c++ statically assert that a function is never called

Question

For some complicated reasons I want to create default constructor (alongside my normal constructors) that always throws. I want it to be there, but I also want it never to be called. It is pretty obvious that during runtime I can check for that thrown exception and for example terminate program when I catch it, but the ideal solution would be to have it checked during compilation.

So my question is: can I statically assert somehow that a function will never be called? I've looked at functions in <type_traits> but I don't see anything there that would help me. Is there some c++ dark magic that I could use to achieve my goal?

I don't have a code example, because what would even be in there?

PS. Yes. I am sure that I want to have a function and disallow everybody of calling it. As I stated previously reasons for that are complicated and irrelevant to my question.

EDIT. I can't delete this constructor or make it private. It has to be accessible for deriving classes, but they shouldn't call it. I have a case of virtual inheritance and want to "allow" calling this constructor by directly virtually derived classes (they won't call it, but c++ still requires it to be there), but no in any other classes deeper in inheritance chain.

EDIT 2. As requested I give a simplified example of my code.

#include <stdexcept>

class Base {
protected:
  Base() { throw std::logic_error{"Can't be called"}; }
  Base(int); // proper constructor
  
private:
  // some members initialized by Base(int)
};

class Left: virtual public Base {
protected:
  Left(int) {}
  // ^ initialize members of Left, but does not call Base()!
  // Though it seems that it implicitly does, Base() is never actually called.
};

class Right: virtual public Base {
protected:
  Right(int) {} // The same as in Left
};

class Bottom: public Left, public Right {
public:
  Bottom(int b, int l, int r): Base{b}, Left{l}, Right{r} {}
  // ^ Explicitly calling constructors of Base, Left, Right.
  // If I forget about calling Base(int) it silently passes
  // and throws during runtime. Can I prevent this?
};

EDIT 3. Added body to Left's and Right's constructors, so that they implicitly "call" Base().

Answer 1

As you've stated in your comments that you never want to instatiate Base, Left or Right object, then you should make them abstract, even by some empty method:

class Base {
private:
  // ...

  virtual void DefineIfNonAbstract() = 0;
};

class Bottom: public Left, public Right {

  void DefineIfNonAbstract() final {};

  // ...
};

Trust your compiler. When it sees that DefineIfNonAbstract is private and none of its parents implemented it, it's not going to put it into a vtable.

You're Bottom class is already 16 bytes in your example for both gcc and clang (likely a pointer for each virtual inheritance). Adding the abstract method doesn't change that.

---

In the comments you expressed concern that this might not be safe, and sent me a link to CppCoreGuidelines:

I.25: Prefer empty abstract classes as interfaces to class hierarchies

Reason

Abstract classes that are empty (have no non-static member data) are more likely to be stable than base classes with state.

They're referring to design choices here, not whether it causes undefined behaviour or something. In our case we're actually enforcing your design, not changing it.

The whole thing likely needs a serious rework in design. Inheritance in general is rarely a good choice - virtual inheritance even rarer.

Answer 2

If your link time optimization is up to the challenge, you might be able to have the problematic function call a never-defined function.

#include "Base.h"

// An anonymous namespace (or a static function) might be caught as missing
// while compiling this translation unit, so use a suggestive namespace name.
namespace DoNotDefine {
    // Declare a function without a definition.
    // The name is intended to make the error message easier to digest.
    void DisallowedConstruction();
} // namespace DoNotDefine

Base::Base()
{
    DisallowedConstruction();
}

In theory – I am not claiming that any particular linker is up to the task – the linker could eliminate unused function definitions before checking for missing definitions. If nothing actually calls Base::Base() then such a linker would eliminate its definition before complaining that DisallowedConstruction() has no definition. After eliminating Base::Base(), there is no longer a call to DisallowedConstruction() so no problem.

If something did actually call Base::Base() then linking would fail because of there is no definition for DisallowedConstruction().

Again, I am not claiming that this will actually work with your compiler chain, only that it could work in theory.

---

For lesser compilers, I would suggest defining a pure virtual function in Base. Keep this pure virtual until you hit a class that uses non-default construction for Base. That ensures that no one instantiates a default Base. A bit awkward, but effective.

However, this has the drawback that the compiler cannot enforce this convention. This convention could be innocently broken. For example, someone might change a class from using non-default construction of Base to default construction and forget to remove the definition of the special function.