内存安全程序设计

Background

Pointer

A pointer is a varaible that contains the memory address of a portion of memory, that is , pointer holds address

• Dereferencing operator: *p refers to the object pointed to by the pointer.
• Address operator: get the address of an object.
• The size of pointer is fixed, it depends on your computer. (64位的机器的指针有8个字节)

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Dynamic and local pointer

注意用new申请的内存要用delete删除，用new申请的内存在heap上；

int *p1,*p2, n;
//p1 p2 n are variables in the stack
p1= new int;
// allocate memory in the heap
*p1= 10;
p2= p1;
//p1 and p2 points to the same memory

Memory-safety Problems

1. Use after free

   指针释放之后继续使用，导致异常

2. Buffer overflow

   访问非法的空间

3. Doule free

   重复释放指针

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记得： 在delete之后将指针赋值为null，在new一个新的pointer之前确定原先的指针指向的内容已经被删除或者还有其他指针占用。new后一定要delete！

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Memory-safety Programming

Tempora Memory Problems

时间类的内存问题，use after free， memory leak, double free

Danglnig Pointer比如：

int main(){
    int* ptr;   
    {
        //create an inner block;
        int a=1;
        ptr=&a;
    }
    cout<<*ptr; // Dangling pointer;
}

In this case, a is a temporary varible in the block;

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Then, how to exscape such problems ???

什么是RAII

如果文件打开但是忘记close可能会带来一些问题，我们可以用过RAII的思想，来封装一个类来解决这个问题。

Resource Acquisition Is Initialisation (Constructor Aquires , Destructor Releases)

class FileObj{
    public:
    File* ptr;
    Fileobj(char* name){
        //打开文件
    }
	~Fileobj(){
        //close the file when deconstructings
    }
}

void printFile(const char* name){
    Fileobj file(name);
    //这个是local地创建，创建在stack上，而非在heap上创建，确保会自动回收；
    
}

int main(){
    FileObj file1("file1");
    //local地创建file而非用new创建，保证会自动地调用deconstructor
}

基于RAII的思想（Resources accusition is initialized )我们可以设计smart pointer；

smartPointer核心背后的逻辑;