Re: How to initialise member variable in template construction

From:

Angus <anguscomber@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 1 Sep 2010 15:28:18 -0700 (PDT)

Message-ID:

<5d6ded87-e6f7-4052-b996-9dbb5b7bf3c2@z7g2000yqg.googlegroups.com>

On 1 Sep, 23:11, red floyd <redfl...@gmail.com> wrote:

On Sep 1, 2:55 pm, Angus <anguscom...@gmail.com> wrote:

On 1 Sep, 22:51, "Francesco S. Carta" <entul...@gmail.com> wrote:

Angus <anguscom...@gmail.com>, on 01/09/2010 14:43:29, wrote:

I am trying to set the strategy (algorithm) used in a context by
template. Here is my context class (which is incorrectly
implemented):
template<class TStrategy>
class Context //TStrategy is the algorithm
{
private:
TStrategy * strategy_; //knows about Strategy=

Interface

     public:
         void execute() const
         {
             strategy_->execute();
         }
};

The strategies:
class StrategyInterface
{
     public:
         virtual void execute() const = 0; //abstract c=

lass - interface

only
};

//the actual concrete algorithms
class ConcreteStrategyA: public StrategyInterface
{
     public:
         virtual void execute() const
         {
             cout<< "Called ConcreteStrategyA exec=

ute method"<< endl;

}
};

And I want to do something like this:
Context<ConcreteStrategyA> contextD;
contextD.execute();

But I need to initialise the context data member properly. how d=

o I

do that?

I'm not sure about "how" you would do that, but I know "where" you
should do that: in a constructor. There is none in the code you poste=

d, why?

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- Show quoted text -

I guess I just need to add this:
Context(){ strategy_ = new TStrategy; }
~Context() { delete strategy_; }

Could I do it without using the heap?

Not if you store a pointer, You defined stategy_ as a TStrategy*.

You could go with a straight-up strategy member.

template <class TStrategy>
class Context //TStrategy is the algorithm
{
    private:
        TStrategy strategy_; //knows about StrategyInterface

    public:
        void execute() const
        {
            strategy.execute();
        }

};

Then there's no allocation needed, and strategy_ is automatically
constructed.- Hide quoted text -

- Show quoted text -

That's interesting I tried that:
template <class TStrategy>
class Context //TStrategy is the algorithm
{
    private:
        TStrategy strategy_; //knows about StrategyInterface

    public:
        void execute() const
        {
            strategy_.execute();
        }
};

And it works without my defining a constructor. I assume the compiler
is constructing strategy_ somehow behind the scenes. Could I create
my own constructor explicitly? What would it look like?