Does C++ support compile-time counters?

c++, metaprogramming, state, templates

Solution

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

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.

Problem

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 << '\n'; // print "0 5" counter_inc_t(); counter_inc_t(); counter_inc_t(); std::cout << counter_read << '\n'; // 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 << '\n'; // print "9 10" } ```

Original source

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