c++ explicit multi-arg constructor ambiguity

Question

I'm encountering an unexpected issue in some code I'm writing now and I'm not sure which compiler is correct.

We have a multi-argument constructor which takes const char*, const char*, but it is declared explicit:

constexpr explicit Wrapper(const char* a, const char* b)  : pair(a,b){}

And then we have a function which takes Wrapper and an overload which takes a std::pair<const char*, const char*>

void q(Wrapper w);
void q(std::pair<const char *, const char *> w);

And then we have code like this, which I would expect to call the second overload:

q({"a", "b"});

This compiles fine on clang, but fails to compile on both GCC and MSVC. I've been trying to look for any mention of explicit multi-arg constructor in the standard and if there's anything mentioning this ambiguity but I haven't found the relevant text. I'm just wondering which behavior is correct and which is wrong?

godbolt link: https://godbolt.org/g/2aYUov

I think the ``explicit`` doesn't count here since it's a constructor that take two arguments and not a constructor that take a pair of argument. — nefas, Jul 25 '17 at 08:18
@nefas according to this other stackoverflow post (https://stackoverflow.com/questions/1118680/explicit-keyword-on-multi-arg-constructor?rq=1) it should count in c++11 and later — Morten242, Jul 25 '17 at 08:20
with ``g++ 7.1.0``, with ``-std=c++1z``, I have the same error — nefas, Jul 25 '17 at 08:36
@Shirkam 7.1 on the godbolt link. Also tested with 5.4.0 and 6.3.0 locally. — Morten242, Jul 25 '17 at 08:37
Seems like GCC has not implemented that difference, as clang. — Shirkam, Jul 25 '17 at 09:02
Why are you tagging C++11 and linking to compilers following C++14 standard? — Passer By, Jul 25 '17 at 09:10
@Shirkam can you point to the documentation that says GCC has not implemented the "difference, as clang" please ? — sop, Jul 25 '17 at 09:16
@PasserBy it's a c++11 feature. I wrote -std=c++14 out of habit. The results are the same when corrected to c++11 — Morten242, Jul 25 '17 at 09:21
@Morten242 Tag C++11 only when you want to ask specifically about C++11 — Passer By, Jul 25 '17 at 09:22
@sop I said "it seems" I'm looking for explicit confirmation. If any, I will edit this comment. — Shirkam, Jul 25 '17 at 09:52

score 2 · Answer 1 · answered Aug 09 '17 at 17:03

Using your provided constructor for Wrapper, g++ 7.1.1 gives me the following error:

main.cpp: In function ‘int main()’:
main.cpp:29:25: error: converting to ‘Wrapper’ from initializer list would use explicit constructor ‘constexpr Wrapper::Wrapper(const char*, const char*)’
     Wrapper w({"a", "b"});
                         ^

So it seems that the explicit keyword on the Wrapper constructor is well taken into account when manually triggering a conversion.

However, the error on the call to q seems to indicate that overload resolution is ignoring the explicit keyword:

main.cpp:34:17: error: call of overloaded ‘q(<brace-enclosed initializer list>)’ is ambiguous
     q({"a", "b"});
                 ^
main.cpp:16:6: note: candidate: void q(Wrapper)
 void q(Wrapper w)
      ^
main.cpp:21:6: note: candidate: void q(std::pair<const char*, const char*>)
 void q(std::pair<const char *, const char *> w)
      ^

This could be a bug in g++, which would need to be verified with other sources.

c++ explicit multi-arg constructor ambiguity

1 Answers1