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 const
const int* foo; int *const bar; //note, you actually need to set the pointer //here because you can't change it later ;)
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
For those who don't know about Clockwise/Spiral Rule: Start from the name of the variable, move clockwisely (in this case, move backward) to the next pointer or type. Repeat until expression ends.
Here is a demo: