virtual constexpr | Together C & C++ | Page 2

gritty saddle Apr 9, 2023, 1:07 AM

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and I'd still use constexpr

tender grove Apr 9, 2023, 1:07 AM

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not in literally any other case

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and yes, you should still use constexpr here

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the fact that this works at all is a relic from times where constexpr was not a thing but people wanted to be able to not have to use #define for defining an array size basically

narrow wave Apr 9, 2023, 1:08 AM

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even C has constexpr nowadays for at least variables

tender grove Apr 9, 2023, 1:09 AM

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well, it will have

wintry agate Apr 9, 2023, 1:09 AM

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tender grove not in *literally any other case*

and i've been lookking for such a case or two. Perhaps a function returning an array size being one?

gritty saddle Apr 9, 2023, 1:10 AM

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I think we've provided lots of examples honestly lol

tender grove Apr 9, 2023, 1:10 AM

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constexpr float blub = 69.42f;

template <float f> void fun();

fun<blub>();

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there you go

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this does not work with const

wintry agate Apr 9, 2023, 1:15 AM

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<source>:10:6: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'f'

template<float f> 
void fun()
{
    
} 

int main()
{
    constexpr float ff = 3.4f;
    fun<ff>();
    return 0;
}

native kindle Apr 9, 2023, 1:15 AM

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what version

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this is fine

wintry agate Apr 9, 2023, 1:16 AM

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20

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'float' is not a valid type for a template non-type parameter

native kindle Apr 9, 2023, 1:17 AM

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thats not a 20 error

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but it doesnt seem to work on clang

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interesting

wintry agate Apr 9, 2023, 1:17 AM

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https://godbolt.org/z/4qWdMh5r1

Compiler Explorer - C++ (x86-64 gcc 10.4)

constexpr int foo(int x)
{
return x * x;
}

template<float f>
void fun()
{

}

int main()
{
constexpr float ff = 3.4f;
fun<ff>();
return 0;
}

native kindle Apr 9, 2023, 1:19 AM

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its always been a bit weird with floats

wintry agate Apr 9, 2023, 1:19 AM

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dot may be able to tell

tender grove Apr 9, 2023, 1:19 AM

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why are you using gcc 10.4?

wintry agate Apr 9, 2023, 1:19 AM

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you can use a latest version of clang too

tender grove Apr 9, 2023, 1:19 AM

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https://godbolt.org/z/da1dvWsTc

wintry agate Apr 9, 2023, 1:20 AM

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so there's a bug in clang?!

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jeez

tender grove Apr 9, 2023, 1:20 AM

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clang seems to not support this yet. it's a C++20 feature…

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¯_(ツ)_/¯

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it should work on gcc and msvc

native kindle Apr 9, 2023, 1:21 AM

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wintry agate so there's a bug in clang?!

yea

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clang has the weirdest template support

wintry agate Apr 9, 2023, 1:22 AM

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fair, so this works only cause templates are evaluated at compile time and const float isn't hence the error?

narrow wave Apr 9, 2023, 1:22 AM

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another clang bug australianblaze surprising

tender grove Apr 9, 2023, 1:23 AM

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wintry agate fair, so this works only cause templates are evaluated at compile time and `cons...

template arguments must be constant expressions. and a const float is not usable in a constant expression.

wintry agate Apr 9, 2023, 1:25 AM

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hm and only integral const values are usage in a constant expression right?

tender grove Apr 9, 2023, 1:26 AM

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integral and enumeration types

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and you ideally forget that's even a thing and just use constexpr

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because it's just a legacy thing anyways

gritty saddle Apr 9, 2023, 1:27 AM

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We're almost there, 30 messages more

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And we made history of standardese

wintry agate Apr 9, 2023, 1:28 AM

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alright thanks dot. appreciate it. the last example was helpful

tender grove Apr 9, 2023, 1:32 AM

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👍

onyx brambleBOT Apr 9, 2023, 1:37 AM

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@wintry agate Has your question been resolved? If so, run !solved :)

wintry agate Apr 9, 2023, 1:37 AM

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!solved

#virtual constexpr