how to construct a class with member type unknown till runtime?

[ckpun1978]

Hi,

I am now writing a class using template. I run into a situation
that, one of my member type is not unknown until run-time (but i dont'
want to specify a template entry for it). How should I handle this?


template <class T>
class myfirst
{
public:

private:
Array<T> myarray;
void * runtime_change_member;
};


Thanks,

Carson

 

[Rolf Magnus]

Hi,

I am now writing a class using template. I run into a situation
that, one of my member type is not unknown until run-time (but i dont'
want to specify a template entry for it). How should I handle this?

Derive them all from a polymorphic base class. Then use a pointer to that
base class in your class template.

 

[ckpun1978]

but if I use pointer to refer, how can I know which type I should cast
to? (cos' i don't know the base class type, as it is polymorphic?)

Carson

 

[red floyd]

but if I use pointer to refer, how can I know which type I should cast
to? (cos' i don't know the base class type, as it is polymorphic?)

Carson

That's what virtual functions are for.

class Shape {
public:
virtual void Draw() = 0;
};

class Circle : public Shape {
public:
void Draw() {
// draw a circle;
}
};

class Rectangle : public Shape {
public:
void Draw() {
// draw a rectangle
}
};

class Polygon : public Shape {
public:
void Draw() {
// guess what this is supposed to do
}
};

template <class T>
class myfirst
{
public:

private:
Array<T> myarray;
void * runtime_change_member;
};

myfirst<Shape*> struct;

 

[manu]

template <class T>
class myfirst
{
public:

private:
Array<T> myarray;
void * runtime_change_member;
};

You may use something like Boost.Any, Boost.Variant.