I always mess up how to use
const int * const, and
int const * correctly. Is there a set of rules defining what you can and cannot do?
I want to know all the do's and all don'ts in terms of assignments, passing to the functions, etc.
Read it backwards (as driven by Clockwise/Spiral Rule):
int*- pointer to int
int const *- pointer to const int
int * const- const pointer to int
int const * const- const pointer to const int
Now the first
const can be on either side of the type so:
const int *==
int const *
const int * const==
int const * const
If you want to go really crazy you can do things like this:
int **- pointer to pointer to int
int ** const- a const pointer to a pointer to an int
int * const *- a pointer to a const pointer to an int
int const **- a pointer to a pointer to a const int
int * const * const- a const pointer to a const pointer to an int
And to make sure we are clear on the meaning of
int a = 5, b = 10, c = 15; const int* foo; // pointer to constant int. foo = &a; // assignment to where foo points to. /* dummy statement*/ *foo = 6; // the value of a cant get changed through the pointer. foo = &b; // the pointer foo can be changed. int *const bar = &c; // constant pointer to int // note, you actually need to set the pointer // here because you can't change it later ;) *bar = 16; // the value of c can be changed through the pointer. /* dummy statement*/ bar = &a; // not possible because bar is a constant pointer.
foo is a variable pointer to a constant integer. This lets you change what you point to but not the value that you point to. Most often this is seen with C-style strings where you have a pointer to a
const char. You may change which string you point to but you can't change the content of these strings. This is important when the string itself is in the data segment of a program and shouldn't be changed.
bar is a constant or fixed pointer to a value that can be changed. This is like a reference without the extra syntactic sugar. Because of this fact, usually you would use a reference where you would use a
T* const pointer unless you need to allow
const int x = 0; const int *const px = &x; const int *const *const p = &px;works just fine. — Aug 08, 2016 at 23:15
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