临时对象的生成

强制类型的转换

class CRectangle {
public:
    CRectangle(CRectangle &to) {
        cout << "***" << endl;
        w = to.w; h = to.h;
        printTotal();
    }
    CRectangle(int i = 1, int j = 2);
    ~CRectangle();
    static void printTotal();
    void print() { cout << w << "--" << h << endl; }
protected:
private:
    int w, h;
    static int nTotalArea;
    static int nTotalNumber;
};
CRectangle::CRectangle(int i, int j) {
    cout << "---" << endl;
    w = i; h = j;
    nTotalNumber++;
    nTotalArea += w*h; printTotal();
}
CRectangle::~CRectangle() {
    nTotalNumber--;
    nTotalArea = nTotalArea - w*h;
}

int CRectangle::nTotalArea = 0;
int CRectangle::nTotalNumber = 0;
void CRectangle::printTotal() {
    cout << nTotalNumber << "," << nTotalArea << endl;
}

int main() {
    CRectangle c1(3, 3), c3;
    CRectangle::printTotal();
    c3 = 40;
    c3.print();
    c1.printTotal();
    return 0;
}

c3 = 40; 这边其实会生成临时对象，调用普通的构造函数，不是拷贝构造函数，只是临时对象很快消亡，我是在构造函数里面“动了手脚”才最终看得到生成新对象的。

int main() {
    CRectangle c1(3, 3), c3=40;
    CRectangle::printTotal();
    c3.print();
    c1.printTotal();
    return 0;
}

此时不会调用构造函数

参数是对象，而不是引用！！！

class CRectangle {
public:
    CRectangle(CRectangle &to);
    CRectangle(int i = 1, int j = 2);
    ~CRectangle();
    static void printTotal();
    void change(CRectangle obj) {
        obj.w *= 2; printTotal();
    }
    void print() { cout << w << "--" << h << endl; }
protected:
private:
    int w, h;
    static int nTotalArea;
    static int nTotalNumber;
};
CRectangle::CRectangle(CRectangle &to){
    cout << "拷贝构造函数" << endl;
    w = to.w; h = to.h;
    printTotal();
}
CRectangle::CRectangle(int i, int j) {
    cout << "构造函数" << endl;
    w = i; h = j;
    nTotalNumber++;
    nTotalArea += w*h; printTotal();
}
CRectangle::~CRectangle() {
    printTotal();
    cout << "析构函数" << endl;
    nTotalNumber--;
    nTotalArea = nTotalArea - w*h;
    printTotal();
}

int CRectangle::nTotalArea = 0;
int CRectangle::nTotalNumber = 0;
void CRectangle::printTotal() {
    cout << nTotalNumber << "," << nTotalArea << endl;
}

int main() {
    CRectangle c1(3, 3);
    c1.change(c1);
    return 0;
}

注意此时不是引用啊，此时，生成临时对象，然后拷贝构造函数里面没有及时加上1，在最后的析构函数里面就减去1。

---

如果改变：

void change(CRectangle &obj) {
        obj.w *= 2; printTotal();
    }

又或者改变拷贝构造函数

CRectangle::CRectangle(CRectangle &to){
    cout << "拷贝构造函数" << endl;
    w = to.w; h = to.h; 
    nTotalNumber++;
    nTotalArea += w*h;
    printTotal();
}

注意，生成的是obj的临时对象，change函数里面还自乘2，所以一下子减去18了。

返回一个对象

class CRectangle {
public:
    CRectangle(CRectangle &to);
    CRectangle(int i = 1, int j = 2);
    ~CRectangle();
    static void printTotal();
    void print() { cout << w << "--" << h << endl; }
protected:
public:
    int w, h;
private:

    static int nTotalArea;
    static int nTotalNumber;
};
CRectangle::CRectangle(CRectangle &to){
    cout << "拷贝构造函数" << endl;
    w = to.w; h = to.h; 
    nTotalNumber++;
    nTotalArea += w*h;
    printTotal();
}
CRectangle::CRectangle(int i, int j) {
    cout << "构造函数" << endl;
    w = i; h = j;
    nTotalNumber++;
    nTotalArea += w*h; printTotal();
}
CRectangle::~CRectangle() {
    printTotal();
    cout << "析构函数" << endl;
    nTotalNumber--;
    nTotalArea = nTotalArea - w*h;
    printTotal();
}

int CRectangle::nTotalArea = 0;
int CRectangle::nTotalNumber = 0;
void CRectangle::printTotal() {
    cout << nTotalNumber << "," << nTotalArea << endl;
}

CRectangle experiment(CRectangle &obj) {
    return obj;
}

int main() {
    CRectangle c1(3, 3),c2;
    c2 = experiment(c1);
    c1.w = 5;
    return 0;
}

这时候，在前面基础上都能够清除了，还有，析构函数总是就近原则，后生成的先析构调。

总结下调用拷贝构造函数的时候

1. 参数是对象
2. 返回一个对象
3. 强制转换

似乎调用拷贝构造函数和临时对象生成是不分开的。