C++ Template Metaprogramming - Is it possible to output the generated code?

Question

I would like to debug some templated code to understand it better.
Unfortunately I\'m new to template metaprogramming and it IS hard for me to get in.

When I try

Answer 1

at general it is not possible to output the entire code. But what I found extremely interesting, is the ability to use Visual C++ debugger to show you the type. Take that simple meta-program:

template
struct type_list
{
  typedef Head head;
  typedef Tail tail;
};

struct null_type
{};

template
struct list_head
{
  typedef typename List::head head;
};

template
struct list_tail
{
  typedef typename List::tail tail;
};

template
struct list_length
{
  static const size_t length = 1+list_length< typename list_tail::tail >::length;
};

template<>
struct list_length
{
  static const size_t length = 0;
};


int main()
{
  typedef 
    type_list
    < int
    , type_list
      < double
      , type_list
        < char
        , null_type
        >
      >
    >       my_types;

  my_types test1;

  size_t length=list_length::length;

  list_head::tail>::tail>::head test2;

}

I just instantiated my meta-types. These are still empty C++ class instances which are at least 1 byte long. Now I can put a breakpoint after the last instantiation of test2 and see, which types/values length, test1 and test2 are of:

Here is what the debugger shows:

length  3   unsigned int
test1   {...}   type_list > >
test2   -52 'Ì' char

Now you you know the head returned you a character, your list contains int, double, char and is terminated by null_type.

That helped me a lot. Sometimes you need to copy the really messy type to a text editor and format it to a readable form, but that gives you the possibility to trace what is inside and how it is calculated.

Hope that helps,
Ovanes