Shriramana Sharma wrote:
> > By declaring a const return type you are promising that the original
> > variable (inside the function's stack frame) will not be modified.
>
> But then this function would have to return the exact same value for each and
> every time it is called, no?
>
> > Marks a function as const allowing it to be called by const objects
> > (btw, const objects can only call const member functions). This
> > construct is usually found as part of member functions declarations,
> > which are known to be immutable.
>
> OK so int f() const means function f() does not change any members of the
> class foo of which it is a member, and therefore it is safe for constant
> instances of the class foo to call this function f().
>
> OTOH, just putting const *before* the function name only means that the
> *output* of the function is a constant value, say for example the function
Putting "const" before the *return type* means that the return value
is a constant.
In that situation, "const" is just a normal type qualifier, with the
same meaning as when used in variable and parameter declarations.
> const int one() { return 1; }
>
> and it does not assure the compiler that it is okay for a *non-const* instance
> of class foo to call this function.
You can use *any* method on a mutable (non-const) instance.
The above indicates that one() returns a "const int", that's all.
> In this case, the compiler will *not* accept calls to the function one() as a
> member of a *non-const* instance of foo, even though the function does not
> change any member of foo, right?
Wrong. It will not allow the method to be used on *const* instances,
as the method itself isn't declared as "const".
If a method doesn't modify the instance, you *should* declare it as
"const". There is no reason not to; const methods can be used on both
const and mutable instances, while methods which aren't declared const
can only be used on mutable instances.
--
Glynn Clements <glynn@xxxxxxxxxxxxxxxxxx>
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