【玩转腾讯云】C++学习笔记-函数const，new，delete

class Zyf
{
public:
    Zyf(string _name, int _age): name(_name), age(_age){}
    string getName() const {return name;}
    string getName() {name += " + not const"; return name;}
    int getAge() const {return  age;}
private:
    string name;
    int age;

};

//在main函数里这样调用
    Zyf z("hanhan1", 12);
    cout << z.getName().c_str() << endl;
    const Zyf z1("han", 12);
    cout << z1.getName().c_str() << endl;
//对象z1，无可厚非是输出有函数const的那一个函数，但是对象z呢？它可以两个函数都调用吗？
//其实在C++语言有个规则,前面无const会优先调用无函数const,而不是调用两个

//比如Zyf *z2 = new Zyf("ss", 12)这一条语句可以拆解为：
Zyf *z2;
void* mem = operator new(sizeof(Zyf));
z2 = static_cast<Zyf*>(mem);
z2->Zyf::Zyf("ss", 12);
//可以看出new语句整个操作是先分配空间，后调用构造函数

//delete z2可以拆解为
Zyf::~Zyf(z2);
operator delete(z2);
//可以看出delete是先调用析构函数，后释放空间

//至于new Zyf[],delete[] ...则是在operator new后面加一个[]，多次分配空间；比如
new Zyf[5]//那么就是分配五次Zyf大小的空间, new [] 一定要搭配delete[]这样才可以把空间释放完全。

void* operator new(size_t size){
    cout << "need to new the memory:" << size << endl;
    return malloc(size);
}

void operator delete(void* ptr){
    cout << "need to delete the memory:"<< sizeof (ptr) << endl;
    free(ptr);

}

void* operator new[](size_t size){
    cout << "need to new the memory:" << size << endl;
    return malloc(size);
}

void operator delete[](void* ptr){
    cout << "need to delete the memory:"<< sizeof (ptr) << endl;
    free(ptr);

}

//main里调用
    Zyf * z2 = new Zyf(12, 11);
    delete z2;
    Zyf * z3 = new Zyf[4];
    delete [] z3;
    Zyf * z4 = new Zyf[4];
    delete z4;
    
//输出
need to new the memory:8
Zyf()
~Zyf()
need to delete the memory:8
need to new the memory:40
Zyf()
Zyf()
Zyf()
Zyf()
~Zyf()
~Zyf()
~Zyf()
~Zyf()
need to delete the memory:8
need to new the memory:40
Zyf()
Zyf()
Zyf()
Zyf()
~Zyf()
need to delete the memory:8
//可以看到如果使用new[], 而不使用delete[]，那么内存就会浪费。
//至于为什么new40字节：两个int 八字节，四个就是32,而在内存·管理中会在第一行告诉系统有4个，这里占据4字节，
//在末尾会有结束符，又是四个，所以一共八个。

//比如，你在new后面加一个long类型,语句为
    Zyf * z2 = new(4) Zyf(12, 11);
    delete z2;
//当然在重载的地方也要加一个参数，size_t是默认参数
void* operator new(size_t size, long dss){
    cout << "need to new the memory:" << size << endl;
    cout << dss << endl;
    return malloc(size);
}
//带有参数的operator delete就不一样，它必须是跟它一样参数的new对应的构造函数抛出异常，
//带有对应参数的delete才会执行(默认参数除外)
//如果    Zyf * z2 = new(4) Zyf(12, 11);抛出异常，那么void operator delete(void*, long)就会执行