How to initialize an std::array in the initializer list of a class without hardcoding the array's size

Question

I've got Foo.h:

#include <array>

class Bar {
public:
  Bar(std::string name) : name(name) {}

  std::string name;

};

class Foo {
public:
  enum {ARRAY_SIZE=10};

  Foo();

  void printElement(int idx);

  std::array<Bar,ARRAY_SIZE> myArray;

};

Foo.cc:

#include "Foo.h"
#include <iostream>

Foo::Foo(): myArray({Bar("9"),Bar("8"),Bar("7"),Bar("6"),Bar("5"),
                     Bar("4"),Bar("3"),Bar("2"),Bar("1"),Bar("0")}) {}


void Foo::printElement(int idx) {
  if (idx < ARRAY_SIZE) {
    std::cout << "Value is " << myArray[idx].name << std::endl;
  } else {
    std::cout << "Index out of bounds" << std::endl;
  }
}

int main() {
  Foo foo;
  foo.printElement(1);
}

The problem is that the {Bar("9"),Bar("8"),Bar("7"),Bar("6"),Bar("5"),Bar("4"),Bar("3"),Bar("2"),Bar("1"),Bar("0")} is too literal, I need to be able to use an expression which expands to the right size based on the value of ARRAY_SIZE (which could live in an external header file).

Note that in this example I must initialize myArray in the initializer list, otherwise I get this:

(...)/gcc/6.3.0/include/c++/6.3.0/array:90:12: error: no matching function for call to 'Bar::Bar()'

What's the best way to do it?

I'm using g++ 6.3.0 by the way.

myarray has the "right size" (i.e. ARRAY_SIZE) when you declare it. That is, you don't need to initialize a std::array to give it a size. The size is part of the array type. — mfnx
– mfnx, Commented Dec 12, 2019 at 16:24
Is the ARRAY_SIZE always fixed? Why not have it as a template parameter? — Jaideep Shekhar
– Jaideep Shekhar, Commented Dec 12, 2019 at 16:38
@Jasancos: This site has an edit history; when you update your question you don't need to keep old versions. That just confuses new readers. — MSalters
– MSalters, Commented Dec 12, 2019 at 16:55

NathanOliver · Accepted Answer · 2019-12-12 17:27:12Z

4

Use a helper function. Add

template <std::size_t N>
auto make_array()
{
    std::array<Bar, N> arr;
    for (std::size_t i = 0; i < N; ++i)
        arr[i] = std::to_string(N - i - 1);
    return arr;
}

To your class and then you can use it in the member initializer list like

Foo::Foo(): myArray(make_array<10>()) {}

This only works through if Bar is default constructable.

For your code what you can do is add another helper that has a std::index_sequence of values to construct the Bar's from and that would look like

// helpers for reversing an integer sequence
template <std::size_t ... Is>
constexpr auto indexSequenceReverse (std::index_sequence<Is...> const &)
   -> decltype( std::index_sequence<sizeof...(Is)-1U-Is...>{} );

template <std::size_t N>
using makeIndexSequenceReverse
   = decltype(indexSequenceReverse(std::make_index_sequence<N>{}));

class Bar {
public:
  Bar(std::string name) : name(name) {}

  std::string name;

};

class Foo {
public:
  enum {ARRAY_SIZE=10};

  Foo();

  void printElement(int idx);

  std::array<Bar,ARRAY_SIZE> myArray;

  // get sequence and expand out initializes 
  template <std::size_t ... Is>
  auto make_array_impl(std::integer_sequence<size_t, Is...>)
  {
      return std::array{ Bar{std::to_string(Is)}... };
  }

  // convenient forwarder to implementation
  template <std::size_t N>
  auto make_array()
  {
      return make_array_impl(makeIndexSequenceReverse<N>{});
  }

};

Foo::Foo(): myArray(make_array<10>()) {}

void Foo::printElement(int idx) {
  if (idx < ARRAY_SIZE) {
    std::cout << "Value is " << myArray[idx].name << std::endl;
  } else {
    std::cout << "Index out of bounds" << std::endl;
  }
}

int main() {
  Foo foo;
  foo.printElement(1);
}

edited Dec 12, 2019 at 17:27

answered Dec 12, 2019 at 16:18

NathanOliver

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11 Comments

Jasancos Over a year ago

Many thanks for the solution, but my original example didn't describe my problem. I've edited the question accordingly

NathanOliver Over a year ago

@Jasancos No problem. I've updated the answer to work for Bar's

MSalters Over a year ago

If you want Bar("nine"), Bar("eight"), etc you'll need a second helper function instead of std::to_string but this indeed is the right solution.

NathanOliver Over a year ago

@Jasancos There isn't one. That function works like std::declval works. It exists solely to get a type that can be used. It's a template meta programming technique.

NathanOliver Over a year ago

@Jasancos If you can use C++17 the code will compile as is thanks to CTAD (Class Template Argument Decuction). If you can't use C++17 then you need return std::array<Bar, sizeof...(Is)>{ Bar{std::to_string(Is)}... };

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