Sensei <senseiwa <at> gmail.com> writes:
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My attempt is now to use a 128bit integers, and extract MSB/LSB as before. The difference is using a const member function. Of course, I've specialized std::less for my type.
This is off-topic, but you don't need to specialize std::less for __uint128_t because the default implementation does exactly the same as your specialization (both use operator<).
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From the documentation I see that I need in const_mem_fun<> in order, the input type, the output type, and the address of a member function, a const member in my case.
const_mem_fun expects a member function *of the class being inserted into the container* (which in your case is __uint128_t), not just any member function of some unrelated class (in your case, you're trying to plug &Data::msb and &Data::lsb). So, turn &Data::msb and &Data::lsb into *static* member functions (as they're really global functions, not needing to access any member of Data): static uint64_t msb(__uint128_t storage) { return static_cast<uint64_t>(storage >> 64); }; static uint64_t lsb(__uint128_t storage) { return static_cast<uint64_t>(storage & 0xFFFFFFFFFFFFFFFF); }; and resort to global_fun rather than const_mem_fun: ordered_non_unique<global_fun<__uint128_t, uint64_t, &Data::msb>>, ordered_non_unique<global_fun<__uint128_t, uint64_t, &Data::lsb>> Joaquín M López Muñoz Telefónica