Why are both "const T" and "T const" valid, and which one should you use?

ghz 1years ago ⋅ 4003 views

Question

To start you probably know that const can be used to make either an object's data or a pointer not modifiable or both.

const Object* obj; // can't change data
Object* const obj; // can't change pointer
const Object* const obj; // can't change data or pointer

However you can also use the syntax:

Object const *obj; // same as const Object* obj;

The only thing that seems to matter is which side of the asterisk you put the const keyword. Personally I prefer to put const on the left of the type to specify it's data is not modifiable as I find it reads better in my left-to- right mindset but which syntax came first?

More importantly why is there two correct ways of specifying const data and in what situation would you prefer or need one over the other if any?

Edit

So it sounds like this was an arbitrary decision when the standard for how compilers should interpret things was drafted long before I was born. Since const is applied to what is to the left of the keyword (by default?) I guess they figured there was no harm in adding "shortcuts" to apply keywords and type qualifiers in other ways at least until such a time as the declaration changes by parsing a * or & ...

This was the case in C as well then I'm assuming?

Answer

why is there two correct ways of specifying const data and in what situation would you prefer or need one over the other if any?

Essentially, the reason that the position of const within specifiers prior to an asterisk does not matter is that the C grammar was defined that way by Kernighan and Ritchie.

The reason they defined the grammar in this way was likely that their C compiler parsed input from left-to-right and finished processing each token as it consumed that. Consuming the * token changes the state of the current declaration to a pointer type. Encountering const after * means the const qualifier is applied to a pointer declaration; encountering it prior to the * means the qualifier is applied to the data pointed to.

Because the semantic meaning does not change if the const qualifier appears before or after the type specifiers, it is accepted either way.

A similar sort of case arises when declaring function pointers, where:

  • void * function1(void) declares a function which returns void *,

  • void (* function2)(void) declares a function pointer to a function which returns void.

Again the thing to notice is that the language syntax supports a left-to-right parser.

#c