if-constexpr

问题 I'm trying to understand if constexpr fully. I understand, that if if constexpr(expr) used in a template, and expr is dependent on a template parameter, then during instantiation, only one of the then / else branches will be instantiated, the other will be discarded. I've got two questions: Is it true, that if expr is not dependent on a template parameter, then no branches of if constexpr(expr) will be discarded? If yes, where does the standard say so? I don't see where the standard has the

问题 In C++17, if constexpr was introduced; however, there doesn't seem to be a switch constexpr (see here). Why is that? That is, if a compiler supports if constexpr , is it not also trivial for it to support switch constexpr (at worst as an if-then-else-if-etc. chain, or multiple if's with some flags to control fallthrough)? 回答1: if constexpr was ultimately derived from a more sane form of the static if concept. Because of that derivation, applying the same idea to switch does not appear to have

问题 I have following code static constexpr bool condition = true; int square(int num) { if constexpr (condition) { return num * num; } else { x return num; } } int main() { return square(3); } compiled with -std=gnu++17 My assumption for if constexpr (condition) was that during compilation the part } else { x return num; } get discarded and I get no error about the undefined x Is my understanding wrong that this 'if constexpr' is something like #ifdef CONDITION return num * num; #else x return

问题 I have following code static constexpr bool condition = true; int square(int num) { if constexpr (condition) { return num * num; } else { x return num; } } int main() { return square(3); } compiled with -std=gnu++17 My assumption for if constexpr (condition) was that during compilation the part } else { x return num; } get discarded and I get no error about the undefined x Is my understanding wrong that this 'if constexpr' is something like #ifdef CONDITION return num * num; #else x return

来源： https://stackoverflow.com/questions/53419133/constexpr-operator-new

来源： https://stackoverflow.com/questions/48826555/not-constexpr-variable-in-if-constexpr-clang-vs-gcc

来源： https://stackoverflow.com/questions/45692665/can-if-constexpr-be-used-to-declare-variables-with-different-types-and-init-ex

来源： https://stackoverflow.com/questions/45692665/can-if-constexpr-be-used-to-declare-variables-with-different-types-and-init-ex

来源： https://stackoverflow.com/questions/45692665/can-if-constexpr-be-used-to-declare-variables-with-different-types-and-init-ex

来源： https://stackoverflow.com/questions/41118861/something-like-if-constexpr-but-for-class-definition