class Persion
{
private:
std::string m_name;
std::string m_addr;
};
class Student : public Persion
{
private:
std::string m_schoolName;
std::string m_schoolAddr;
};
bool IsValidStudent(Student s) {return true;}
Student s1; bool b_s = IsValidStudent(s1);IsValidStudent函数调用行为如下：编译器调用student拷贝构造函数拷贝实参到形参，然后形参使用完后再调用析构函数，但该类调用远不止这些，如student的父类也需要调用拷贝构造函数，而persion中有两个string成员也需要调用拷贝构造函数，还有student也有两个string成员需要调用拷贝构造函数，而且这些调用完后还需要调用析构函数。就因为一个pass-by-value操作，而需要花费这么多成本。
bool IsValidStudent(const Student &s) {return true;}这样没有调用任何的构造函数和析构函数，形参实际是实参的副本，因此需要加上const保护实参免修改。class Persion
{
public:
Persion(const string &name, const string& addr) : m_name(name), m_addr(addr) {}
virtual ~Persion() {} //作为基类，需要定义虚析构函数
string GetName() const {return m_name;}
string GetAddr() const {return m_addr;}
virtual void DisPlay() {cout<<"Person\n";}
private:
std::string m_name;
std::string m_addr;
};
class Student : public Persion
{
public:
Student(const Persion &persion, const string &sName, const string &sAddr)
:Persion(persion.GetName(), persion.GetAddr())
,m_schoolName(sName)
,m_schoolAddr(sAddr)
{
}
string GetName() const {return m_schoolName;}
virtual void DisPlay() {cout<<"Student\n";}
private:
std::string m_schoolName;
std::string m_schoolAddr;
};
void DisPlayPersion(Persion persion)
{
std::cout<<persion.GetName()<<endl;
persion.DisPlay();
}调用 Persion p("cuancuan", "guangzhou");
Student s1(p, "shiyan", "yuexiu");
DisPlayPersion(s1);输出void DisPlayPersion(const Persion &persion)
条款20：宁以pass-by-reference-to-const替换pass-by-value,布布扣,bubuko.com
条款20：宁以pass-by-reference-to-const替换pass-by-value
原文地址：http://blog.csdn.net/hualicuan/article/details/27564945
