Dont expand BOOST_PP_SEQ_FOR_EACH if SEQ is empty

Question

If the argument list is empty

#define key_evaluate(r, key, sig) key |= 1 << sig;
#define getKey(...)\
({\
      ComponentKey key = 0;\
      BOOST_PP_SEQ_FOR_EACH(key_evaluate, key, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))
      key;\
})
    
int main(void)
{
     getKey();
}

expands to

int main(void)
{
    ({ 
         ComponentKey key = 0; 
         key |= 1 << ;
         key; 
    })
}

but I would like to have it expand to nothing if _VA_ARGS_ is empty. This answers a question of this type, but if I use this instead

#define key_evaluate(r, key, sig) key |= 1 << sig;
#define convertToKey(...) BOOST_PP_SEQ_FOR_EACH(key_evaluate, key, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))
#define components_convertToKey(...)\
    ({\
        ComponentKey key = 0;\
        BOOST_PP_IF(BOOST_PP_EQUAL(BOOST_PP_TUPLE_SIZE((,##__VA_ARGS__)), 1), \
        BOOST_PP_EXPAND, convertToKey) (__VA_ARGS__) \
        key;\
    })

I get this error: error: ‘BOOST_PP_EXPAND’ undeclared. This means BOOST_PP_EXPAND is not replaced by the Preprocessor, because BOOST_PP_EXPAND is used elsewhere, which means it is defined.

Answer 1

I don't know.

I DO know

• that macros are often problematic
• that braced-group expressions are non-standard

It looks like you're trying to combine a number of integral sigs into a key mask, so making up an example could look like:

enum SigKeys {
    key0 = 1,
    key1 = 2,
    key2 = 4,
    key3 = 8,
    key4 = 16,
};

Live On Coliru

#include <iostream>
int main(void) {
    std::cout << "key1, key2: " << (key1 + key2) << " == " << getKey(1, 2) << "\n";
    std::cout << "key2, key4: " << (key2 + key4) << " == " << getKey(4, 2) << "\n";
}

Prints

key1, key2: 6 == 6
key2, key4: 20 == 20

RETAKE, NO MACROS

Assuming your compiler is up to snuff, you could simply write that as:

template <typename... T>
constexpr ComponentKey getKey(T&&... sig) {
    return (0 | ... | (1 << sig));
}

It will be standard, portable, compile in a fraction of the time, be more readable, contexpr and optimizable.

See it Live On Coliru

What If Your Compiler Old

Even then you can have it better:

ComponentKey getKey(std::initializer_list<int> sig) {
    return std::accumulate(sig.begin(), sig.end(), 0,
            [](int r, int s) { return r | (1 << s); });
}

See it Live On Coliru as well. Unsurprisingly it will optimize just the same.

If you prefer without std::accumulate: Coliru and Compiler Explorer

ComponentKey getKey(std::initializer_list<int> sig) {
    ComponentKey r = 0;
    for (auto& s : sig)
        r |= 1 << s;
    return r;
}