CODEWITHSUNDEEP
c++
small_potato
small_potato

Reputation: 3207

pass by const reference of class

void foo(const ClassName &name)
{
    ...
}

How can I access the method of class instance name?

name.method() didn't work. then I tried:

void foo(const ClassName &name)
{
    ClassName temp = name;
    ... ....
}

I can use temp.method, but after foo was executed, the original name screwed up, any idea? BTW, the member variable of name didn't screwed up, but it was the member variable of subclass of class screwed up.

Upvotes: 4

Views: 6811

Answers (2)

Péter Török
Péter Török

Reputation: 116306

If I understand you correctly, you want to call name.method() inside foo() and the compiler doesn't let you. Is ClassName::method() a non-const method by any chance? Since name is declared as a const parameter to foo(), you can only call const functions on it.

Update: if ClassName::method() is non-const, but does not actually change state, the best would be of course to make it const. In case you can't for some reason, I see the following ways:

  • (the obvious way, as @Naveen pointed out - thanks :-) declare name as a non-const method parameter.
  • make a non const copy of name, and call method on it - as you actually did. However, this works only if the assignment operator and/or copy constructor is properly implemented for ClassName. However, you write "after foo was executed, the original name screwed up", which is a very vague description, but it can be interpreted so that copying did have some unwanted side effects on name, which suggests that those functions are not implemented correctly or at all.
  • (the nasty way) cast away constness with const_cast - this should really be a last resort, and only if you are sure that ClassName::method() does not actually change any state.

Update2, to @AKN's comment - example of casting away constness:

void foo(const ClassName &name)
{
    ClassName& temp = const_cast<ClassName&>(name);
    ... ....
}

Upvotes: 4

Naveen
Naveen

Reputation: 73503

Going by the description you provided, it looks like method() in ClassName is a non-const method. If you try to call a non-const method on a const object compiler will throw an error. When you did ClassName temp = name; you created a copy of the variable into a temporary non-const object temp. You can call any method on this object but since it is a copy, the modification done on this object will not be reflected in name object.

EDIT

The best way to solve this, is to make the ClassName::method const if it doesn't modify any member variables. If it does, then you should not take your parameter in function foo as a const-reference. You should take parameter as a non-const reference.

Upvotes: 6

Related Questions