Question

I am a bit in stuck and need a help from C++ template guru. There is a template struct:

template<typename T, typename ID>
struct TypeMapping
{
    T Type;
    char* Name;
    ID Id;
};

and a few template functions like this:

template<typename T, typename ID>
bool TryGetTypeByNameImp(const TypeMapping<T, ID> map[], size_t mapSize,
    const char* name, T& type)
{
    for (size_t i = 0; i < mapSize; i++)
    {
        if (strcmp(map[i].Name, name) == 0)
        {
            type = map[i].Type;
            return true;
        }
    }

    return false;
}

Map (the first parameter) is defined as (there are a few similar maps)

namespace audio
{ 
    const TypeMapping<Type, AMF_CODEC_ID> Map[] = 
    {
        {AAC, "aac", AMF_CODEC_AAC},
        {MP3, "mp3", AMF_CODEC_MP3},
        {PCM, "pcm", AMF_CODEC_PCM_MULAW}
    };

    const size_t MapSize =  sizeof(Map)/sizeof(Map[0]);
}

Map is passed to a function as an argument and I am looking for how to pass it as template parameter so I can use functions like in this sample:

 audio::Type type;
 bool r = TryGetTypeByNameImp<audio::Map>("aac", type);

The only solution I found it is to define a struct which holds static Map and MapSize and use the struct as template parameter but I do not like this solution and I am looking for another one. Does anybody know how to do this?

Answer 1

bool r = TryGetTypeByNameImp<audio::Map>("aac", type);

This is trying to use audio::Map as a type – but it isn’t, it’s a variable. Just pass it to the function as a normal argument:

bool r = TryGetTypeByNameImp(audio::Map, "aac", type);

That said, I have three remarks about your code:

• Be aware that declaring a function argument as an array (x[]) does in reality declare it as a pointer. Your code uses this correctly, but using the array syntax is misleading. Use a pointer instead.
• This code is slightly too C-heavy for my taste. While I agree that using raw C-strings is appropriate here, your usage of char* is illegal in C++11, and deprecated in C++03 (since you are pointing to string literals). Use char const*. Furthermore, I’d suggest using a std::string argument in the function, and using the comparison operator == instead of strcmp.

• You are using an out-parameter, type. I abhor this technique. If you want to return a value, use the return type. Since you also return a success value, use a pair as the return type, unless there’s a very compelling reason not to:

  template<typename T, typename ID>
  std::pair<bool, T> TryGetTypeByNameImp(
      const TypeMapping<T, ID> map[], size_t mapSize,
      const char* name)
  {
      for (size_t i = 0; i < mapSize; i++)
          if (strcmp(map[i].Name, name) == 0)
              return std::make_pair(true, map[i].Type);
  
      return std::make_pair(false, T());
  }
  

Ah, and I’d also consider using a std::vector or std::array here instead of a C array. Then you don’t need to manually shlep the array size around through all the functions which use the array.