Modeling Initializer Lists in C++03

Question

I'm working on a tuple class for C++03. I want to be able to construct it from a list, which is tricky since the tuple's length can vary.

#ifndef N_TUPLE_H
#define N_TUPLE_H


template <typename T, int N>
class N_Tuple
{
 public:
  N_Tuple(T values[N]);
 private:
  T values_[N];
};


template <typename T, int N>
N_Tuple< T, N >::N_Tuple(T values[N])
{
  for (int i = 0; i != N; ++i)
    values_[i] = values[i];
}


#endif

Currently, I have to do this:

int arr[] = { 0, 1, 2 };
N_Tuple< int, 3 > t(arr);

It seems clunky and not very user-friendly. Is it possible to reduce this to a single line in C++03?

Thanks in advance.

According to the answer in stackoverflow.com/questions/13267277/… "In C++03 you can only use list-initialization for aggregates (C++03 [dcl.init.aggr]) and scalar (C++03 [dcl.init]/13) types" Which means you can't do it for arrays. — Jerry Jeremiah
– Jerry Jeremiah, Commented Jun 22, 2015 at 3:45
@JerryJeremiah "Which means you can't do it for arrays." Huh? Arrays are aggregates. — dyp
– dyp, Commented Jun 22, 2015 at 7:50
@Patrick Your tuple looks very unlike a std::tuple but much more like a std::array class. The boost.assign library contains list_of, which can be used to initialize a boost::array like this: boost::array<int, 3> arr = list_of(0)(1)(2); -- live demo — dyp
– dyp, Commented Jun 22, 2015 at 7:56

Luis Machuca · Accepted Answer · 2015-06-28 20:43:37Z

It seems clunky and not very user-friendly. Is it possible to reduce this to a single line in C++03?

Since you are basically imitating std::array, just rid of the constructor:

template <typename T, int N>
class N_Tuple
{
 public:
  T values_[N];
};

Note, in this, you have to make the values_ public.

That way you can initialize them just as you expect:

N_Tuple<int,3> p = {1,2,3};

This is valid in both C++03 and C++11, no extensions of any kind (Tr1, Boost or whatever) required.

Another alternative is to hide the helper array behind a macro. The basic idea is to initialize your helper array and from there your tuple-esque array. But since you'd want the array's contents to come after in the initializing notation (var x = contents) you'd have to use something like a prolog-epilog macro which will require some repetition and you'll have to be careful with the comma in your particular type's case:

SOME_MACRO_BEGIN(My_Sequence_type, t) = {1, 2, 3} 
SOME_MACRO_END(My_Sequence_type, t);

I've worked on such a solution that is compatible with both C++03 and C++11 (provided you do implement an initializer_list constructor for the C++11 case) without any particular requirements on the C++03 side of things.

(I am not sure how would it ever be possible to do it in a single macro, since a list of elements would contain commas which are processed specially for the macro and variadic macros are not a thing in C++03)

But unless your list is short or you abbreviate the names for the prolog-epilog macros a lot, it won't likely fit in a single line.

(Moreover, it still requires copying of the data, even though for the most simple kinds of types that won't ever matter)

Community · Accepted Answer · 2017-05-23 12:30:03Z

0

If you have access to Boost, you can use Assignment Library

There even is info on how to use it right here on SO.

edited May 23, 2017 at 12:30

CommunityBot

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answered Jun 22, 2015 at 17:07

MatiasFG

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