创建一个const share_ptr <pure_virtual_class> 会员

Question

I have a number of classes that derive from a pure virtal base:

class base {
public:
    virtual int f() = 0;
};

class derived_0 : public base {
public:
    int f() {return 0;}
};

class derived_1 : public base {
public:
    int f() {return 1;}
};

I only put two derived classes for brevity, but in practice I have more.

And I would like to create a class that has a const shared pointer to the base. I would like do to the following but I can't as I must initialize the const pointer in the initialization list:

class C{
public:
    C(bool type) { 
        if(type) {
            derived_0* xx = new derived_0;
            x = shared_ptr<base>( xx );
        }
        else {
            derived_1* xx = new derived1;
            x = shared_ptr<base>( xx );
        }
    } 
private:
    const share_ptr<base> x;
};

How can I get this functionality?

Answer 1

You encapsulate the creation of the object in a function, like this:

shared_ptr<base> create_base(bool type) {
     if(type) {
         return make_shared<derived_0>();
     }
     else {
         return make_shared<derived_1>();
     }
}

And then you can use it in your initialization-list:

class C{
public:
    C(bool type)
    : x(create_base(type))
    {}
private:
    const share_ptr<base> x;
};

Answer 2

In simple cases like this precise example:

class C
{
    shared_ptr<Base> const x;
public:
    C( bool type ) 
        : x( type
            ? static_cast<Base*>( new Derived_0 )
            : static_cast<Base*>( new Derived_1 ) )
    {
    }
};

(And yes, the static_cast , or at least one of them, are necessary.)

In more general cases, where the decision logic is more complex, you might want to create a static function which returns the shared_ptr , eg:

class C
{
    shared_ptr<Base> const x;
    static shared_ptr<Base> makeSharedPtr( bool type );
public:
    C( bool type )
        : x( makeSharedPtr( type ) )
    {
    }
};

This will allow any imaginable logic (and a more complex set of parameters as well).