template function in struct

[zepher]

Hi All,

I understand template functions and how to call them explicitly when no arguments are passed to them but can't seem to instantiate them explicitly when it resides within a class or struct.

struct Creator
{

   template <class T>
   T * New(T * p)
   {
      return new T;
   }

};


Creator cr;
MyClass * pMy = cr.New<MyClass>();     //gives compile error 

The compiler complains about MyClass (or any other) being used illegally in expression ...

Can someone please explain how this can be done.

[Peter87]

You didn't pass the function argument.

[zepher]

Hi Peter - I made a mistake its supposed to be:

struct Creator
{

   template <class T>
   T * New()
   {
      return new T;
   }

};


Creator cr;
MyClass * pMy = cr.New<MyClass>();     

[Peter87]

Has the MyClass class been defined?

[JLBorges]

There are a lot of great features in C++11, but unique_ptr stands out in the area of code hygiene. Simply put, this is a magic bullet for dynamically created objects. It won't solve every problem, but it is really, really good at what it does: managing dynamically created objects with simple ownership semantics.
http://www.drdobbs.com/cpp/c11-uniqueptr/240002708

#include <iostream>
#include <memory>

namespace creator
{
    template < typename T, typename... ARGS >
    std::unique_ptr<T> make_unique( ARGS&&... args )
    { return std::unique_ptr<T>( new T( std::forward<ARGS>(args)... ) ) ; }
}

int main()
{
    struct A
    {
        A() { std::cout << this << " - A::default_constructor\n" ; }

        A( int i, double d = 2.3, const char* cstr = "hello" )
        { std::cout << this << " - A::A( " << i << ", " << d << ", '" << cstr << "' )\n" ; }

        ~A() { std::cout << this << " - A::destructor\n" ; }
    };

    auto pa1 = creator::make_unique<A>() ;
    auto pa2 = creator::make_unique<A>(1) ;
    auto pa3 = creator::make_unique<A>( 22, 33.44 ) ;
    auto pa4 = creator::make_unique<A>( 333, 555.666, "bye" ) ;

    std::cout << "\nexit main\n\n" ;
}

http://coliru.stacked-crooked.com/a/185b87d75f94d6ab

Note: std::make_unique<>() (C++14)
http://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique

[zepher]

@Peter87: Yes - MyClass has been defined.

@JLBorges: thanks for the reply - I however note the make_unique template in your sample above resides within the namespace creator.

Can this creator be a class or struct instead of a namespace?

[JLBorges]

> Can this creator be a class or struct instead of a namespace?

Yes.

But is an object really required?