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c++variablesconstants
Naruto
Naruto

Reputation: 9634

What is the differnce between "const X a" and "X const a" if X is the class

i have a class name X, what is the difference between "const X a" and "X const a"

Upvotes: 4

Views: 1547

Answers (3)

Paul E.
Paul E.

Reputation: 1909

If you use simple types (embedded or custom) then that is a matter of taste.

In case of using pointers there is a simple general rule: if const is placed before '*' then the data pointed is constant and otherwise the pointer itself is constant, you can't change its value.

For example:

const int  a=1;  // 'a' value can't be changed
const int* q;    // the data that 'a' point to is constant
int const* q;    // the same
int* const p=&a; // the pointer is constant: const is behind '*'

so

int b=2;
p = &b; // error: trying to change constant pointer

Upvotes: 2

Jerry Coffin
Jerry Coffin

Reputation: 490178

In this case, there's no difference at all.

When you have a pointer or a reference, a change that might look almost the same is significant though. Given something like:

T * a;

The position of const (or volatile) relative to the asterisk is significant:

T const * a;
T * const a;

The first one says that a is a pointer to a const T (i.e., you can't modify the T object that a refers to). The second one says that a is a const point to a (non-const) T -- i.e., you can modify what a points at, but you can't modify the pointer itself, so you can't point it at a different object. Of course, you can also do both:

T const * const a;

This means you can't change the pointer itself or the T object it refers to.

Upvotes: 6

James McNellis
James McNellis

Reputation: 355079

Nothing.

A const qualifier applies to whatever is immediately to its left. If there is nothing to its left then it applies to whatever is immediately to its right.

Upvotes: 13

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