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In C++ template code is "generated" at compile time. `constexpr` code is ran at compile time.
by arunc 4y ago
In C++ template code is "generated" at compile time.
`constexpr` code is ran at compile time.
- Ferret7446 4y agoWhat is generating code at compile time, if not code running at compile time?
- anonymoushn 4y agoIf one wishes to be pedantic, constexpr is a C++-like language for generating C++ values at compile time and templates are a quite different language for doing the same thing (and some other things). The pitch of the comptime feature is that you can do all of the things you would do with templates and constexpr (and more things due to some limitations being relaxed and type introspection existing) in the language that is similar to the language you use for runtime code. You don't have to do anything in a weird metalanguage.
- gpderetta 4y agoC++ has been trying to get compile time introspection and synthesis into the language for a while. There is a proposal going slowly through standardization, but it is not yet know when it will be ready (I believe it missed 2 or more release trains already). Circle, mentioned elsewhere, is a C++ dialect with fully working compile time metaprogramming.
- rurban 4y agoC++ has only half of a fully working compile time metaprogramming. constexpr are limited, consteval, would be the full part, but is usually an error. And the error handling is even only 10% working. clang is fine, gcc is broken for years.
- duped 4y agoThe big thing is the macro language is restricted to generating additional code when it's evaluated whereas compile time expressions are more about reducing expressions into literals.
- kazinator 4y agoNot any code that you can program yourself. It's a bunch of template deduction/instantiation rules specified in the C++ standard, baked into the compiler, which you can steer only to the extent that you can grab the template declaration syntax by whatever horns it it is trying to gore you with.
- Deukhoofd 4y agoconstexpr *can be ran at compile time, consteval is the one that's always ran at compile time.