c++ - How can I create a macro which uses a value multiple times, without copying it? -

i'd create macro unpacks pair 2 local variables. i'd not create copy of pair if it's variable, accomplish:

#define unpack_pair(v1, v2, pair) \     auto& v1 = pair.first; \     auto& v2 = pair.second;  unpack_pair(one, two, x); 

however, i'd not evaluate expression it's given multiple times, e.g. should call expensive_computation() once:

unpack_pair(one, two, expensive_computation()); 

if do:

#define unpack_pair_a(v1, v2, pair) \     auto tmp = pair; \     auto& v1 = tmp.first; \     auto& v2 = tmp.second; 

then works expensive_computation() case, makes copy in x case. if do:

#define unpack_pair_r(v1, v2, pair) \     auto& tmp = pair; \     auto& v1 = tmp.first; \     auto& v2 = tmp.second; 

then works in x case without making copy fails in expensive_computation() case. if do:

#define unpack_pair_cr(v1, v2, pair) \     const auto& tmp = pair; \     auto& v1 = tmp.first; \     auto& v2 = tmp.second;  #define unpack_pair_rr(v1, v2, pair) \     auto&& tmp = pair; \     auto& v1 = tmp.first; \     auto& v2 = tmp.second; 

these both compile , run, suspect invoke undefined behavior - correct that? also, either of these make sense?

#define unpack_pair_rr(v1, v2, pair) \     auto&& tmp = std::move(pair); \     auto& v1 = tmp.first; \     auto& v2 = tmp.second;  #define unpack_pair_rr(v1, v2, pair) \     auto&& tmp = std::forward<decltype(pair)>(pair); \     auto& v1 = tmp.first; \     auto& v2 = tmp.second; 

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is there way create macro works both of these use cases - not copying x yet not invoking undefined behavior when given result of expression or function call?

auto&& creates forwarding reference, i.e. accepts anything. not (always) create rvalue reference. this:

#define unpack_pair(v1, v2, pair) \     auto&& tmp = pair; \     auto& v1 = tmp.first; \     auto& v2 = tmp.second; 

however, suggest against (unless scope of use of unpack_pair limited , operation really ubiquitous in scope). looks obscurity no real benefit me. imagine returning project after 6 months, 2 hours find critical bug. thanking using nonstandard macro-based syntax instead of readable?