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 << '\n';
std::cout << counter_read( my_cnt ) << '\n';
}
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
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.