Message from C, C++ talks

July 2019

— Because you didn't mark the function as constexpr

— Constexpr implies const

constexpr int f{ 3 }; // f has type const int

— Now list all the conditions a func must meet to may be evaluated at compile time

— It may be harder than virtual functions

— 

You can find them in cppreference. In C+11 were restricted to a single return statement, but they relaxed that in C++14. You can do pretty much anything that doesn't involve dynamic allocations

— Remember, we're talking about novices: people, who hasn't even heard about classes e.g. They're struggling with unformatted text output

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— It's as simple as "constexpr implies compile time execution instead of runtime"

— If you have constexpr int sum(int lhs, int rhs){ return lhs + rhs; }, what you're telling the compiler is "Hey, do this at compile time if you can"

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— In C++20 we will have inmediate functions, which will be marked as consteval. This forces the function to be executed at compile time

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— First bullet:
> its return type must be a LiteralType
Okay. Let's take a look
> possibly cv-qualified (da hell?) void
Well, that's relatively easy to google
> trivial dtor, aggregate, closure
Et cetera

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— One need to know hella lot terms and principles to understand conxtexpr conditions

— It may sound easy for one who faced this things a lot

— I don't even understand all of them. I just program functions normally, mark them as contexpr and wait for the compiler to tell me what's wrong and adapt as I go

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— Just avoid dynamic allocations and you're good to go

— They're not used for everything, but if you think your function is quite simple, just mark it as constexpr and the compiler will execute at compile time everytime it can

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