C++ : create a tuple with vector of strings

Question

I have the following API to be implemented:

tuple<vector<string>, ItemStatus> SomeFcn() const
{
    vector<string> result_item;
    return tuple(result_item, ItemStatus::ACTUAL );
}

But it does not compile and compile output is not clear at all

/usr/include/c++/9/bits/stl_pair.h:436:9: note: candidate: ‘template<class ... _Args1, long unsigned int ..._Indexes1, class ... _Args2, long unsigned int ..._Indexes2> std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>)’
  436 |         pair(tuple<_Args1...>&, tuple<_Args2...>&,
      |         ^~~~
/usr/include/c++/9/bits/stl_pair.h:436:9: note:   template argument deduction/substitution failed:

Something is wrong at tuple instance creation. I have this code compiled with -std=c++17. ItemStatus is an enum class.

What's wrong here?

Your question is tagged with C++11 (which does not have class template argument deduction) whereas you mention you are compiling with C++17: what language level are you using? — dfrib, Oct 26 '20 at 15:37
I have no compiler at hand currently, so I cannot test this: try wrapping ˋresult_itemˋ with parentheses in the return call. — Mike Lischke, Oct 26 '20 at 15:43
Did you simply try `return {result_item, ItemStatus::ACTUAL };` ? — Damien, Oct 26 '20 at 15:44
Have you tried [`std::make_tuple()`](https://en.cppreference.com/w/cpp/utility/tuple/make_tuple) yet? — Remy Lebeau, Oct 26 '20 at 15:47
Cannot reproduce, see: https://wandbox.org/permlink/QSlMKwScu5dLaEdn. Maybe you just need to remove that `const`, if it's a non-member function. — Paul Sanders, Oct 26 '20 at 15:51

score 4 · Answer 1 · answered Oct 26 '20 at 16:05

4

Basically, you have three options:

Copy-initialization :

return {result_item, ItemStatus::ACTUAL };

Create a tuple object (make_tuple) (AndersK's answer):

return std::make_tuple(result_item, ItemStatus::ACTUAL );

Use tuple ctor :

return std::tuple<std::vector<std::string>, ItemStatus>{result_item, ItemStatus::ACTUAL };

answered Oct 26 '20 at 16:05

Jose

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Would using `std::move` improve (or harm) the performance of any of these (assuming `result_item` is in fact a huge `vector`, since a function that always returns an empty one seems kind of pointless)? – ShadowRanger Oct 26 '20 at 16:10
2

@ShadowRanger: It's already a return statement. The compiler knows that local variables go out of scope and can safely be moved from. – MSalters Oct 26 '20 at 16:11
As @MSalters said, I wouldn't do that. In addition, highly related with copy elision and NRVO : https://stackoverflow.com/questions/12953127/what-are-copy-elision-and-return-value-optimization – Jose Oct 26 '20 at 16:18
Just to be explicit: you don't want to call `std::move` on the return expression. Calling `std::move` on the arguments to `std::make_tuple` on the other hand would make sense. – MSalters Oct 26 '20 at 16:25
@ShadowRanger: Thinking about it more, and seeing that there was no anwer on that yet, I've created a new question: https://stackoverflow.com/questions/64541229/stdmove-in-sub-expressions-of-return-statement – MSalters Oct 26 '20 at 16:34
@MSalters: Yeah, that's what I meant. `std::move` of the result of `std::make_tuple` is clearly pointless (copy-elision/RVO handles that), but whether it helps avoid deep-copies of the `vector` wasn't clear; can the compiler adequately optimize the call `std::make_tuple` with the knowledge that `result_item` is expiring, or is that knowledge not available or too complex for most compilers to handle, so `std::move` would be indicated? – ShadowRanger Oct 26 '20 at 16:45
@Jose: Looks like you should use `move` though; obviously you shouldn't `move` the `tuple` itself (copy-elision handles that), but the items being added to the `tuple` [should be `move`d to avoid being copied](https://stackoverflow.com/q/51521031/364696). – ShadowRanger Oct 26 '20 at 18:57

score 2 · Accepted Answer · answered Oct 26 '20 at 15:48

2

Write like this instead, assuming ItemStatus is an enum

std::tuple<std::vector<std::string>, ItemStatus> SomeFcn() const
{
  std::vector<std::string> result_item;
  return std::make_tuple(result_item, ItemStatus::ACTUAL );
}

(* i don't like leaving namespace out *)

answered Oct 26 '20 at 15:48

AndersK

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@Dmitry, then perhaps you should consider accepting the answer? – Elliott Oct 27 '20 at 07:23
ups, sorry - I upvoted but forgot to accept...my fault – Dmitry Oct 27 '20 at 08:10

C++ : create a tuple with vector of strings

2 Answers2