_string<char, std::char_traits<char>,
std::allocator<char> > >(std::basic_string<char, std::char_traits<char>,
std::allocator<char> >, std::basic_string<char, std::char_traits<char>,
std::allocator<char> >&)in /var/folders/GL/GLgvvL6NETK1iaM7tYVFwk+++TI/-Tmp-
//ccKgJ9C0.o and /var/folders/GL/GLgvvL6NETK1iaM7tYVFwk+++TI/-Tmp-//cccZPuby.o
collect2: ld returned 1 exit status

---

I wonder, why the compiler creates multiple objects for it and what template function specialization is for, if such things fail...?

Thanks in advance!

If it's in a header, try the inline keyword which instructs the linker that multiple definitions are OK. (Template functions are implicitly inline, but I suppose this doesn't apply to specializations.)

Next, it doesn't have to be a specialization. A non-template overload would be simpler (still declare inline, or define in a source file).

You probably also wanted to pass the input by const reference.

And lastly, this functionality is also readily available in the boost library (lexical_cast).

Thanks, it can be so easy :/

The function specialization results in the same function signature as the template. Do not specialize template functions, overload them instead.

Code:

---

bool string2any( const std::string input, std::string &output )
{
  output = input;
  return true;
}

---

(And it's wasteful to pass the input argument by value. You should consider using a const reference)