Choice quote from cppreference: This runtime polymorphism allows objects using polymorphic_allocator to behave as if they used different allocator types at run time despite the identical static allocator type The issue with “regular” allocators is that they change the type of the container. If you want a vector with a specific allocator, you can make … Read more
The first sentence of the proposal: ” The inline specifier can be applied to variables as well as to functions. The ¹guaranteed effect of inline as applied to a function, is to allow the function to be defined identically, with external linkage, in multiple translation units. In practice that means defining the function in a header, that can be included in multiple translation units. The proposal … Read more
There’s now a drop down (at least since VS 2017.3.5) where you can specifically select C++17. The available options are (under project > Properties > C/C++ > Language > C++ Language Standard) ISO C++14 Standard. msvc command line option: /std:c++14 ISO C++17 Standard. msvc command line option: /std:c++17 Visual Studio 2022 (MSVC C++20 and the … Read more
Up to and including C++17 pi is not a constant introduced into the language, and it’s a pain in the neck. I’m fortunate in that I use boost and they define pi with a sufficiently large number of decimal places for even a 128 bit long double. If you don’t use Boost then hardcode it … Read more
In C++11 the act of “assignment”, i.e. the side-effect of modifying the LHS, is sequenced after the value computation of the right operand. Note that this is a relatively “weak” guarantee: it produces sequencing only with relation to value computation of the RHS. It says nothing about the side-effects that might be present in the … Read more
The purpose of any and all kinds of “string reference” and “array reference” proposals is to avoid copying data which is already owned somewhere else and of which only a non-mutating view is required. The string_view in question is one such proposal; there were earlier ones called string_ref and array_ref, too. The idea is always … Read more
The scoped_lock is a strictly superior version of lock_guard that locks an arbitrary number of mutexes all at once (using the same deadlock-avoidance algorithm as std::lock). In new code, you should only ever use scoped_lock. The only reason lock_guard still exists is for compatibility. It could not just be deleted, because it is used in … Read more
std::string_view is faster in a few cases. First, std::string const& requires the data to be in a std::string, and not a raw C array, a char const* returned by a C API, a std::vector<char> produced by some deserialization engine, etc. The avoided format conversion avoids copying bytes, and (if the string is longer than the … Read more
std::launder is aptly named, though only if you know what it’s for. It performs memory laundering. Consider the example in the paper: struct X { const int n; }; union U { X x; float f; }; … U u = {{ 1 }}; That statement performs aggregate initialization, initializing the first member of U … Read more
(The approach of the better_enums library) There is a way to do enum to string in current C++ that looks like this: ENUM(Channel, char, Red = 1, Green, Blue) // “Same as”: // enum class Channel : char { Red = 1, Green, Blue }; Usage: Channel c = Channel::_from_string(“Green”); // Channel::Green (2) c._to_string(); // … Read more