pointer to an int array gives the same address as when it is dereferenced

问题

I have the following code:

#include <iostream>
using namespace std;
int main()
{
    int g[] = {9,8};
    int (*j)[2] = &g;
    cout << "*(j):" << *(j) << endl;
    cout << "j:" << j << endl;
    cout << "&j:" << &j << endl;
    cout << "&(*j)" << &(*j) << endl;
    cout << "*(*j):" << *(*j) << endl;
    return 0;
}

which ouputs:

*(j):0x7fff5ab37c7c
j:0x7fff5ab37c7c
&j:0x7fff5ab37c70
&(*j)0x7fff5ab37c7c
*(*j):9

I think that j is a pointer to an array of two integer.

And &g is the address of the whole array.

Then j store the address of the whole array.

And so I use the *(j), it will dereference the first element in the array.

But the result said that *(j) store the array address the same value as j.

I cannot figure out how this happened.

回答1:

"I think that j is a pointer to an array"

Yes, it is. And that's also the reason why *j output the same address as outputting g would do. In this case an array decays into the pointer and that's why even outputting j yields the same result.

The fact that the same address is outputted might make you think that j and *j are the same pointers, however they are not. Their type is different (a fact that actually matters):

int g[] = {9,8};     // int[]
int (*j)[2] = &g;    // int (*)[2]

so using *j becomes equivalent to using g directly, just like *(*j) becomes equivalent to *g.
And &(*j) is nothing but j, which is initialized with ~~an address of an array~~ (an address taken from decayed g, i.e. an address of the first element of this array).

So why j and *j outputs the same address and *(*j) outputs the value of first element?

Because of the type of j being int (*)[2]. A simple example:

int g[] = {9,8};
int (*j)[2] = &g;     // j points to the first element as well
cout << *((int*) j);

outputs 9.

回答2:

I think that j is a pointer to an array of two integer.
And &g is the address of the whole array.

That is correct.

And so I use the *(j), it will dereference the first element in the array.

This is not. *j gives you the array itself. When you insert it to cout, it decays to a pointer again (this time to a pointer to its first element, type int*) and its value is printed.

It's in effect the same as if you wrote cout << g.

来源：https://stackoverflow.com/questions/19620218/pointer-to-an-int-array-gives-the-same-address-as-when-it-is-dereferenced

标签

c++

arrays

pointers

memory-address

dereference