C++ Const Double Pointer – Explanation and Usage

c++const-pointerconstants

I want to make a constant double pointer points to a constant pointer points to a constant double. I started to make it (of course I make a little search at books and I googled it) from scratch and think what the following three make:

const double* cp; //pointer to a constant double
double *const cp; //constant pointer
const double *const cp; //constant pointer to a constant double

I thought the next step is to write a constant double pointer

double **const cp;// double constant pointer

then I combine the last two statements and I write

const double *const cp = arr[0];
double **const cp1 = arr ;

where arr is a dynamically allocated double dimension array.
After that I tried to verify what I have done and I wrote the below statements expecting to produce error all of them.

**cp1 = 1;    // didn't produce error  
*cp1 = arr[4];    // didn't produce error
cp1 = new double*[5]; //produce error

So the thing is that I couldn't make what I described above , a constant double pointer points to a constant pointer points to a constant double. How can I make it ?

Thanks in advance.

Best Answer

There's only one const in

double **const cp1 = arr ;
//       ^^^^^

so I'm not sure why you're expecting the other two assignments to produce an error.

If you want it to be const on all levels, you need

const double *const *const cp1 = arr;
//                         ^ cp1 is ...
//                  ^ a const pointer to ...
//           ^ a const pointer to ...
// ^ a const double

Related Solutions

C++ Const Pointer – How to Declare Const Pointer in C++

It means "myPtr is a const pointer to a const myClass". It means that you can neither modify what the pointer is pointing at through this pointer nor can you make the pointer point somewhere else after it's initialised (by the return value of myClass->getPointer()). So yes, you're basically right, with the addition that it also points to a const object (as far as you know; it could really be non-const underneath).

Remember that const applies to the item to its left (or if there is no item to its left, the item to its right). The first const makes the myClass const (where you can't modify what the pointer points at) and the second const makes the * const (where you can't modify the pointer itself).

C Programming – What Does 2 Const Mean

First const tells you can not change *key, key[i] etc

Following lines are invalid

*key = 'a';
*(key + 2) = 'b';
key[i] = 'c';

Second const tells that you can not change key

Following lines are invalid

key = newkey;
++key;

Also check how to read this complex declaration

Adding more details.

const char *key: you can change key but can not change the chars pointed by key.
char *const key: You can not change key but can the chars pointed by key
const char *const key: You can not change the key as well as the pointer chars.

Best Answer

Related Solutions

C++ Const Pointer – How to Declare Const Pointer in C++

C Programming – What Does 2 Const Mean

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