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Re: get_i [message #54273 is a reply to message #54272]

Wed, 17 June 2020 21:03

mirek
Messages: 13975
Registered: November 2005

Ultimate Member

Novo wrote on Wed, 17 June 2020 19:14

Another fix to avoid extra-copying ...

	template <class I, class V>
	void iter_set(I& t, V&& v)
	{
		*t = std::forward<V>(v);
	}
	
	template <class I, class V, typename... Args>
	void iter_set(I& t, V&& v, Args&& ...args)
	{
		*t++ = std::forward<V>(v);
		iter_set(t, std::forward<Args>(args)...);
	}
	
	template <class C, typename... Args>
	C gather(Args&& ...args)
	{
		C x;
		x.SetCount(sizeof...(args));
		auto iter = x.Begin();
		iter_set(iter, std::forward<Args>(args)...);
		return x;
	}

Thanks. Trunk version are a bit different now, can you review?

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Re: get_i [message #54274 is a reply to message #54273]

Wed, 17 June 2020 23:21

Novo
Messages: 1358
Registered: December 2006

Ultimate Contributor

mirek wrote on Wed, 17 June 2020 15:03

Thanks. Trunk version are a bit different now, can you review?

In your last commit you've added std::forward not to all places.
I've attached my version.
I also changed a functor signature in "void iter_get(I s, Args& ...args)".
Just a reference is fine there ...
IMHO, functors and iterators should be passed by reference to avoid extra-copying ...

Attachment: Fn.h
(Size: 3.28KB, Downloaded 148 times)

Regards,
Novo

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Re: get_i [message #54275 is a reply to message #54274]

Wed, 17 June 2020 23:59

Novo
Messages: 1358
Registered: December 2006

Ultimate Contributor

A more generic version of

template <typename... T>
constexpr const char *get_i(int i, const char* p0, const T& ...args)
{
	const char *list[] = { p0, args... };
	return list[clamp(i, 0, (int)sizeof...(args))];
}

	template <typename A, typename... T>
	constexpr A* get_i(int i, A* p0, const T& ...args)
	{
		A* list[] = { p0, args... };
		return list[clamp(i, 0, (int)sizeof...(args))];
	}

Example:

	const char* cr = get_i(1, "0", "11", "222");
	RLOG(cr);
	cr = get_i(1, "0", String("11"), "222");
	RLOG(cr);
	const wchar _0[] = {0};
	const wchar _3[] = {2, 2, 2};
	const wchar* wcr = get_i(1, _0, WString("11"), _3);
	RLOG(wcr);

Regards,
Novo

[Updated on: Thu, 18 June 2020 00:00]

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Re: get_i [message #54279 is a reply to message #54275]

Thu, 18 June 2020 08:39

mirek
Messages: 13975
Registered: November 2005

Ultimate Member

Thanks, applied...

Mirek

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Re: get_i [message #54282 is a reply to message #54279]

Thu, 18 June 2020 23:56

Novo
Messages: 1358
Registered: December 2006

Ultimate Contributor

mirek wrote on Thu, 18 June 2020 02:39

Thanks, applied...

Mirek

No problem.
There is still a bug with

template <typename P, typename... T>
constexpr const P *get_i(int i, const P* p0, const T& ...args)
{
	const char *list[] = { p0, args... };
	return list[clamp(i, 0, (int)sizeof...(args))];
}

It has to look like below.

template <typename P, typename... T>
constexpr const P *get_i(int i, const P* p0, const T& ...args)
{
	const P *list[] = { p0, args... };
	return list[clamp(i, 0, (int)sizeof...(args))];
}

Regards,
Novo

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Re: get_i [message #54348 is a reply to message #54247]

Mon, 29 June 2020 19:23

mirek
Messages: 13975
Registered: November 2005

Ultimate Member

I have just found that this fails with Visual C++ compiler:

	String n = " 2";
	ASSERT(decode(4, 1, "one", 2, "two", 3, "three", "unknown" + n) == String("unknown 2"));

The problem is that temporaty object gets destroyed too early...

I _believe_ this is a compiler error. In any case, it is pretty bad.

EDIT: Not a compiler bug. The problem is in your decode. Fixed it with

template <class T, class V>
constexpr const V& decode(const T& sel, const V& def)
{
	return def;
}

template <class T, class K, class V, typename... L>
constexpr const V& decode(const T& sel, const K& k, const V& v, const L& ...args)
{
	return sel == k ? v : decode(sel, args...);
}

template <class T>
constexpr const char *decode(const T& sel, const char *def)
{
	return def;
}

template <class T, class K, typename... L>
constexpr const char *decode(const T& sel, const K& k, const char *v, const L& ...args)
{
	return sel == k ? v : (const char *)decode(sel, args...);
}

[Updated on: Mon, 29 June 2020 23:08]

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Re: get_i [message #54350 is a reply to message #54247]

Thu, 02 July 2020 22:08

Novo
Messages: 1358
Registered: December 2006

Ultimate Contributor

Sorry for the late response.
My code is correct. Temporary String lives only during function call. This is how C++ works.
Return type is a value, not a reference. So, no temporaries ...

"The type of the ternary ?: expression is the common type of its second and third argument. If both types are the same, you get a reference back. If they are convertable to each other, one gets chosen and the other gets converted (promoted in this case). Since you can't return an lvalue reference to a temporary (the converted / promoted variable), its type is a value type."

Basically, the ternary ?: is needed to convert "const char[N]" and "const char[M]" to "const char*".

On the other side, templates is a complicated thing.
If you have a non-template version of "decode" declared before template instantiation point, compiler will choose it ...
Also MSVC is very well known for broken "two-phase name lookup". Even till these days, I believe ...
IMHO, a safer version would look like this:

namespace details {
	template <class T, class V>
	constexpr auto decode(const T& sel, const V& def)
	{
		return def;
	}
	
	template <class T, class K, class V, typename... L>
	constexpr auto decode(const T& sel, const K& k, const V& v, const L& ...args)
	{
		return sel == k ? v : details::decode(sel, args...);
	}
}
	
template <class T, class K, class V, typename... L>
constexpr auto decode(const T& sel, const K& k, const V& v, const L& ...args)
{
	return details::decode(sel, k, v, args...);
}

Regards,
Novo

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Re: get_i [message #54353 is a reply to message #54350]

Fri, 03 July 2020 09:38

mirek
Messages: 13975
Registered: November 2005

Ultimate Member

Novo wrote on Thu, 02 July 2020 22:08

Sorry for the late response.
My code is correct. Temporary String lives only during function call. This is how C++ works.

I would not be fixing if it was correct. Whole thing was actual error in actual application.

Indeed, temp string lives only during function call. What happened here is that in some circustances when mixing String and const char * parameters, const char * gets converted to String temporary, then back to const char *, then temporary is detroyed and dangling const char * returned.

Quote:

Return type is a value, not a reference. So, no temporaries ...

And that is exactly the problem. That return value is temporary one level up and gets converted to const char *....

It is very tricky indeed. Actually the version posted here was not final, it needed more fixes for other situations (namely enums). Hopefully trunk version is now ok. Full test is in "autotest/decode". Also the error only appears with MSC, but I have checked, the problem is not in the compiler.

[Updated on: Fri, 03 July 2020 10:10]

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