Runtime type information/ identification (RTTI)
Runtime type information/ identification (RTTI)
Operator: typeid again
Operator: typeid again
Class: type_info
Class: type_info
Operator: dynamic_cast
Operator: dynamic_cast
Embedded Files
When you use objects of polymorphic types, so those types with at least one virtual method, C++ attaches a few things to the objects:
Vtables to address virtual functions.
RTTI object locators to store the actual types of objects.
Data types can be:
Data types can be:
Static: Determined by the compiler.
Dynamic: Determined at runtime, stored in RTTI.
What is the cost?
What is the cost?
From Stroustrup’s 2012 ETAPS keynote address.
An aside ...
An aside ...
Again from Stroustrup’s presentation...
C++: Likely size 4*sizeof(double)“Pure object-oriented likely cost ...
3*sizeof(reference)+3*sizeof(heap_overhead)+4*sizeof(double)
A Vtable example ...
A Vtable example ...
class Shape {
public:
virtual void draw() { int i=0; }
};
class Circle : public Shape {
public:
virtual void draw() { int i=0; }
};
class Rectangle : public Shape {
public:
virtual void draw() { int i=0; }
};
void drawShape( Shape *sp )
{
sp->draw();
}
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