Using macros to define forward declarations-CodePudding

I am writing a pretty huge program with lots of templated functions. Naturally, the program has a long compile-time, which is why I wanted to use forward declaration. Now there are a lot of functions and I do not want to write the forward declaration for each single one of them. Even more so, I want to be able to add some functions without having to add the forward declarations manually.

A quick example:

#define max_dim 3
template bool match_any<1>();
template bool match_any<2>();
template bool match_any<3>();

If I set max_dim to another value, I do not want to manually add the additional forward declarations. Until now I have used a python-script, to just generate a file with all the forward declarations for me.

My goal now is to avoid the python-script and to do this using the cpp-preprocessor only (if this is possible). More concretly, I want to be able to just change the max_dim and all neccessary forward-declarations are generated.

I now that loops etc. are possible just with the preprocessor; what I do not know is how to actually make the preprocessor generate the declarations.

CodePudding user response：

I'm not the right one to ask if what you want to do is possible in a better way, but this is definitely solvable using the preprocessor.

I'll use file iteration, although other methods are also possible:

// slot.h
#ifndef A_0
# include "a.h"
#else
# include "b.h"
#endif


// a.h
#if (SLOT) & 1
# define A_0 1
#else
# define A_0 0
#endif
#if (SLOT) & 2
# define A_1 2
#else
# define A_1 0
#endif
#if (SLOT) & 4
# define A_2 4
#else
# define A_2 0
#endif
#if (SLOT) & 8
# define A_3 8
#else
# define A_3 0
#endif

#undef VAL
#define VAL (A_3|A_2|A_1|A_0)

#undef B_0
#undef B_1
#undef B_2
#undef B_3


// b.h
#if (SLOT) & 1
# define B_0 1
#else
# define B_0 0
#endif
#if (SLOT) & 2
# define B_1 2
#else
# define B_1 0
#endif
#if (SLOT) & 4
# define B_2 4
#else
# define B_2 0
#endif
#if (SLOT) & 8
# define B_3 8
#else
# define B_3 0
#endif

#undef VAL
#define VAL (B_3|B_2|B_1|B_0)

#undef A_0
#undef A_1
#undef A_2
#undef A_3



// impl.cpp
#ifndef VAL
#define VAL 12
#endif

template bool match_any<VAL>();

#define SLOT (VAL-1)
#include "slot.h"
#if VAL != 0
#include __FILE__
#endif

(https://godbolt.org/z/csehns44j)

If you need more than 4 bits to represent the value of max_dim, then you'd need to add a few lines to a.c and b.c. This uses self recursive includes, which only works for a few hundred iterations (without special compiler flags). To circumvent this, you can use the following structure:

// iter1.h
#if CONTINUE
#include "iter2.h"
#if CONTINUE
#include "iter2.h"
#if CONTINUE
#include "iter2.h"
#if CONTINUE
#include "iter2.h"
#if CONTINUE
#include "iter2.h"
// ...
#endif
#endif
#endif
#endif
#endif

// iter2.h
#if CONTINUE
#include "iter3.h"
#if CONTINUE
#include "iter3.h"
#if CONTINUE
#include "iter3.h"
#if CONTINUE
#include "iter3.h"
#if CONTINUE
#include "iter3.h"
// ...
#endif
#endif
#endif
#endif
#endif

// iter3.c
#if CONTINUE
#include FILE
#if CONTINUE
#include FILE
#if CONTINUE
#include FILE
#if CONTINUE
#include FILE
#if CONTINUE
#include FILE
// ...
#endif
#endif
#endif
#endif
#endif



// impl.cpp
#ifndef VAL
#define VAL 100
#define FILE "impl.cpp"
#endif

template bool match_any<VAL>();

#define SLOT (VAL-1)
#define CONTINUE VAL != 0
#include "slot.h"

#ifndef ONCE
#define ONCE
#include "iter1.h"
#endif

(https://godbolt.org/z/h74jPb11c)

With 3 iter.h files, of which each has 5 includes that results in 5^3 iterations. Expanding this should be trivial.

CodePudding user response：

It is doubtful if the idea below is an improvement for you, but here is how it could be done.

// in some header file:
#define MAX_DIM 3
#ifndef __INSTANCE_IMPL
#  define TINSTANCE(n) \
   extern template bool match<(n)>;
#else
#  define TINSTANCE(n) \
   template bool match<size_t N = (n)> {
     // however it is implemented
   }
#endif

TINSTANCE(1)
TINSTANCE(2)
TINSTANCE(3)

// in the .cpp file:
#define __INSTANCE_IMPL
#include <extra-header>