Re: Template specialisation with multiple typenames

From:

Barry <dhb2000@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 25 Sep 2007 18:59:17 +0800

Message-ID:

<fdapnv$2o3$1@aioe.org>

Adam Nielsen wrote:

Hi everyone,

I've having some trouble trying to specialise a template. My original
code, with one typename, worked:

  template <typename T>
  class MyClass
  {
    private:
      T data;

    public:
      const std::string& toString() const
      {
        // convert this->data to a string and return
      }
  };

  const std::string& MyClass<std::string>::toString() const
  {
    return this->data; // don't bother with unnecessary conversion
  }

However I then tried to extend the class with a second template
parameter, and it stopped working:

  template <class C, typename T>
  class MyClass

class MyClass<Type1>

for example:

template <class F = void()>
class Func
{
};

template <class Ret, class Arg>
class Func<Ret(Arg)>
^^^^^^^^^^
{
};

without "<Ret(Arg)>", the compiler assumes that you're defining another
*primary class template* with the same name, which violates the ODR.
with "<Ret(Arg)>", it's a partial specialization. Be aware the number if
template parameters in the <> must be same as the primary class template.

You can't have a template class with different template arguments
(default arguments also count). So you can't do it.

  ...

  template <class C>
  const std::string& MyClass<C, std::string>::toString() const
  {
    return this->data; // don't bother with unnecessary conversion
  }

It would appear to work if I write code for every possible value of C,
but then that's the whole reason for having a template! I'm assuming
I've just gotten the syntax wrong, so if someone would be kind enough to
point out the correct way of doing this it would be much appreciated!

--
Thanks
Barry