I've done a shady thing

Question

Are (seemingly) shady things ever acceptable for practical reasons?

First, a bit of background on my code. I'm writing the graphics module of my 2D game. My module contains more than two classes, but I'll only mention two in here: Font and GraphicsRenderer.

Font provides an interface through which to load (and release) files and nothing much more. In my Font header I don't want any implementation details to leak, and that includes the data types of the third-party library I'm using. The way I prevent the third-party lib from being visible in the header is through an incomplete type (I understand this is standard practice):

class Font
{
  private:
    struct FontData;
    boost::shared_ptr<FontData> data_;
};

GraphicsRenderer is the (read: singleton) device that initializes and finalizes the third-party graphics library and also is used to render graphical objects (such as Fonts, Images, etc). The reason it's a singleton is because, as I've said, the class initializes the third-party library automatically; it does this when the singleton object is created and exits the library when the singleton is destroyed.

Anyway, in order for GR to be able to render Font it must obviously have access to its FontData object. One option would be to have a public getter, but that would expose the implementation of Font (no other class other than Font and GR should care about FontData). Instead I considered it's better to make GR a friend of Font.

Note: Until now I've done two things that some may consider shady (singleton and friend), but these are not the things I want to ask you about. Nevertheless, if you think my rationale for making GR a singleton and a friend of Font is wrong please do criticize me and maybe offer better solutions.

The shady thing. So GR has access to Font::data_ though friendship, but how does it know exactly what a FontData is (since it's not defined in the header, it's an incomplete type)? I'll just show the code and the comment that includes the rationale...

// =============================================================================
//   graphics/font.cpp
// -----------------------------------------------------------------------------

struct Font::FontData
    : public sf::Font
{
    // Just a synonym of sf::Font
};

// A redefinition of FontData exists in GraphicsRenderer::printText(),
// which will have to be modified as well if this definition is modified.
// (The redefinition is called FontDataSurogate.)
// Why not have FontData defined only once in a separate header:
// If the definition of FontData changes, most likely printText() text will
// have to be altered also regardless. Considering that and also that FontData
// has (and should have) a very simple definition, a separate header was
// considered too much of an overhead and of little practical advantage.


// =============================================================================
//   graphics/graphics_renderer.cpp
// -----------------------------------------------------------------------------

void GraphicsRenderer::printText(const Font& fnt /* ... */)
{
    struct FontDataSurogate
        : public sf::Font {
    };

    FontDataSurogate* suro = (FontDataSurogate*)fnt.data_.get();
    sf::Font& font = (sf::Font)(*suro);

    // ...
}

So that's the shady thing I'm trying to do. Basically what I want is a review of my rationale, so please tell me if you think I've done something horrendous or if not confirm my rationale so I can be a bit surer I'm doing the right thing. :) (This is my biggest project yet and I'm only at the beginning so I'm kinda feeling things in the dark atm.)

Answer 1

In general, if something looks sketchy, I've found that it's often worth going back a few times and trying to figure out exactly why that's necessary. In most cases, some kind of fix pops up (maybe not as "nice", but not relying on any kind of trick).

Now, the first issue I see in your example is this bit of code:

struct FontDataSurogate
    : public sf::Font {
};

occurs twice, in different files (neither being a header). That may come back and be a bother when you change one but not the other in the future, and making sure both are identical will very likely be a pain.

To solve that, I would suggest putting the definition to FontDataSurogate and the appropriate includes (whatever library/header defines sf::Font) in a separate header. From the two files that need to use FontDataSurogate, include that definition header (not from any other code files or headers, just those two).

If you have a main class declaration header for your library, place the forward declaration for the class there, and use pointers in your objects and parameters (regular pointers or shared pointers).

You can then use friend or add a get method to retrieve the data, but by moving the class definition to its own header, you've created a single copy of that code and have a single object/file that's interfacing with the other library.

Edit: You commented on the question while I was writing this, so I'll add on a reply to your comment.

"Too much overhead" - more to document, one more thing to include, the complexity of the code grows, etc.

Not so. You will have one copy of the code, compared to the two that must remain identical now. The code exists either way, so it needs documented, but your complexity and particularly maintenance is simplified. You do gain two #include statements, but is that such a high cost?

"Little practical advantage" - printText() would have to be modified every time FontData is modified regardless of whether or not it's defined in a separate header or not.

The advantage is less duplicate code, making it easier to maintain for you (and others). Modifying the function when the input data changes is not surprising or unusual really. Moving it to another header doesn't cost you anything but the mentioned includes.

Answer 2

friend is fine, and encouraged. See C++ FAQ Lite's rationale for more info: Do friends violate encapsulation?

This line is indeed horrendous, as it invokes undefined behavior: FontDataSurogate* suro = (FontDataSurogate*)fnt.data_.get();