Pointing to a const pointer (or a pointer to a const)

Question

I have this code: ``` #include int main(){ int v1 = 20; int *p1 = &v1; int **p2 = &p1; return 0; } ``` What I want to do here is pointing a pointer to another pointer, and it works fine in this case. I make p1 to a `const` pointer OR a pointer to a `const`: ``` int *const p1 = &v1; OR const int *p1 = &v1; ``` Now my code does not work. Considering the first case. `int *const p1 = &v1;`. Here is my updated code, which I think should be right: ``` #include int main(){ int v1 = 20; int *const p1 = &v1; const int **p2 = &p1; return 0; } ``` Making the pointer itself `const` means that it cannot point to another object, but the value of that object which it was pointing can be changed. So if I want to point another pointer to the previous pointer `p1`, I need to make sure that this second pointer cannot be used to change the value of `p1` So I make it (`p2`) a pointer to a `const`. But it does not work. Error: : invalid conversion from 'int* const*' to 'const int**' […

Accepted Answer

Your first rewrite did not work, because you tried to assign an address of a constant pointer to a pointer-to-pointer where the final "pointee" (i.e. the `int`) is constant, rather than the pointer in the middle. In other words, if C allowed this

```
int *const p1 = &v1;
const int **p2 = &p1; // Not allowed
```

it should allow this:

```
*p2 = &v2; // This would be legal.
```

However, this would have modified the pointer `p1`, which is allegedly constant.

Here is what you need to change for the first rewrite to work:

```
int *const p1 = &v1;
int * const *p2 = &p1;
```

Note how `const` is moved from before the declaration to between the asterisks. Now you are making a pointer to a constant pointer to int.

Your second rewrite fails because it drops `const`-ness altogether. To fix it, put `const` back at the beginning of the declaration:

```
const int *p1 = &v1;
const int **p2 = &p1;
```

Pointing to a const pointer (or a pointer to a const)

Solution

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