How to automatically fill a std::array base on sizeof a variadic template?

Question

In this simplified code :

template <int... vars>
struct Compile_Time_Array_Indexes
{
    static std::array < int, sizeof...(vars)> indexes;//automatically fill it base on sizeof...(vars)
};
template <int ... vars>
struct Compile_Time_Array :public Compile_Time_Array_Indexes<vars...>
{
};

I want to automatically fill indexes base on the vars... size .

Example :

Compile_Time_Array <1,3,5,2> arr1;//indexes --> [0,1,2,3]
Compile_Time_Array <8,5> arr2;   // indexes --> [0,1]

Any idea ?

C++14 brought std::integer_sequence (though if these are indices, you might consider using std::size_t, which has a nice premade std::index_sequence for it). — Qaz
– Qaz, Commented Jun 10, 2014 at 18:00
something like this Q&A? (also works without constexpr IIRC) — TemplateRex
– TemplateRex, Commented Jun 10, 2014 at 18:26

nosid · Accepted Answer · 2014-06-10 18:24:49Z

7

The following definition apparently works with GCC-4.9 and Clang-3.5:

template <typename Type, Type ...Indices>
auto make_index_array(std::integer_sequence<Type, Indices...>)
    -> std::array<Type, sizeof...(Indices)>
{
    return std::array<Type, sizeof...(Indices)>{Indices...};
}

template <int... vars>
std::array<int, sizeof...(vars)> 
Compile_Time_Array_Indexes<vars...>::indexes
    = make_index_array<int>(std::make_integer_sequence<int, sizeof...(vars)>{});

edited Jun 10, 2014 at 18:24

answered Jun 10, 2014 at 18:02

nosid

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7 Comments

Qaz Over a year ago

This doesn't fill the array with indices, though. Rather, it fills array with the same numbers as in the parameter pack.

nosid Over a year ago

@chris: You are right. I didn't see that in the OP's question. I have fixed the answer.

Qaz Over a year ago

@OP, Just so you know, there are many implementations of such an integer sequence for C++11 such that you could grab one and use it with this code.

uchar Over a year ago

@nosid I'm trying to compile it in vs 2013 I add an implementation of make_integer_sequence but this line give me an error Compile_Time_Array_Indexes<vars...>::indexes : missing ';' before '<'

Mooing Duck Over a year ago

@xyz Here, I used the standard "indecies trick" to make this work in C++11: coliru.stacked-crooked.com/a/0113a31abe51b68f. It ought to work in VS2013.

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