关于C++类的拷贝构造函数

#include <iostream>

using namespace std;

class A
{
public:
   A() {cout << "A()" << endl;}
   A(const A& a){ cout << "A(const A& a)" << endl;}
   void print() const {cout << "print in A" << endl;}
};

class B : public A
{
public:
   B() {cout << "B()" << endl;}
   B(const B& a){ cout << "B(const A& a)" << endl;}
   void print() const {cout << "print in B" << endl;}
};

void printByValue(A a)
{
   a.print();
}
void printByRef(const A& a)
{
   a.print();
}
int main(int argc, char* argv[])
{
   A a;
   printByValue(a);
   printByRef(a);
   B b;
   printByValue(b);
   printByRef(b);
   a = b;
   printByValue(a);
   printByRef(a);

   return 0;
}

结果为：
A()
A(const A& a)
print in A
print in A
A()
B()
A(const A& a)
print in A
print in A
A(const A& a)
print in A
print in A

注：函数参数直接传值和“=”赋值均会调用拷贝构造函数。