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MSVS15x64: Compile Error C3256 C3259 C3250 in Fn.h findarg [message #57131]

Fri, 28 May 2021 12:34

wqcmaster
Messages: 37
Registered: March 2013

Member

Hi,

is there an error here?
No error is thrown under the CLANG compiler; if the MSVS15x64 compiler is used, several errors are thrown.

Upp Version: 15947
Package Core
File "Fn.h"

template <class T, class K, typename... L>
constexpr int findarg(const T& sel, const K& k, const L& ...args)
{
	if(sel == k)
		return 0;
	int q = findarg(sel, args...);
	 return q >= 0 ? q + 1 : -1;
}

threw the following error:
...Fn.h (58): error C3256: "q": Variablenverwendung ergibt keinen konstanten Ausdruck
...Fn.h (53): error C3259: "constexpr"-Funktionen können nur eine return-Anweisung aufweisen.
...Fn.h (57): error C3250: "q": Deklaration ist im "constexpr"-Funktionstext nicht zulässig

But if the function is changed as follows:

template <class T, class K, typename... L>
constexpr int findarg(const T& sel, const K& k, const L& ...args)
{
	return (sel == k) ? 0 : findarg(sel, args...) >= 0 ? findarg(sel, args...) + 1 : -1;
}

-> no error is displayed when compiling.

[Updated on: Fri, 28 May 2021 12:40]

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Re: MSVS15x64: Compile Error C3256 C3259 C3250 in Fn.h findarg [message #57149 is a reply to message #57131]

Mon, 31 May 2021 20:06

Novo
Messages: 1430
Registered: December 2006

Ultimate Contributor

In your fix you are using C++11. It supports very limited syntax with constexpr, basically, only what fits into a return statement.
Current code in U++ is using C++14, which allows to use almost all C++ statements with constexpr.

Regards,
Novo

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