Because the variable isn’t odr-used. You have a constexpr if there that always discards the branch that could use it. One of the points of constexpr if is that the discarded branch need not even compile, only be well-formed. That’s how we can place calls to non-existing member functions in a discarded branch.
On my 10.11 El Capitan, Xcode 7.3.1, clang has been updated to: Apple LLVM version 7.3.0 (clang-703.0.31) which is almost equivalent to llvm version 3.8. clang++ hasn’t -std=c++17 option, but -std=c++1z, working well at present, though only supporting some features of C++1z. For gcc, you can install a very new one by: brew install gcc … Read more
The new overload was added by P0272R1 for C++17. Neither the paper itself nor the links therein discuss why only data was given new overloads but c_str was not. We can only speculate at this point (unless people involved in the discussion chime in), but I’d like to offer the following points for consideration: Even … Read more
#include <iostream> #include <limits> #include <tuple> int main() { auto step = std::numeric_limits<double>::infinity(); auto as = std::numeric_limits<int>::infinity(); std::tie(step, as) = std::tuple{ 0.1, 2 }; std::cout << step << “, ” << as << std::endl; }
If the remainder isn’t zero when dividing the address with the desired alignment, then the address isn’t aligned. inline bool is_aligned(const void * ptr, std::uintptr_t alignment) noexcept { auto iptr = reinterpret_cast<std::uintptr_t>(ptr); return !(iptr % alignment); } Ths can’t be constexpr though, because of the cast. Also, this relies on the implementation-defined fact that the … Read more
This is a clang and gcc bug, we have a clang bug report for this: default member initializer for ‘m’ needed within definition of enclosing class for default argument of function which has the following example: #include <limits> class A { public: class B { public: explicit B() = default; ~B() = default; private: double … Read more
Yes, in c++20. The problem was that determining uniqueness of a template non-type argument was difficult. c++20 adds in a <=> spaceship operator comparison. If it is non-user provided (and based only off non-user provided <=> in turn, repeat recursively) (and a few other requirements; see p0732), the type can be used as a non-type … Read more
try_emplace can indeed replace most uses of emplace, but if you have an unusual use case of a map with a non-copyable and immovable key type, try_emplace will not work because it copies or moves the key. In that case, you must use emplace with std::pair‘s piecewise construction constructor to avoid copies and moves. Even … Read more
How to read: This article is rather long. If you want to know about both aggregates and PODs (Plain Old Data) take time and read it. If you are interested just in aggregates, read only the first part. If you are interested only in PODs then you must first read the definition, implications, and examples … Read more
Why not use the standard library? #include <bitset> int bits_in(std::uint64_t u) { auto bs = std::bitset<64>(u); return bs.count(); } resulting assembler (Compiled with -O2 -march=native): bits_in(unsigned long): xor eax, eax popcnt rax, rdi ret It is worth mentioning at this point that not all x86 processors have this instruction so (at least with gcc) you … Read more