条款20：宁以pass-by-reference-to-const替换pass-by-value

// 条款20： 宁以pass-by-reference-to-const替换pass-by-value// 1.尽量以pass-by-reference-to-const替换pass-by-value。前者通常比较高效，可避免切割问题。// 2.以上规则并不适用于内置类型，以及STL的迭代器和函数对象。对它们而言，pass-by-value往往比较恰当。#include <iostream>#include <string>// 缺省状况下C++以值方式传递对象时，传递的是对象的副本，系由copy构造函数产出，可能使得以值传递// 成为费时操作。class Face20 {public:Face20(const Face20& f) {std::cout << "I am Face" << std::endl;}Face20() {}};class Person20 {public: Person20(const Person20& p): face(p.face) { std::cout << "I am Person" << std::endl;}Person20() {}virtual void Name() const {std::cout << "My name is Person20" << std::endl;}private:Face20 face;};class Student20 : public Person20 {public:Student20() {}Student20(const Student20& s) {std::cout << "I am Student" << std::endl;}virtual void Name() const {std::cout << "My name is Student20" << std::endl;}};// 测试以值传递的耗时性// I am Student// I am Face// I am Person// 以值传递参数的传递成本就是多次调用构造函数，最后调用析构函数void TestStudent20(Student20 s) {}// 测试以值传递的切割性// My name is Person20// 以值传递时，参数p会被构造成一个Person20的对象，p原本的特化信息被切割。导致多态不能形成。void TestStudent202(Person20 p) {p.Name();}// 测试引用传递的完整性，可以实现多态// My name is Student20void TestStudent203(const Person20& p) {p.Name();}int main() {Student20 s;TestStudent20(s);TestStudent202(s);TestStudent203(s);//输出为：// I am Student// I am Face// I am Person// ------------// My name is Person20// ------------// My name is Student20return 0;}