What is the curiously recurring template pattern (CRTP)?

Question

Without referring to a book, can anyone please provide a good explanation for CRTP with a code example?

Answer 1

Here you can see a great example. If you use virtual method the program will know what execute in runtime. Implementing CRTP the compiler is which decide in compile time!!! This is a great performance!

template 
class Writer
{
  public:
    Writer()  { }
    ~Writer()  { }

    void write(const char* str) const
    {
      static_cast(this)->writeImpl(str); //here the magic is!!!
    }
};


class FileWriter : public Writer
{
  public:
    FileWriter(FILE* aFile) { mFile = aFile; }
    ~FileWriter() { fclose(mFile); }

    //here comes the implementation of the write method on the subclass
    void writeImpl(const char* str) const
    {
       fprintf(mFile, "%s\n", str);
    }

  private:
    FILE* mFile;
};


class ConsoleWriter : public Writer
{
  public:
    ConsoleWriter() { }
    ~ConsoleWriter() { }

    void writeImpl(const char* str) const
    {
      printf("%s\n", str);
    }
};