How to initialize a const int array by a function in a header file?

Question

I can easily declare and initialize a constant array member in a header file like so:

class MyClass {
 public:
  const int arr[4] = {1, 2, 3, 4};
}

But when the data is defined by a function, I cannot initialize it in the header:

#include <cmath>
#define BASE 2
class MyClass {
 public:
  const int arr[4];
  for (i=0;i<4;i++) {
        arr[i] = pow(BASE, i);
  }
}

When I try to initialize the array in the class constructor in the .cpp file, I get the obvious uninitialized member with 'const' typeerror, as the array should already be initialized.

How can I initialize a const int array in the header file with a preprocessor macro and cmath functions?

If the values are known in advance you could just calculate them, and write just the result in the code. Maybe add some comments to explain the values. — Dialecticus, Jul 30 '17 at 22:22
You can't have a loop in a class definition outside of a function like that. — juanchopanza, Jul 30 '17 at 22:24
@Dialecticus they are not known in advance because they depend on preprocessor variables, as in this example BASE. — uzumaki, Jul 30 '17 at 22:24
er, `const` is not usually what you want for member variables ... — o11c, Jul 30 '17 at 22:25
Have you considered the fact an plain array is not the correct tool for the job? — StoryTeller - Unslander Monica, Jul 30 '17 at 22:25
And requiring a function call doesn't mean you need a loop https://ideone.com/WPZ3a1 (BTW, ditch those macros for type-safe `constexpr` variables). — StoryTeller - Unslander Monica, Jul 30 '17 at 22:27
@StoryTeller You do not realize that this is just an example. In reality the array might have thousands of entries and does need to be initialized in a loop. After all, that's what loops are made for! — uzumaki, Jul 30 '17 at 22:29
A side note: watch out for `pow` not always giving you the correct result. — PaulMcKenzie, Jul 30 '17 at 22:30
I don't really care what your use case is if you don't present it. Refer to my first comment. If you still insist on using a plain array despite all advice to the contrary, then write a custom preprocessor in your favorite scripting language to modify the source and generate the initializer. After all, that's what loops are made for! — StoryTeller - Unslander Monica, Jul 30 '17 at 22:32
@PaulMcKenzie What do you mean by "not correct"? The precision is not a problem in my case. A precision of +- 1 is ok as long as the boundaries are correct. — uzumaki, Jul 30 '17 at 22:33
@uzumaki [This is what I mean](https://stackoverflow.com/questions/25678481/why-does-pown-2-return-24-when-n-5-with-my-compiler-and-os). — PaulMcKenzie, Jul 30 '17 at 22:33
@uzumaki -- This section is for comments, and StoryTeller commented. — PaulMcKenzie, Jul 30 '17 at 22:37
If you are gonna be rude to whomever tries to spare you headaches, you deserve to maintain such code. — StoryTeller - Unslander Monica, Jul 30 '17 at 22:38

score 4 · Answer 1 · answered Jul 30 '17 at 22:35

It's possible to use BOOST_PP_REPEAT, if your array can have at most 256 elements (fewer if you're stuck with MSVC). Something like:

#define my_elem(z, n, data) pow(BASE, n)

const int data[4] = {BOOST_PP_REPEAT(4, my_elem, "ignored - extra data not needed")};

But you should really, really, ask yourself why you need a non-static butconst member variable, since that is almost never a useful thing to do, and places major limitations on what your program can do (e.g. it deletes the assignment operator).

score 0 · Answer 2 · answered Jul 30 '17 at 22:41

You can do something like this (if the array is not too long):

#include <iostream>
#include <cmath>
#include <functional>

constexpr int BASE = 2;

class A {
public:

    A(std::function<int(int)> f);

    const int arr[4];
};

A::A(std::function<int(int)> f) :
    arr{ f(1),f(2), f(3), f(4) }
{
}

int main() {
    auto f = [](int i) { return pow(BASE, i); };
    A a(f);
    for (const auto val : a.arr) {
        std::cout << val << std::endl;
    }
    return 0;
}

How to initialize a const int array by a function in a header file?

2 Answers2