CODEWITHSUNDEEP
c++constructorprivate
Max Frai
Max Frai

Reputation: 64366

Private constructor

I have an object of class which has private constructor:

class CL_GUIComponent
{
    // ...
    private:
    CL_SharedPtr<CL_GUIComponent_Impl> impl;
    CL_GUIComponent(CL_GUIComponent &other);
    CL_GUIComponent &operator =(const CL_GUIComponent &other);
    CL_GraphicContext dummy_gc;
};

I have a class which has a pointer to the object of the type I described before.

class Some
{
   private:
       CL_GUIComponent *obj;
   public:
       CL_GUIComponent getComp() { return *obj; }
}

But this code calls the error:

In member function ‘CL_GUIComponent Some::getComp()’:
error: ‘CL_GUIComponent::CL_GUIComponent(CL_GUIComponent&)’ is private
error: within this context

How can I store and get that object?

Upvotes: 1

Views: 1793

Answers (6)

Lakshmi
Lakshmi

Reputation: 179

Since the constructor is declared private, you have to use a public member function to create an object of class that uses the private constructor. 

class CL_GUIComponent 
{ 
    // ... 
    private: 
    CL_GUIComponent();
    CL_GUIComponent(CL_GUIComponent &other); 
    CL_GUIComponent &operator =(const CL_GUIComponent &other); 
    public:
    CL_GUIComponent* CreateInstance()
         {
           CL_GUIComponent *obj = new CL_GUIComponent();
         }

};
class Some 
{ 
   private: 
       CL_GUIComponent *obj; 
   public: 
       CL_GUIComponent* getComp() { return (obj->CreateInstance()); } 
};

Upvotes: 0

Potatoswatter
Potatoswatter

Reputation: 137930

Use getComp() to initialize a reference.

CL_GUIComponent const &mycomp = getComp();

Then the language doesn't try to invoke the copy constructor inside the calling function. (getComp does still create and return a copy, though.)

Upvotes: 0

Nikolai Fetissov
Nikolai Fetissov

Reputation: 84239

This is non-copyable idiom in action. Return by pointer or reference.

Upvotes: 0

Patrick
Patrick

Reputation: 23629

getComp returns an instance of CL_GUIComponent. This means that getComp will actually make a copy of the instance pointed to by obj. If you want getComp to return the instance where obj is pointing to, return a reference to CL_GUIComponent, like this:

CL_GUIComponent &getComp() {return *obj;}

Upvotes: 0

Edward Strange
Edward Strange

Reputation: 40897

By pointer or reference. You can't construct a new one and thus can't return copies, as your get attempts to do.

Upvotes: 2

Peter Alexander
Peter Alexander

Reputation: 54300

Return a reference instead:

CL_GUIComponent& getComp() { return *obj; }

and/or

const CL_GUIComponent& getComp() const { return *obj; }

The code you have their is trying to return a copy, but the copy constructor is private so it can't access it (hence the error). In any case, for non trivial objects, it's almost always better to return a const& instead (in general, not always).

Upvotes: 5

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