在类中广泛使用函数typedef是不好的编程习惯吗？

Question

Would this general concept be considered "bad"? The concept of using function typedefs for precalculating which function's are better optimized to handle the data stored? Or should I just stick to if and switch statements to keep other programmers from cringing? Aside from cringing at the names in this throw together example:

#ifndef __PROJECT_CIMAGE_H_
#define __PROJECT_CIMAGE_H_

#define FORMAT_RGB 0
#define FORMAT_BGR 1

typedef unsigned char ImageFormat;

class CImage
{
    protected:
        // image data
        Components* data;
        ImageFormat format;

        // typedef the functions
        typedef void(*lpfnDeleteRedComponentProc)();
        typedef void(*lpfnDeleteGreenComponentProc)();
        typedef void(*lpfnDeleteBlueComponentProc)();

        // specify the different functions for each supported format
        void DeleteRedComponentRGB();
        void DeleteGreenComponentRGB();
        void DeleteBlueComponentRGB();

        void DeleteRedComponentBGR();
        void DeleteGreenComponentBGR();
        void DeleteBlueComponentBGR();

        // Add in references to which functions to use.
        lpfnDeleteRedComponentProc   DRC;
        lpfnDeleteGreenComponentProc DGC;
        lpfnDeleteBlueComponentProc  DBC;
    public:
        Image();  // Allocate some basic data
        ~Image(); // Deallocate stored data

        // change the image format
        void SetImageFormat(ImageFormat format)
        {
            // shift through the data and adjust it as neccissary.

            switch (format)
            {
                case FORMAT_RGB:
                    // use functions specially suited for the RGB format
                    DRC = DeleteRedComponentRGB;
                    DGC = DeleteGreenComponentRGB;
                    DBC = DeleteBlueComponentRGB;
                    break;
                case FORMAT_BGR:
                    // use functions specially suited for the BGR format
                    DRC = DeleteRedComponentBGR;
                    DGC = DeleteGreenComponentBGR;
                    DBC = DeleteBlueComponentBGR;
                    break;
            }
        }

        // Set's the specifyed component to 0 throughout the entire image
        void DeleteRedComponent()   { DRC(); }
        void DeleteGreenComponent() { DGC(); }
        void DeleteBlueComponent()  { DBC(); }

        // more, similarly pourposed, functions here...
};

#endif // __PROJECT_CIMAGE_H_

Answer 1

There are many problems with the above code.

You use #define uselessly, and typedef where you should have an enum

enum class ImageFormat:unsigned char { // unsigned char optional
  FORMAT_RGB, // =0 optional
  FORMAT_BGR // =1 optional
};

Second, you have a cluster of virtual behavior you want to swap out in a clump. How does this not scream interface class to you?

struct ChannelSpecific {
  virtual void DeleteGreen( CImage* ) = 0;
  virtual void DeleteBlue( CImage* ) = 0;
  virtual void DeleteRed( CImage* ) = 0;
  // etc
};
template< ImageFormat format >
struct ChannelSpecificImpl;
template<>
struct ChannelSpecificImpl<FORMAT_RGB>:ChannelSpecific {
  void DeleteGreen( CImage* ) final { /* etc...*/ }
  // etc...
};
template<>
struct ChannelSpecificImpl<FORMAT_BGR>:ChannelSpecific {
  // etc...
};

The overhead to calling the above virtual functions is marginally higher than a function pointer (due to the vtable being less likely to be in the cache), but in cases where you are doing a whole pile of operations in a row you can find the format and explicitly cast the worker and call the final methods with no function-pointer or virtual table overhead (up to and including allowing the methods to be inlined).

As a second advantage, a whole pile of the operations you want to perform on channels ends up being exceedingly uniform, and just a matter of what the offset of each channel is. So I can do away with the two above specializations by simply doing this:

enum class Channel { Red, Green, Blue };

template<ImageFormat, Channel> struct channel_traits;
template<> struct channel_traits<FORMAT_RGB, Red>:std::integral_constant< size_t, 0 > {};
template<> struct channel_traits<FORMAT_RGB, Green>:std::integral_constant< size_t, 1 > {};
template<> struct channel_traits<FORMAT_RGB, Blue>:std::integral_constant< size_t, 2 > {};
template<> struct channel_traits<FORMAT_BGR, Red>:std::integral_constant< size_t, 2 > {};
template<> struct channel_traits<FORMAT_BGR, Green>:std::integral_constant< size_t, 1 > {};
template<> struct channel_traits<FORMAT_BGR, Blue>:std::integral_constant< size_t, 0 > {};

and now I get to write my ChannelSpecificImpl<ImageFormat> without specializations -- I just need to access the above traits classes, and I get to write my code once, and use it multiple times.

Inside CImage I store a single ChannelSpecific pointer, which holds no data, just algorithms. When I swap out the image format, the ChannelSpecific pointer is swapped out. If I find I have a bottleneck in how I'm using ChannelSpecific because of too much vtable overhead, I get to refactor and put a mega-function in it.

If I hate the fact that I'm passing in the CImage all the time, I can give ChannelSpecific state of a pointer to the CImage internally, and now the code gets to use this->cimage to access the CImage .

On the other hand, code like what you wrote above has its place. I'd consider it better than massive case switch statements.

Note that a bit of the above code is C++11 specific ( enum class , enum with storage specifier, final ), but if you drop those features the solution is still viable.

Also note that your switch statement ends up looking like:

switch (format) {
  case FORMAT_RGB:
    channelSpecific.reset(new ChannelSpecificImpl<FORMAT_RGB>());
  case FORMAT_BGR:
    channelSpecific.reset(new ChannelSpecificImpl<FORMAT_BGR>());

which is far less to maintain and less likely to contain bugs. If you hate the free store (and more specifically, have found that format changes are common enough that the ::new call is a performance hit that matters), create a boost::variant or a C++11 union of each of the possible ChannelSpecificImpl . ( std::unique_ptr<ChannelSpecific> channelSpecific , or std::shared_ptr , depending on various things -- use unique_ptr by default.)

Finally, if you get tired of maintaining that switch statement (as I tend to), making a cascading if based magic switch via template metaprogramming isn't that hard -- or even an array of function pointer factories that produce a ChannelSpecific* and an explicit array lookup to call one of them. (sadly, there isn't a variardic template expansion that produces an actual switch statement, but compilers might optimize chained sequential ifs into the same structure anyhow).

If you get nothing from the above code, the important part is that you don't want to hand write each of the zero functions. You want to not repeat yourself, write it once, factor out the differences between the foramts into a traits class, and have template functions on the format and channel produce the function that does the work and be written once. If you don't do this, you will either have to generate your code via macros and have an undebuggable mess, generate your code via some other method (and not be able to debug the generator, just the generated code), or you will have some corner case bug that occur only when doing some specific operation to some specific channel that your QA will miss. Maybe not today, maybe not tomorrow, but someday when a change is made and someone screws up the update to the 18th format specific function but only in the blue channel.

I'm in the midst of attacking an old per-channel imaging library that was done in this "virtual C-style function pointer swap out" pretty much exactly as you are proposing, and each function I touch gets rewritten using the above technique. I am reducing the amount of code by huge swaths, increasing reliability, and sometimes even getting performance boosts. Why? Because I was able to check common assumptions -- pixelstride equal to pixel packing, pixelstride in source and dest equal -- and generate a less-branchy version for that case, and fall back to a more-branchy for the corner cases, then apply that to a whole myriad of different pixel iteration code in one fell swoop. Maintaining N different pixel iterating code with that kind of micro optimization on top of the existing micro optimizations would be expensive: doing it this way means that I get to write it once, and reap the benefits N fold.

Answer 2

Typedefs are a good way to make the code much more readable. If you have a problem with the typedefs then it means it just does not contribute to the code's readability. Just changing the typedef name will solve the problem but you would need to change it everywhere in the existing codebase.

Answer 3

@Yakk comments on using virtual instead of function pointers are on the money; as well as the better solution also offered.

Given the reservations on the design here, its worth noting that:

  // typedef the functions
  typedef void(*lpfnDeleteRedComponentProc)();
  typedef void(*lpfnDeleteGreenComponentProc)();
  typedef void(*lpfnDeleteBlueComponentProc)();

creates distinct new type names for each component, even though they have the same signature. If I were going down this path I'd have a single type name which would make clear the expected common behavior.