C ++重载运算符=指针无法编译

Question

I am trying to implement a template Class with an Operator Overload for = so far it works for non pointer elements, but if I try it for pointers it does not compile

My template Class:

template <class T>
class IBag {
public:
    T _val;
    void Set(T val) { _val = val; } 
    T Get() { return _val; }

    IBag& operator=(T val) {
        this->Set(val);
        return *this;
    }

    operator T() {
        return this->Get();
    }
};

How it works using it:

class IBagExample
{
    static void showExample() {
        IBag<QString>* bag = new IBag<QString>();
        bag->Set(QString("Blub"));
        *bag = QString("Blub");
    }
};

how it does not compile:

class IBagExample
{
    static void showExample() {
        IBag<QString*>* pbag = new IBag<QString*>();
        pbag->Set(new QString("Blub")); // This works
        pbag = new QString("Blub"); // This doesn't works ?!?
    }
};

The compiler Error I get is :

error: assigning to 'IBag<QString *> *' from incompatible type 'QString *'
    pbag = new QString("Blub");
         ^ ~~~~~~~~~~~~~~~~~~~

So my question is how do I get this work, I expected that the overload of the = operator will force c++ to pipe all requests including the new request to my custom function if its not the new constructor of the current class (so IBag* x = new IBag() will allocate IBag but x = new QString("sdjf"); will be forwarded to my custom overload as it is a different Object)

(Note:The IBag example is just a simplification of the Code I am trying to implement.)

Thanks a lot,

Answer 1

The error message says it well:

error: assigning to 'IBag<QString *> *' from incompatible type 'QString *'
    pbag = new QString("Blub");
         ^ ~~~~~~~~~~~~~~~~~~~

pbag is a pointer, not an IBag . You overloaded operator = for IBag , not for IBag * (which is not possible).

This should compile because it calls your operator:

*pbag = new QString("Blub");

Answer 2

Well

pbag = new QString("Blub"); // This doesn't works ?!?

try

*pbag = new QString("Blub");

Since pbag is of type IBag* which is not of type IBag . You're esentially calling the assignment operator of IBag* which is built-in I guess but probably "looks" like

Ibag*& operator=(Ibag* const &);

which is not your IBag& operator=(T val); since that one is part of IBag not IBag* .

You cannot alter the C++ pointer types and operator= must be a member function of the specified type.

and

IBag<QString> bag = QString("Blub");
IBag<QString*> pbag = new QString("Blub");

do not work because you don't have a custom constructor from T .

Answer 3

After various changes in my Code using your suggestions and reviewing what I need, I have following Result :

class IBagExample
{
   void showExample() {
        IBag<QString*> pbag;
        pbag = new QString("Blub"); // This works !
    }
};

This does compile and work, but what does not work is following and I will ask this in another follow up question:

class IBagExample
{
   void showExample() {
        IBag<QString*> pbag = new QString("Blub"); // This doesn't work !
    }
};

So thanks a lot for your help!

Follow up Question: C++ Overload Operator = for Pointers does not work/compile properly

Answer 4

You overload operator for a class to be used by an object. Here pBag is just a pointer and not an object. C++ standard ISO/IEC 14882 § 13.5.3 mention an example which takes pointers, but used with de-referencing (by *).