c++ Virtual Derivation Grandpa Constructing Propagation Problem: Solution

> Start
> c++ example code
> What will if . ?
> What is the . ?
> Back: Home

Q: c++: What is the solution of virtual derivation grandpa constructing propagation problem?

A: The solution is very simple: delete default constructor of the very super-base class.

c++ example Code

// code-(2)

#include <iostream>

namespace manda
{
	// The very super-base class.
	class grandpa
	{
	protected:
		int x{0};
	public:
		grandpa() = delete;
		grandpa(int x): x{x} {}
	};

	class father:
		virtual public grandpa
	{
	protected:
		int y{0};
	public:
		father(int x, int y):
			grandpa{x},
			y{y}
		{}
	};

	class man:
		public father
	{
	protected:
		int z{0};
	public:
		man(int x, int y, int z):
			grandpa{x},
			father{x, y},
			z{z}
		{}
	public:
		void print() const
		{
			std::cout
				<< "(x, y, z) = "
				<< "(" <<  x << ", " << y << ", " << z << ")"
				<< std::endl
			;
		}
	};
}

int main()
{
	manda::man man{7, 4, 9};
	man.print();
}

Output:

(x, y, z) = (7, 4, 9)

What will happen if man does not propagate grandpa?

What will happen if constructor of manda::man does not propagate to manda::grandpa?

// code-(2)

class man
	public father
{
private:
	int z{0};
public:
	man(int x, int y, int z):
		// grandpa{x},
		father{x, y},
		z{z}
	{}

		...
	...
};

The compiling will help you to correct code by an error message:

error: use of deleted function ‘manda::grandpa::grandpa()

What is the problem?

What is the virtual derivation grandpa constructing propagation problem?

When c++ cascaded virtual derivation is enabled,
manda::man constructor transfers parameters to manda::father constructor,
we expect then the manda::father constructor will transfer parameters to manda::grandpa constructor,
however it does not work:
manda::father constructor does not transfer parameters to manda::grandpa constructor,
Because they are virtual derivation !
Instead, manda::man constructor will implicitly call a default constructor of manda::grandpa directly,
which will let wrong parameters be transferred:

Like code-(2). (and if the default constructor of manda::grandpa is not deleted.)

To let the correct parameters be transferred from manda::man constructor to manda::grandpa constructor,
we can let the manda::man consructor propagate manda::grandpa constructor explicitly:

// code-(3)

class man
	public father
{
private:
	int z{0};
public:
	man(int x, int y, int z):
		grandpa{x}, // explicit propagate
		father{x, y},
		z{z}
	{}

		...
	...
};

However, it has an easy-to-make-mistake problem:
user programming will be more easy to miss propagating to grandpa constructor,
and especially, if the grandpa code is coded at a third-party library,
user programming will not realize it at all.

The solution is delete the default constructor of grandpa like this article,
then the compiling will point out the error by an error message.

//////////////////////////////////////////////////////////////////////

Home

//////////////////////////////////////////////////////////////////////

Tue Aug 11 12:46:26 AM UTC 2026

//////////////////////////////////////////////////////////////////////

Helpful

Spaceship 50 Years Alienated

Role

Donate

@cppfx.xyz