What is the difference between const int*, const int * const, and int const *?
I always mess up how to use const int* , 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 NULL pointers.
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:
I think everything is answered here already, but I just want to add that you should beware of typedef s! They're NOT just text replacements.
For example:
typedef char *ASTRING;
const ASTRING astring;
The type of astring is char * const , not const char * . This is one reason I always tend to put const to the right of the type, and never at the start.
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