cpp class4 ( test blog )

Class4

指针不安全性

• fly pointer(null/right value; in func first use if(null); )

• memory leak

• return adr of local var(warning)

• multi-pointers for one object!!!!!!!!!!

int main() {
    int *p; //fly pointer 野指针
    int i=10;
    int j=20;
    p = &i;
    p = &j;
    //可能和预想不同
    //reference 引用(cpp new pointer)
    int& r = i;     //引用必须初始化，否则编译报错; r是i的引用(forever)
    r = j;          //不能表明引用改变，只能表示j赋值给r引用的对象
    return 0;
}

value or address(pointer | reference)

read read/write

sizeof(object) sizeof(int)

copy-constructor nothing

reference:

void fun(int a) {
    a++;
}
​
int main() {
    int m = 10;
    fun(m);
    cout << m <<endl;   //output still is 10
    return 0;
}
// use reference
void fun(int& a) {
    a++;
}
​
int main() {
    int m = 10;
    fun(m);
    cout << m <<endl;   //output is 11
    return 0;
}

copy:

class Test {
    int i;  //value
    int *j; //address handle
public:
    //....
    //copy construct
    Test(Test& t);
/*
写一个private拷贝函数声明，卡编译
private:
    Test(Test& t);
*/
}
​
//copy construct
Test::Test(Test& t) {
    //default(bitwise copy)
    this->i = t.i;
    //this->j = t.j;
    
    //deep copy addr(logic copy)
    this->j = (int*) malloc(sizeof(int));
}
​
void func(int& a) {
    
}
​
int main() {
    Test t1(1,2);   //构造t1
    Test t2(t1);    //语法看起来不ok，但默认支持copy
                    //address变量 会指向同一个空间
    //pass value, default use copy construct
    //care about if defaut copy ?
}
​

一些琐碎：

//new & delete
​
int main() {
    //no-use. without constructor
    Test* a = (Test*)malloc(sizeof(Test));
    
    //work
    //(new = malloc(void*) + constructor(this))
    //'new' is an operator(like +-*/)
    Test *p = new Test();
    //要同时构造多个，类必须支持默认构造（无参数）
    Test *q = new Test[10];
    
    //delete = destrucot + free
    delete p;
    delete []q;
    
    return 0;
}

//const
class Test{
    //...
    void func() const;
    //...
}
//...
void Test::func() const{
    //...(read-only)
}
​
int func2();    //default const
//const pass by address
//约定只要不write 就 const
void fun(const Test *p) {
    //p's value is read-only
}
​
int main() {
    const Test t;
    test.func();    //error,const object cannot use func
    //unless: void func() const;
    const double pi = 3.14;
    
    int a = func2();    //work
    func2()++;  //error
}

 

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