templates - Does C++ support compile-time counters?

Question

For the purpose of introspection, sometimes I've wanted to automatically assign serial numbers to types, or something similar.

Unfortunately, template metaprogramming is essentially a functional language, and as such lacks global variables or modifiable state which would implement such a counter.

Or does it?

---

Example code by request:

#include <iostream>

int const a = counter_read;
counter_inc;
counter_inc;
counter_inc;
counter_inc;
counter_inc;

int const b = counter_read;

int main() {
    std::cout << a << ' ' << b << '
'; // print "0 5"
    
    counter_inc_t();
    counter_inc_t();
    counter_inc_t();
    
    std::cout << counter_read << '
'; // print "8"
    
    struct {
        counter_inc_t d1;
        char x[ counter_read ];
        counter_inc_t d2;
        char y[ counter_read ];
    } ls;
    
    std::cout << sizeof ls.x << ' ' << sizeof ls.y << '
'; // print "9 10"
}

Answer 1

Well… yes, template metaprogramming lacks side effects as it is intended. I was misled by a bug in older versions of GCC and a little unclear wording in the Standard to believe that all those features were possible.

However, at least the namespace-scope functionality can be achieved with little use of templates at all. Function lookup can extract numeric state from the set of declared functions, as demonstrated below.

Library code:

template< size_t n > // This type returns a number through function lookup.
struct cn // The function returns cn<n>.
    { char data[ n + 1 ]; }; // The caller uses (sizeof fn() - 1).

template< typename id, size_t n, size_t acc >
cn< acc > seen( id, cn< n >, cn< acc > ); // Default fallback case.

/* Evaluate the counter by finding the last defined overload.
   Each function, when defined, alters the lookup sequence for lower-order
   functions. */
#define counter_read( id ) 
( sizeof seen( id(), cn< 1 >(), cn< 
( sizeof seen( id(), cn< 2 >(), cn< 
( sizeof seen( id(), cn< 4 >(), cn< 
( sizeof seen( id(), cn< 8 >(), cn< 
( sizeof seen( id(), cn< 16 >(), cn< 
( sizeof seen( id(), cn< 32 >(), cn< 0 
/* Add more as desired; trimmed for Stack Overflow code block. */ 
                      >() ).data - 1 ) 
                      >() ).data - 1 ) 
                      >() ).data - 1 ) 
                      >() ).data - 1 ) 
                      >() ).data - 1 ) 
                      >() ).data - 1 )

/* Define a single new function with place-value equal to the bit flipped to 1
   by the increment operation.
   This is the lowest-magnitude function yet undefined in the current context
   of defined higher-magnitude functions. */
#define counter_inc( id ) 
cn< counter_read( id ) + 1 > 
seen( id, cn< ( counter_read( id ) + 1 ) & ~ counter_read( id ) >, 
          cn< ( counter_read( id ) + 1 ) & counter_read( id ) > )

Quick demo (see it run):

struct my_cnt {};

int const a = counter_read( my_cnt );
counter_inc( my_cnt );
counter_inc( my_cnt );
counter_inc( my_cnt );
counter_inc( my_cnt );
counter_inc( my_cnt );

int const b = counter_read( my_cnt );

counter_inc( my_cnt );

#include <iostream>

int main() {
    std::cout << a << ' ' << b << '
';

    std::cout << counter_read( my_cnt ) << '
';
}

C++11 Update

Here is an updated version using C++11 constexpr in place of sizeof.

#define COUNTER_READ_CRUMB( TAG, RANK, ACC ) counter_crumb( TAG(), constant_index< RANK >(), constant_index< ACC >() )
#define COUNTER_READ( TAG ) COUNTER_READ_CRUMB( TAG, 1, COUNTER_READ_CRUMB( TAG, 2, COUNTER_READ_CRUMB( TAG, 4, COUNTER_READ_CRUMB( TAG, 8, 
    COUNTER_READ_CRUMB( TAG, 16, COUNTER_READ_CRUMB( TAG, 32, COUNTER_READ_CRUMB( TAG, 64, COUNTER_READ_CRUMB( TAG, 128, 0 ) ) ) ) ) ) ) )

#define COUNTER_INC( TAG ) 
constexpr 
constant_index< COUNTER_READ( TAG ) + 1 > 
counter_crumb( TAG, constant_index< ( COUNTER_READ( TAG ) + 1 ) & ~ COUNTER_READ( TAG ) >, 
                                                constant_index< ( COUNTER_READ( TAG ) + 1 ) & COUNTER_READ( TAG ) > ) { return {}; }

#define COUNTER_LINK_NAMESPACE( NS ) using NS::counter_crumb;

template< std::size_t n >
struct constant_index : std::integral_constant< std::size_t, n > {};

template< typename id, std::size_t rank, std::size_t acc >
constexpr constant_index< acc > counter_crumb( id, constant_index< rank >, constant_index< acc > ) { return {}; } // found by ADL via constant_index

http://ideone.com/yp19oo

The declarations should be put inside a namespace, and all names used in the macros except counter_crumb should be fully qualified. The counter_crumb template is found via ADL association with the constant_index type.

The COUNTER_LINK_NAMESPACE macro can be used to increment one counter in the scope of multiple namespaces.