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i've continues test come around multi-level pointers, and got trouble too:
my question is why and where i need use const in allocation a pointer to constant, and why i don't need const in allocation of a constant pointer?Code:int main() {
const int*** x;
x = new const int**;
*x = new const int*;
**x = new int;
// const pointer
int*** const y = new int**;
*y = new int*;
**y = new int;
return 0;
}
thanks for reply.
The difference is whether the pointer is constant, or the thing being pointed to is constant.
In the first case, the following statement would be illegal:Code:const int *x; // x is a pointer to a CONSTANT integer
int * const x; // x is a CONSTANT pointer to an integer
In the second case, the following statement would be illegal:Code:*x = 1;
Code:x = &y;
No, i mean why *x = new const int*; has 'const' if not error occur , **x = new int; hasn't 'const' if has error occur.
sorry for my description.
For the reason I just stated.Quote:
Originally Posted by PuKa
const int*** x;
x = new const int**; // if i use this statement x = new int**; // error: invalid conversion from 'int***' to 'const int***' , ok i understand this.
*x = new const int*; // the same as previous line.
**x = new int; // this is problem. if i use this statement **x = new const int; error occur, why two lines above must be have 'const' while this line must be not have 'const'
I use g++ ver 4.3 to compile.
Mod, please change my topic'name by "Keyword 'const' in allocate memory a multi-level pointers" thanks a lot.
>>**x = new int; // this is problem. if i use this statement **x = new const int; error occur, why two lines above must be have 'const' while this line must be not have 'const'
Separate compile errors from runtime errors.
The line must have const because you have a pointer to const int.
But as x is uninitialized in your code, dereferencing it is a no-no.
1. i still don't understand, as you said and error compile i didn't initialize a value to ensure has got a memory area is allocated and initialized, but how can i do two jobs in the same time?
2. what about these statement
int*** const y = new int**;
*y = new int*;
**y = new int;
why i don't need const in act allocation, if need then where i put 'const' in these statement.
thanks!
1) I don't understand what this question is trying to ask.
2) Go back to brewbuck's example. It explains well the two type of "const" available.
*x = new const int*; // no error
*x = new int*; // error
**x = new int; // no error
**x = new const int; // error
i've just wanted to ask why i declares *x = new const int*; is OK while **x = new const int; is occur error. hic.
Because new takes a non-const type, a type that does not match "const T". const int matches that form. const int * does not, the pointer is not const, it just points to const data.Quote:
Originally Posted by PuKa
yes, it's basically, but... i did not understand it. thanks much.
What King Mir is trying to say is that new takes a modifiable type. T is a type that you can modify. const T is not a type you can modify.
Hence, x = new int is okay, while x = new const int is not.
Now, separate the constness for pointers. There is const int* which is a type that points to an int. You can still change the variable itself, but you can't change what it points to.
Example:
int a, b;
const int * x = &a;
x = &b; // OK
*x = 10; // Not OK; x points to const int, hence *x is a const int.
There is also the type int* const which is is a constant type that points to int. It means you can't change the variable itself, but you can change what it points to.
Example:
int a, b;
const * const x = &a;
x = &b; // Not OK; x is const.
*x = 10; // OK
I think *x = 10 is illegal too, x is a const pointer points to const, so can't modify value that x points to through *x.Quote:
Originally Posted by Elysia
Ah, no, that was a typo.
It should say
int * const x = &a;
there.