a function follow by const

[moongeegee]

what does it mean/purpose ?

class xxx
{

}

 

[moongeegee]

press the send by mistake.

What does the "const" mean/purpose ?? Thanks.

class xxx
{
double getNum() const;
}

 

[moongeegee]

Sent by mistake.

What does the "const" mean/purpose ?? Thanks

class xxx
{
double getNum() const;
}

 

[Vincent Jacques]

moongeegee a écrit :

Sent by mistake.

What does the "const" mean/purpose ?? Thanks

class xxx
{
double getNum() const;
}

it means that, is you have a const instance of the class xxx, you will
be able to call getNum on it:

const xxx theXXX;
double d = theXXX.getNum();

When you declare a member function as const, the compiler check that the
function does not modify the object it is called on.

You can see those const functions as 'observers' of you class, or
'getters' (hence the name 'getYYY')

Cheers,

 

[Stefan Ram]

You seem to imply that for a non-const instance of 'xxx'
'getNum' cannot be called.

A -> B does not imply ¬A -> ¬B.

That's not true.

This comment applies to your reading,
not to Vincents text.

 

[James Kanze]

moongeegee a écrit :

it means that, is you have a const instance of the class xxx,
you will be able to call getNum on it:

const xxx theXXX;
double d = theXXX.getNum();

When you declare a member function as const, the compiler
check that the function does not modify the object it is
called on.

Formally, the const has two effects: you cannot call a non-const
function on an object designated by an expression with const
type (e.g. a const object, or a reference to const), and the
type of this in the function will be ClassName const*, rather
than simply ClassName* (i.e. you cannot modify the object in a
const function, with a few special exceptions).

Formally, too, the compiler applies what has usually been called
"bit-wise const": if your class is something like:

class C
{
char* p ;
public:
void f() const ;
} ;

C::f() can modify what p points to. Generally accepted
practice, however, is to implement logical const: if what p
points to is part of the "value" of your object, you don't
modify it in a const function, even if the compiler allows it.
(Similarly, if some direct member of your object is not part of
its conceptual value, you may modify it in a const function,
either by declaring it mutable or casting away const. This case
occurs far less frequently, however.)