Should safe pointers be used in strategy pattern?

Question

Given a typical strategy pattern

class Strategy
{
public:
    virtual int execute() const = 0;
}

class StrategyA : public Strategy
{
public:
    int execute() const override;
}

class StrategyB : public Strategy
{
public:
    int execute() const override;
}

I believe the 'pre-C++11' way to implement a context class would be something like

class ContextRaw
{
public:
    ContextRaw(Strategy* the_strategy);
    ~ContextRaw(); // Should this delete the_strategy_?
    int execute() const;
private:
    Strategy* the_strategy_;
}

To me, in this design it's not clear if Context should take responsibility for Strategy, and unless there is clear documentation stating otherwise, bad things might happen if it does

void trouble()
{
    StrategyA a_concrete_strategy;
    ContextRaw a_context(&a_concrete_strategy); // Oops, Context may try to delete stack variable
}

void more_trouble()
{
    Strategy* a_concrete_strategy = new StrategyA;
    ContextRaw* a_context       = new ContextRaw(a_concrete_strategy);
    ContextRaw* another_context = new ContextRaw(a_concrete_strategy);
    delete a_context;
    std::cout << another_context.execute() << std::endl; // Oops, the_strategy is deleted
}

In light of safe-pointers, should it now be preferable to inject a safe pointer, and have Context take ownership of the Strategy?

class ContextUnique
{
public:
    ContextUnique() = delete;
    ContextUnique(std::unique_ptr<Strategy> the_strategy);
    ~ContextUnique();
    int execute() const;
private:
    std::unique_ptr<Strategy> the_strategy_;
}

or if Strategy can be shared amongst different Context?

class ContextShared
{
public:
    ContextShared() = delete;
    ContextShared(std::shared_ptr<Strategy> the_strategy);
    ~ContextShared();
    int execute() const;
private:
    std::shared_ptr<Strategy> the_strategy_;
}

This design of course introduces problems of it's own, in particular only dynamically allocated Strategy's can be injected into Context.

Answer 1

You're doing it wrong.

In the light of std::function, everything you've just written is completely obsolete and you should just use std::function<int()> and some lambdas.