class Test { public: Test(int a = 10) : ma(a) { cout << this << endl; cout << "constructor" << endl; //cout << __FUNCTION__ << endl; } ~Test() { cout << this << endl; cout << "destructor" << endl; //cout << __FUNCTION__ << endl; } Test(const Test &t) : ma(t.ma) { cout << this << endl; cout << "copy constructor" << endl; //cout << __FUNCTION__ << endl; } Test& operator=(const Test &t) { cout << this << endl; cout << "operator constructor" << endl; //cout << __FUNCTION__ << endl; ma = t.ma; return *this; } private: int ma; }; int main() { Test t1; Test t2(t1); Test t3 = t1; /* c++编译器对于对象构造的优化，用临时对象生成新对象的时候 临时对象就不产生了，直接构造新对象就可以了 */ Test t4 = Test(20);//Test t4(20);//没有区别 t4 = t2; //t4.operator=(const Test &t) cout << "=========" << endl; //显式生成临时对象 t4 = Test(30); cout << "=========" << endl; t4 = (Test)30; //int -> Test(int) //隐式生成临时对象 t4 = 30; //int -> Test(int) Test *p = &Test(40); //p指向一个已经析构的对象 const Test &ref = Test(50); cout << "=====end=====" << endl; return 0; }
程序输出：
0047FB20 constructor 0047FB14 copy constructor 0047FB08 copy constructor 0047FAFC constructor 0047FAFC operator constructor ========= 0047FA0C constructor 0047FAFC operator constructor 0047FA0C destructor ========= 0047FA00 constructor 0047FAFC operator constructor 0047FA00 destructor 0047F9F4 constructor 0047FAFC operator constructor 0047F9F4 destructor 0047F9E8 constructor 0047F9E8 destructor 0047FAD8 constructor =====end===== 0047FAD8 destructor 0047FAFC destructor 0047FB08 destructor 0047FB14 destructor 0047FB20 destructor
class Test { public: //Test() Test(a) Test(a, b)三种构造 Test(int a = 5, int b = 5) : ma(a) , mb(b) { cout << this << endl; cout << "constructor" << endl; //cout << __FUNCTION__ << endl; } ~Test() { cout << this << endl; cout << "destructor" << endl; //cout << __FUNCTION__ << endl; } Test(const Test &t) : ma(t.ma), mb(t.mb) { cout << this << endl; cout << "copy constructor" << endl; //cout << __FUNCTION__ << endl; } void operator=(const Test &t) { cout << this << endl; cout << "operator constructor" << endl; //cout << __FUNCTION__ << endl; ma = t.ma; mb = t.mb; } private: int ma; int mb; }; Test t1(10, 10);//1.Test(int, int) int main() { Test t2(20, 20); //3.Test(int, int) Test t3 = t2; //4.Test(const Test &) //static Test t4(30, 30); static Test t4 = Test(30, 30); //5.Test(int, int) t2 = Test(40, 40);//6.Test(int, int) operator= ~Test() //逗号表达式强转(50, 50) = (Test)50; t2 = (Test)(50, 50); //7.Test(int, int) operator= ~Test() t2 = 60; //Test(int) 8.Test(int, int) operator= ~Test() Test *p1 = new Test(70, 70); //9.Test(int, int) Test *p2 = new Test[2]; //10.Test(int, int) Test(int, int) Test *p3 = &Test(80, 80); //11.Test(int, int) ~Test() const Test &p4 = Test(90, 90); //12.Test(int, int) cout << "============" << endl; delete p1; //13.~Test(); delete[] p2; //14.~Test() ~Test() return 0; } Test t5(100, 100);//2.Test(int, int)
00CAF2D8 //t1 constructor 00CAF2E0 //t5 constructor 0096F908 constructor 0096F8F8 copy constructor 00CAF2E8 //t4 constructor 0096F7E8 constructor 0096F908 operator constructor 0096F7E8 destructor 0096F7D8 constructor 0096F908 operator constructor 0096F7D8 destructor 0096F7C8 constructor 0096F908 operator constructor 0096F7C8 destructor 009DF060 constructor 009DE8BC constructor 009DE8C4 constructor 0096F788 constructor 0096F788 destructor 0096F8B8 constructor ============ 009DF060 destructor 009DE8C4 destructor 009DE8BC destructor 0096F8B8 destructor 0096F8F8 destructor 0096F908 destructor 00CAF2E8 //t4 destructor 00CAF2E0 //t5 destructor 00CAF2D8 //t1 destructor
class Test { public: //Test() Test(a)两种构造 Test(int a = 5) : ma(a) { cout << this << endl; cout << "constructor" << endl; } ~Test() { cout << this << endl; cout << "destructor" << endl; } Test(const Test &t) : ma(t.ma){ cout << this << endl; cout << "copy constructor" << endl; } void operator=(const Test &t) { cout << this << endl; cout << "operator constructor" << endl; ma = t.ma; } int getData() const { return ma; } private: int ma; }; //不能返回局部的或者临时对象的指针或引用 Test GetObject(Test t)//3.Test(const Test &) { int val = t.getData(); Test tmp(val);//4.Test(int) return tmp;//5.Test(const Test &)返回值临时变量拷贝构造 } //6.tmp析构 //7.形参析构 int main() { Test t1;//1.Test(int) Test t2;//2.Test(int) t2 = GetObject(t1); //8.operator= //9.~Test(),返回值临时变量拷贝析构 //10.t2析构 //11.t1析构 return 0; }
程序输出：
0113F798 constructor 0113F78C constructor 0113F684 copy constructor 0113F654 constructor 0113F6B4 copy constructor //返回值的拷贝构造 0113F654 destructor 0113F684 destructor 0113F78C operator constructor 0113F6B4 destructor //返回值的析构 0113F78C destructor 0113F798 destructor
三条对象优化原则
1.函数参数传递过程中，对象优先按引用传递，不要按值传递 2.函数返回对象的时候，应该优先返回一个临时对象，而不要返回一个定义过的对象
//不能返回局部的或者临时对象的指针或引用 Test GetObject(Test &t)//1 { int val = t.getData(); /*Test tmp(val); return tmp;*/ //返回临时对象， 2 //用临时对象拷贝构造一个新对象时，临时对象不会构造，而是直接构造新对象 return Test(val); } int main() { Test t1; Test t2; t2 = GetObject(t1); return 0; }
程序输出：
00BAF830 constructor 00BAF824 constructor 00BAF758 constructor 00BAF824 operator constructor 00BAF758 destructor 00BAF824 destructor 00BAF830 destructor
如果是这样调用呢：
int main() { Test t1; Test t2 = GetObject(t1);//3 //t2 = GetObject(t1); return 0; }
程序输出：
0137FD4C constructor 0137FD40 constructor 0137FD40 destructor 0137FD4C destructor
可以看到函数返回值的对象也没有生成而是直接初始化了t2，因此第三条：
3.接收返回值是对象的函数调用的时候，优先按初始化的方式接收，不要按赋值的方式接收
