CODEWITHSUNDEEP
c++c++11
Baruch
Baruch

Reputation: 21508

initialize an array of compile-time constant size with incrementing numbers

I have an array whose size is set using a compile-time constant (a pre-processor #define in my case). I need to initialize it using consecutive numbers at compile-time. How can I do this?

Simplified example:

#define ARR_SZ 5
struct C {
  C(int a) : a(a) {}
  int a;
};
C arr[ARR_SZ] = {{0},{1},{2},{3},{4}}; // This needs to adapt to any number

I can use C++11, but not newer (although I would be interested to learn of newer techniques even if I can't use them for this project)

Upvotes: 2

Views: 336

Answers (2)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170163

Since boost was mentioned in the comment section, here's another totally different solution based on Boost.PP. It's also entirely C++03.

#include <boost/preprocessor/repetition/repeat.hpp>
#include <boost/preprocessor/punctuation/comma_if.hpp>

#define ARR_SZ 5
struct C {
  C(int a) : a(a) {}
  int a;
};

#define INIT(z, n, d) BOOST_PP_COMMA_IF(n) C(n)

C arr[ARR_SZ] = { BOOST_PP_REPEAT(ARR_SZ, INIT, ?) };


int main () {

}

BOOST_PP_REPEAT will expand to INIT(z, 0, ?) ... INIT(z, 4, ?). The z is not relevant to our goal, and the ? token is just a placeholder. Since INIT in turn expands to C(n) for n from 0 to 4 (comma delimited), we get an initializer for a regular C style array.

Upvotes: 1

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170163

C++14 code (because of std::integer_sequence):

#include <type_traits>
#include <array>

#define ARR_SZ 5
struct C {
  C(int a) : a(a) {}
  int a;
};

template<int ...Is>
auto make_C_arr(std::integer_sequence<int, Is...>) -> std::array<C, sizeof...(Is)> {
    return {{ {Is}... }};
}

auto arr = make_C_arr(std::make_integer_sequence<int, ARR_SZ>{});

int main () {

}

std::integer_sequence and the like are implementable in C++11 however as noted in a comment, so substituting the standard version for a home-brewed one will give a C++11 specific solution.

Upvotes: 4

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