template question

[Linara]

Hi, I am not very familiar with templates and have a question.

Say I have a class template like this:

namespace {
   template <class T>
   class A
   {
      public:
         T m_var;
   };
}
 

And in a separate file I have a class like this:

namespace MyNamespace
{
   class B
   {
   public:
      B(int x=0) : m_x(x) {}
      void SetX(int x) { m_x = x; }
   private:
      int m_x;
   };
}
 

And then in the same file as the class template I have a function to return a pointer to the class:

A* Create()
{
   A* p = new A<myNamespace::B>();
   return p;
}
 

My question is, when the class A object is created, is there any way to pass an int to the class B object that gets created so that m_x is initialized as 5 or 7, etc.? If not, is there a way to just set x through p?

[weaknessforcats]

Yes.

The way your template is defined, the member is a T m_var so that means the default contstructor for T will be called. This will be the default constructor for class B in your example.

So, in the default constructor for class B, you can acquire the starting value by reading a disc file, usibng a singleton, etc. You just can't pass it as an argument.

Other observations:
1) What's with the public data?? This exposes the name of your data variable. Should you ever need to re-design the class, you will need to change every scrap of code written that uses your class. Not a good idea. Make all data private.
2) What's with the m_ ??? Using an m_ does not identify a variable as a class member. You shoulduse this->var. All naming conventions don't work becuse they are not enforceable. Anything not enforceable is a waste of time. I realize this m_ stuff started years ago when everyine was using vi oe somesuch but today we're all using IDE's. In my case, I just place my mouse cursor on top of the variable name and it's declaration appears. I don't have to go look it up.
3) Your Create() function should take an argument that says what type of obkect to build or should be renamed to CreateB().

[Linara]

Thanks for the info.

Is there a way to just call B's SetX function through p? I'm not sure what the syntax would be.

[weaknessforcats]

Is there a way to just call B's SetX function through p? I'm not sure what the syntax would be.

No. p is a pointer to A and there is no SetX in A.

You would need to change your design:
1) make B a member of A and write a SetX for A
or
2) derived B from A and have a virtual SetX in A
or
3) derived B from A and use a Visitor. Read the article in the C/C++ Articles forum.