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《c++编程思想》上说当编译器知道一个对象的确切类型时,为了提高效率,可能会选择early binding, 而不是late binding。那具体是什么时候呢,没有采用指针或者引用进行upcasting的时候吗?
书中给出例子的代码如下:
//: C15:Early.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
// 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;
cout << sizeof (p3) << endl;
return 0;
} ///:~
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