Linxutopia - Thinking in C++ - 15: Polymorphism & Virtual Functions

Thinking in C++
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Objects are different

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It’s important to realize that upcasting deals only with addresses. If the compiler has an object, it knows the exact type and therefore (in C++) will not use late binding for any function calls – or at least, the compiler doesn’t need to use late binding. For efficiency’s sake, most compilers will perform early binding when they are making a call to a virtual function for an object because they know the exact type. Here’s an example:

//: C15:Early.cpp
// Early binding & virtual functions
#include <iostream>
#include <string>
using namespace std;

class Pet {
public:
  virtual string speak() const { return ""; }
};

class Dog : public Pet {
public:
  string speak() const { return "Bark!"; }
};

int main() {
  Dog ralph;
  Pet* p1 = &ralph;
  Pet& p2 = ralph;
  Pet p3;
  // Late binding for both:
  cout << "p1->speak() = " << p1->speak() <<endl;
  cout << "p2.speak() = " << p2.speak() << endl;
  // Early binding (probably):
  cout << "p3.speak() = " << p3.speak() << endl;
} ///:~

In p1–>speak( ) and p2.speak( ), addresses are used, which means the information is incomplete: p1 and p2 can represent the address of a Pet or something derived from Pet, so the virtual mechanism must be used. When calling p3.speak( ) there’s no ambiguity. The compiler knows the exact type and that it’s an object, so it can’t possibly be an object derived from Pet – it’s exactly a Pet. Thus, early binding is probably used. However, if the compiler doesn’t want to work so hard, it can still use late binding and the same behavior will occur.

Thinking in C++
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