Wild pointer, null pointer, invalid pointer

1. Wild pointer and null pointer

Put these two together , Because there is really no important knowledge point , Let's have a look first .

int main()
{
    int *p;
    int *p1=nullptr;
    return 0;
}

The pointer p For the wild pointer , That is, when defining a pointer, it is not initialized to nullptr, Then this pointer will point to a random address , This is the wild pointer , Wild pointers are very dangerous .

p1 Initialize to null when we define ,p1 Is a null pointer .

Notice that I'm using nullptr, No NULL,nullptr stay C++ Is a keyword used to initialize or assign a null pointer , Why not NULL Well ？ Let's talk about it briefly , Later in this column, there will be a special article to introduce C++ in nullptr.

stay C In the language compiler ,NULL For the definition of #define NULL 0 perhaps #define NULL  (void*)0;NULL Is a literal constant defined by a macro 0 Or a typeless pointer 0 Constant , Then this situation sometimes leads to ambiguity , There is no example here , Later articles can be read carefully , To solve this situation , stay C11 Medium standard medium , In consideration of compatibility , Literal constants 0 The ambiguity of has not been eliminated , But the standard still gives a new answer to ambiguity , Namely nullptr（ A so-called “ Pointer null type ” The constant ）.

2. Invalid pointer

The failure pointer is a key point of this chapter , So what is an invalid pointer ？ Let's give an example of ：

 int main()
{
    int *r=(int *)malloc(sizeof(int));
    *r=100;
    free(r);