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I understand move constructor, but in some places I see move constructor is deleted. In what scenarios is deleting a move constructor is useful/makes sense?
Even if the the object needs to be singleton, we can delete the copy constructor but not the move constructor.
Its generally not needed, other than for purposes of being verbose. Just an example of "can doesnt mean should". In your example of deleting the copy constructor (or even just defined), the move constructor is implicitly deleted. In scenarios where this is done, you could imagine explicitly deleting the move constructor as well.
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I know that using copy and swap to define assignment operators ensures exception security and avoids self-assignment.
However, when a large number of move assignments are used, copy and swap will cause performance waste:Why is it not efficient to use a single assignment operator handling both copy and move assignment?
When should I use copy and swap and define copy/move assignment operators separately?
After seeing the answer, I still have questions. Does copy-and-swap use efficiency in exchange for convenience, and define the copy/move assignment operator separately to use convenience in exchange for efficiency?
copy and swap will cause performance waste
[citation needed]
You don't guess performance issues: you bench them, you measure them. After you've done that and you know you have a performance issue you can ask yourself how to improve things.
When should I use copy and swap and define copy / move assignment operators separately?
This should be your default choice. If one day you do have performance issues, profile your code and see from there.
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How can I dynamically allocate a class object that has no defined constructor?
I tried:
A * newPtr = new A();
But it's giving me some kind of memory leak on gdb.
Cheers!
If there's no defined constructor and standard rules allow it then you get a implicitly-declared default constructor, otherwise your code wouldn't compile at all.
So if you are not defining any constructor and your code compiles then for sure a default one is declared and defined (so you actually have a constructor).
You get a leak because you need to delete the pointer to free its memory from the heap.
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How should i do the move constructor for a vector with pointers?
for example :
I have this in class A as a field - vector<A*> AList;
In the function (A &&otherA) ( which is the move constructor) ,
should i write like this :
AList(std::move(other.AList)) or in an other way?
Yes, you can do it like that.
Or, if you don't need the move assignment operator to do anything else, just leave it out and the compiler will do this for you.
This goes for moving any member vector.
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I know that we can directly assign same class objects in C++,but what actually happen behind the scene?
There's something called "default copy-constructor" and "default assignment-operator". Unless you overload these methods in the class, the default behavior is that all non-static members of the class are copied one-by-one from the source to the target class.
A little more: This includes pointers, btw. Which is why you generally should overload these operators and follow the rule of three if you have pointers as members.
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If you don't provide a copy constructor, one is created automatically. This is called the default copy constructor.
A default constructor is a constructor that doesn't take any parameters.
If you don't provide any constructors, a default constructor is created automatically.
Is this called the default default constructor?
No, the default constructor is called a “default constructor” in both cases, whether it is provided explicitly or is automatically generated by the compiler.
Well, to be fair the C++ Standard does speak, in paragraph 12.1-5, of defaulted default constructors.
No. It's always called the default constructor.
It can be called an "implicitly defined default constructor", if you insist. C++11 §12.1/6
A default constructor that is defaulted and not defined as deleted is implicitly defined ...