CODEWITHSUNDEEP
c++templates
Marcel Bonzelet
Marcel Bonzelet

Reputation: 238

Template explicit instantiation of a method with a different body

I have a template class with a constructor that takes a std::vector<T>. For every object except one I want it to make operation A. But for that one object, I want it to do some other stuff B.

Is there a possibility to create an explicit instantiation of only the constructor for a template class? I hope its described precisely enough.

Regards

Update: I have now implemented a test case:

//header

Container(const std::vector<T>& source)
{...}

//source code

template <> Container<int>::Container(const std::vector<int>& source)
{
    throw 42;
}

This example compiles but doesn't work. I export this into a dll and want to have it being called whenever I try to create an instance of the class with generic parameter int. But as it is now, it only calls the standard constructor used for every other object. Is there a change I have to make to the declaration?

Update: I succeeded! Just had to copy it to the header file.

Update: OK, now I have another problem. I am able to make a specialization for 'simple' types but not for templates. I tried it this way:

template<typename T>
Container<MyClass<T>>::Container(const std::vecror<MyClass<T>>& source)
{...}

I want to specialize it for every MyClass object, but MyClass itself shall be able to live on as a template.

Upvotes: 0

Views: 520

Answers (4)

BЈовић
BЈовић

Reputation: 64293

If I understood your question correctly, you want to do this : 

template< typename T >
struct A
{
  template< typename P >
  A( std::vector< P > &v )
  {
  }
};

Upvotes: 0

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272802

Your question isn't clear. Perhaps you mean something like the following?

template <typename T>
class Foo
{
public:
    Foo() { std::cout << "standard" << std::endl; }
};

template <>
Foo<float>::Foo() { std::cout << "random" << std::endl; }  // Special case

...

Foo<int>    f1;  // Prints "standard"
Foo<float>  f2;  // Prints "random"

Upvotes: 5

Nick
Nick

Reputation: 5805

There is no way to do explicit constructor template instantiation -- this was covered here: C++ invoke explicit template constructor

Upvotes: 0

VGE
VGE

Reputation: 4191

Which compiler? I have had trouble with template instanciation with some old version of g++ compiler and with Solaris and HPUX c++ compiler. The template <> is used to specify explicit instanciation. It is a long time I do not have specialized only a method of a class.

Did you try ?

template <> TemplateClass<InstanciedType>::TemplateClass()  {
   ...
}

Where TemplateClass is the template class instanciation you want to override.

Upvotes: 0

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