Re: How to initialise member variable in template construction

From:

red floyd <redfloyd@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 1 Sep 2010 15:11:59 -0700 (PDT)

Message-ID:

<80f100d0-6724-47c4-8f54-7c07ad85e38d@e14g2000yqe.googlegroups.com>

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 StrategyIn=

terface

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

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

ss - interface

only
};

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

e 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 do =

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 posted,=

why?

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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.