Justin's Code Haus
Ramblings of a compiler engineer

The Beauty of C++ Templates

Every so often, I'll get a random C++ question from a friend or colleague.  Most of the time the answers are trivial, at least for someone who has a history with the language.  Other questions make me stop and ponder, searching for the best "C++" way to do something.  Yesterday, the question was simple and the solution turned out to be equally simple, but getting to the solution made me stop and appreciate some of the cool things one can do with C++ templates.

The Problem

The problem was simple.  Suppose you have a C++ template class/struct that is parameterized by a single type, e.g.

template<typename T>
class my_data {
  // ...
private:
  T element_;
};

The Solution

Now, the question is, "how do I write a method for this class/struct that maps the type of T to an enumeration value?"  For context, the real problem involved mapping T to an MPI data type, e.g. (float -> MPI_FLOAT), (double -> MPI_DOUBLE), etc..

The first thought for anyone familiar with containers may be to explicitly generate a map, e.g. std::map in this case, to hold all possible mappings from the C++ type (via typeid()) to the MPI type (really just an integer).  Such a solution is certainly valid and may be the best way to approach the problem in another language such as C# or Java.  After pondering the "C++" solution to the problem for a few minutes, my colleague and I came up with a fairly elegant solution involving templates.  Or, at least I found it quite elegant.

/**
 * This struct wrappers the MPI data type value for the given C++ type.
 *
 * Any valid MPI data type value must have a corresponding explicit template
 * instantiation below.
 */
template<typename T>
struct mpi_type_wrapper {
  int mpi_type;
  mpi_type_wrapper();
};

// Explicit instantiation for `float'
template <>
mpi_type_wrapper::mpi_type_wrapper()
: mpi_type(MPI_FLOAT) {}

// Explicit instantiation for `double'
template <>
mpi_type_wrapper::mpi_type_wrapper()
: mpi_type(MPI_DOUBLE) {}

The mpi_type_wrapper struct is a convenient way to convert an arbitrary C++ type to an equivalent MPI type.  All one has to do is declare a local variable of type mpi_type_wrapper<T> (with appropriate T) and read the value of its mpi_type field.  Of course, none of this is specific to MPI in any way.  The only requirement is that an explicit instantiation of the constructor must be provided for any C++ types that are to be converted.

Why This Solution?

This solution strikes me as elegant for two reasons.  First, it is a solution that would be difficult, if not impossible, to express in many other languages.  Second, and most interesting to me, there is no run-time overhead associated with this solution.  You can even compile this with RTTI turned off.  Any reasonable compiler automatically inlines the appropriate constructor, then constant propagation replaces any uses of the mpi_type field with the appropriate MPI_* enumeration value.  There is no memory overhead associated with explicitly keeping a map at run-time, nor any time overhead of performing a map look-up.  The final code just uses the constant value!  If you do not believe me, check out this example:

/**
 * Some template class that needs to know the MPI_DataType value for its
 * template parameter type.
 */
template<typename T>
struct some_type {
  void printType() {
    mpi_type_wrapper<T> wrap;

    printf("My Type: %d", wrap.mpi_type);
  };
};

int main() {
  some_type<float> floatClass;
  some_type<double> doubleClass;

  floatClass.printType();
  doubleClass.printType();

  return 0;
}

And the generated code?

_main:
  pushq %rbx
  leaq L_.str(%rip), %rbx
  movq %rbx, %rdi
  xorl %esi, %esi
  xorb %al, %al
  callq _printf
  movl $1, %esi
  movq %rbx, %rdi
  xorb %al, %al
  callq _printf
  xorl %eax, %eax
  popq %rbx
  ret

Conclusion

While this example is probably trivial for most experienced C++ programmers out there, including myself, I always find myself stopping and appreciating such solutions.  In this case, C++ templates provide such an elegant and efficient solution that I cannot help feeling giddy.

Posted Fri 01 April 2011 by Justin Holewinski in Programming (C++)