Strategy Pattern in C++ with template

Question

This is a sample implementation of Strategy Pattern in C++:

ConcreteStrategy.h

class ConcreteStrategy {
public:
    ConcreteStrategy();
    ~ConcreteStrategy();
    const OtherObject* doSomething(const OtherObject &obj);
};

ConcreteStrategy.cpp

#include "ConcreteStrategy.h"

ConcreteStrategy::ConcreteStrategy() { // etc. }
ConcreteStrategy::~ConcreteStrategy() { // etc. }
const OtherObject* ConcreteStrategy::doSomething(const OtherObject &obj) { // etc. }

MyContext.h

template <class Strategy> class MyContext {
    public:
        MyContext();
        ~MyContext();
        const OtherObject* doAlgorithm(const OtherObject &obj);
    private:
        Strategy* _the_strategy;
};

MyContext.cpp

#include "MyContext.h"

template <typename Strategy>
MyContext<Strategy>::MyContext() {
    _the_strategy = new Strategy;
}

template <typename Strategy>
MyContext<Strategy>::~MyContext() {
    delete _the_strategy;
}

template <typename Strategy>
const OtherObject* MyContext<Strategy>::doAlgorithm(const OtherObject &obj) {
    obj = _the_strategy(obj);
    // do other.
    return obj;
}

main.cpp

#include "MyContext.h"
#include "ConcreteStrategy.h"
#include "OtherPrivateLib.h"

int main(int argc,char **argv) {
    OtherObject* obj = new OtherObject;
    MyContext<ConcreteStrategy>* aContext = new MyContext<ConcreteStrategy>;
    obj = aContext.doAlgorithm(obj);

    // etc.

   delete aContext;
   delete obj;

   return 0;
}

Is this implementation right? It's my first approach with template in C++ and I've some doubt, specially about construction and destruction of template object (Strategy) inside the context (MyContext).

UPDATE: I've this error at compile-time:

undefined reference to `MyContext<Strategy>::MyContext()'

Answer 1

First, the class template implementations should go in the header file or in a file included by the header, not in a .cpp file to be compiled. The compiler needs to see the template code in order to create the MyContext<ConcreteStrategy> required by main. This is the cause of the compiler error.

Second, and unrelated to templates, you have many uses of dynamically allocated objects for no apparent reason. I would change doSomething and doAlgorithm to return by value, for example

OtherObject doSomething(const OtherObject &obj);

and remove all uses of new from the main, for example:

int main(int argc,char **argv) {
    OtherObject obj;
    MyContext<ConcreteStrategy> aContext;
    obj = aContext.doAlgorithm(obj);
    return 0;
}

If you really must use dynamically allocated objects, then I suggest using smart pointers, particularly C++11's std::unique_ptr.