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const question

Posted on 1998-12-08
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Last Modified: 2010-04-02
I have a class that has a method:
class map
{
friend ostream& operator<<(ostream&, const map&);
public:
int want_map()const;  // what is the purpose of this const
private:

};
the method is declared in the class
defined outside the the class:
int loan::want_bigmap()const
{                    

}
in this declaration I need to use an overloaded inseration
operator to display a map.
the friend function above should work right??
how?

thanks for any help.

 
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Question by:strmtrpr
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Answers2000 earned 20 total points
ID: 1179634
1. The const means that want_map won't change the state of any members variables of the class.  Actually this is not strictly true, const members can change members marked mutable (VC5 or later req'd to use mutable - but this is part of the standard)

e.g.
class X
{
private:
int a ;
mutable int b ;
public:
void some_func() const ;
} ;

This would mean that some_func could change b, but not a.

The reason for mutable is you might have some totally internal mechanism to the class which does at all effect external behaviour, which needs scratch variables (typical examples are caches, reference counting etc)


2. The friend looks okay to me.  Because the const in the friend declaration, the overloaded << can only call const members (like example 1) of the class.
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by:strmtrpr
ID: 1179635
thanks for the help
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