Unable to pass multi-dimensional array to base class in g++

Question

I am new to C++ but have many years in C#, so excuse me if you see C# style coding, I am trying to grasp the concept of pointers, dereference, handles, references, array parameters and I still have difficulties.

The following is for an atmel microcontroller compiled with g++ 11 (arduino).

I am trying to pass a two-dimensionnal fixed size array of unsigned int as a parameter for a constructor of a base class, but no matter how I try, I can't get it transfer the array content in the base class. Here is the raw code, without the includes, only the essential.

In this exemple, the "Derived" constructor is called and I expect it to call the "DisplayDriver" constructor and pass the _colorTable array to it as a parameter. :

DisplayDriver.hpp

class DisplayDriver
{
public:
    DisplayDriver(const int xResolution, const int yResolution, const unsigned int colorTable[18][2]);
}

Derived.hpp

class Derived : public DisplayDriver
{
public:
    Derived(const int xResolution, const int yResolution);

private:
    const unsigned int _colorTable[18][2] =
    {
        {4, 2},
        {9, 31},
        ... // 16 other lines.
    }
}

Derived.cpp

Derived(const int xResolution, const int yResolution) : DisplayDriver(xResolution, yResolution, _colorTable)
{
    // Here, _colorTable[1][0] gives 9 and _colorTable[1][1] gives 31.  As expected.
}

DisplayDriver.cpp

DisplayDriver(const int xResolution, const int yResolution, const unsigned int colorTable[18][2])
{
    // Here, colorTable[1][0] return 0, not 9.
    // colorTable[1][1] return 0, not 31.
}

I tried many different parameter types for the array :

• unsigned int (*colorTable)[18][2]
• unsigned int colorTable[][2]
• unsigned int (&colorTable)[18][2]
• with or without "const"

Answer 1

The problem is nothing to do with types (which are correct). The issue is that you are using a data member in Derived in the constructor for DisplayDriver. When the DisplayDriver constructor is called the Derived object has not been constructed yet, so the values are garbage.

Simple solution is to make _colorTable static.

class Derived : public DisplayDriver
{
public:
    Derived(const int xResolution, const int yResolution);

private:
    static const unsigned int _colorTable[18][2];
};

and

const unsigned int Derived::_colorTable[18][2] =
{
    {4, 2},
    {9, 31},
    ...
};

Answer 2

As others have said, your problem is that your using _colorTable before it has been initialised. If you don't mind all instances Derived sharing a common _colorTable (which is what static in the context of a class declaration means) then making _colorTable static works.

If you however want each instance to have its own _colorTable then one way to get around your problem is to use the base-from-member idiom. This idiom utilize the initialisation order of base classes. When a class has multiple base classes...:

class A {  };
class B {  };
class C {  };

class D : public A, public C, public virtual B {  }; // note virtual

...C++ says that the initialisation order is:

1. All virtual base classes in the order of declaration (left to right).

2. All non-virtual base classes in the order of declaration (left to right).

Therefore if you if create an instance of D then D's base classes gets initialised in the following order: first B, then A and then C. If your just starting out (and perhaps more so if you're using C++ on microcontrollers) then you don't have have to worry about virtual base classes.

So in your case, using your knowledge of base class initialisation order, you can use the base-from-member idiom. Create a new class and move _colorTable from Derived to your new class:

// Derived.hpp

// namespace detail {

class DerivedDMembers 
{
protected:
  const unsigned int _colorTable[18][2] =
  {
    {4, 2},
    {9, 31},
    ... // 16 other lines.
  };
};

// } // namespace detail

// if you feel that DerivedDMembers is polluting your namespace
// you can put it in a namespace called "detail". there's nothing
// special about 'detail' other than that "internal details" by
// convention are put in it.    

Have Derived derive from your new class (DerivedDMembers):

// Derived.hpp

// note how DerivedDMembers is listed before DisplayDriver, which
// is crucial
class Derived : protected DerivedDMembers, public DisplayDriver
{
public:
  Derived(const int xResolution, const int yResolution);

  // note, _colorTable is now in DerivedDMembers  
};

Now you can safely pass _colorTable to your base class DisplayDriver:

// Derived.cpp

Derived(const int xResolution, const int yResolution) : DisplayDriver(xResolution, yResolution, _colorTable) {  }

You don't have to explicitly call DerivedDMembers's default constructor in Derived's constructors initialiser list because if a base class constructor call is omitted its default constructor is implicitly called.

Now each instance of Derived has its own _colorTable and you can use _colorTable to initialise DisplayDriver.

Answer 3

The problem here is the order of initialization:

Derived instance;

is expanded to:

1. Call to constructor Derived::Derived()
2. To construct derived class the base classes must be constructed first. So DisplayDriver(xResolution, yResolution, _colorTable) is run.
3. All member variables are constructed with their constructors in order in which they were declared in the class. In this case you specified that instance._colorTable is initialized to that array.
4. Body of Derived::Derived is executed.

So, in 2. you are using an uninitialized variable which is undefined behaviour - it can contain anything. In your case the variable contains array of zeroes.

To fix this you can mark _colorTable static.

Also there's no need to use const for int as it is passed by value.