What is Compiler Generated constructor for abstract class no data members

Question

I'm running a static analysis tool and getting an error because an abstract class, with no data members, has no constructors.

Given an abstract class with no data members:

class My_Interface
{
  public:
    virtual void interface_function(void) = 0;
};

1. Are any constructors generated by the compiler?
2. If a constructor is generated, what would it's content be?
3. If a constructor is generated, would it be eliminated by an optimization level?

The rule documentation in the static analysis says:
If you do not write at least one constructor in a class, the compiler will write a public constructor for you by default. This rule detects if you do not declare at least one constructor.

The rule documentation references Scott Meyers, "Effective C++: 55 Specific Ways to Improve your Programs and Design", third edition.

My understanding is that the compiler will not generate constructors for the above case.

Edit 1:
This is not a duplicate of many constructor questions because:

1. This one has no data members.
2. This is not asking if a constructor is necessary, but what happens when a constructor is not provided.
3. This is C++ language.

Answer 1

The compiler at least theoretically synthesizes a constructor even in this case. Even though you can't create an instance of this class, the constructor will be invoked in the process of creating a derived class (that overrides interface_function, so it can be instantiated).

Given that this is basically a pure interface class, the constructor probably won't do anything, so most compilers will probably optimize it out (quite possibly even when you don't tell it to optimize the code).

Answer 2

1. Are any constructors generated by the compiler?

Yes. Several. First, from [class.ctor]:

A default constructor for a class X is a constructor of class X that either has no parameters or else each parameter that is not a function parameter pack has a default argument. If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted (8.4). An implicitly-declared default constructor is an inline public member of its class. A defaulted default constructor for class X is defined as deleted if:

Several bullet points follow, none of which apply. So we have the equivalent of:

My_Interface() = default;

Then, from [class.copy]:

If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4).

So we have:

My_Interface(const My_Interface&) = default;

Also:

If the definition of a class X does not explicitly declare a move constructor, a non-explicit one will be implicitly declared as defaulted if and only if
(9.1) — X does not have a user-declared copy constructor,
(9.2) — X does not have a user-declared copy assignment operator,
(9.3) — X does not have a user-declared move assignment operator, and
(9.4) — X does not have a user-declared destructor.

So we also have:

My_Interface(My_Interface&& ) = default;

2. If a constructor is generated, what would it's content be?

All three are generated, all three are = default;

3. If a constructor is generated, would it be eliminated by an optimization level?

None of the three constructors are trivial because My_Interface has a virtual function. As such, at the very least, the vtable will need to be initialized/copied. So something will have to happen, even if there aren't any members to initialize/copy/move.

Answer 3

Q1. Are any constructors generated by the compiler?

Answer: Yes. From the C++11 Standard:

12.1 Constructors

5 A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4). An implicitly-declared default constructor is an inline public member of its class.

I don't see anything in the standard that answers the other two questions. However, in your case, since there is a virtual member function, the default constructor must, at least, set the virtual table of the object.