取消引用此指针有什么问题？

Question

While debugging i found that my program stops at: "cout << *ptr ; cout << "\\n";" what is wrong with this code?

#include<iostream>

using namespace std;
int *ptr = 0;
void myfun(void);

int main()
{
    void myfun();
    for(int j = 1; j < 3; j++)
    {
        ptr = ptr-j ;       
        cout << *ptr ; cout << "\n";

    }
    return(0);  
}

void myfun(void)
{
    int x[3] = {11,12,13};
    for(int i = 0; i <3; i++)
    {
        ptr = &x[i];
        ptr = ptr+1;
    }
}

Answer 1

You initialized your pointer with zero

int *ptr = 0;

which means that it is a null pointer.

Then you are applying pointer arithmetic to a null pointer. The behavior is undefined. The you are dereferencing the nonsensical pointer obtained in that way. The behavior is undefined.

Note that

void myfun();

in main is not a function call, it is a function declaration. A call to myfun (which was apparently your intent) would look as follows

myfun();

Why did you put that void there in your version of the code?

Answer 2

Your variable ptr is a wild pointer.

Although you let the ptr porint to array x, when the myfun() finish and return the memory of x had been destory. So in the main function, ptr become wild pointer and your program will crash.

You should know that the local variable will be destoried when they are not in their scope.

Answer 3

main function doesn't have access to the data, as scope of the array x[3] is myfun(void). After myfun call (it should be myfun(); instead of void myfun(); ), ptr will be pointing to an unallocated memory location.

To print the data in main function, you can declare the int x[3] as a global variable.

int *ptr = 0;
void myfun(void);

int x[3] = {11,12,13};

int main()
{
    int *temp_ptr;
    myfun();
    temp_ptr = ptr;
    for(int j = 1; j <= 3; j++)
    {
        temp_ptr = ptr-j ;       
        cout << *temp_ptr ; cout << "\n";

    }
    return(0);  
}

Answer 4

After following the above answers and a little bit of googling I found out the exact solution to my problem: To return variable sized array from a function.

This is the code:

#include<iostream>

using namespace std;
int * myfun(void);

int main()
{
    int *ptr;
    ptr = myfun();
    int len = (sizeof(ptr)/sizeof(*ptr));
    for(int j = 0; j <= len; j++)
    {
        cout << ptr[j];
    }
    return(0);  
}
int * myfun()
{
    static int x[3] = {11,12,13};
    return x;
}