Can you initialize a static array with a function? [duplicate]

Question

I have a class with a const static C-array in it

// Header file
class test
{
    const static char array[];
};

I am trying to initialize it somewhere in the code but not in the straight-forward way like

// Source file
const char test::array[] = {'1','2','3'};

because the values are calculated using some other constant values so i need to use a function to do that. something like

CONST_VALUE = 4;
void func(int a[3]){
    a[0]=2*CONST_VALUE;
    a[1]=10*CONST_VALUE;
  ...
}

The point is that i don't know where to define and use such a function, should it be a member function? a global function? and when should i call it so it happens only once?

It's very easy if you have a std::array instead, as then you could call a function which returns the array so it can be used in the initialization (i.e. const std::array<char, 3> test::array = function_for_initialization();) — Some programmer dude
– Some programmer dude, Commented Aug 12, 2019 at 8:37
Is it your intent to have the array size deduced from the initializer? — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Aug 12, 2019 at 8:41
@shlmig maybe you can elaborate why a c-style array is required here ? as you may access the array of the vector with the data() function. — darune
– darune, Commented Aug 12, 2019 at 9:13

pptaszni · Accepted Answer · 2019-08-12 09:15:52Z

If you want to use dynamic allocation it might look like that:

class test
{
    const static char* array;
};
char* foo(int arr_size) {
    char* arr = new char[arr_size]();
    // other logic
    return arr;
}
const char* test::array = foo(10);

There is no practical need to delete this memory, unless you want to use your module as a shared library loaded and unloaded in runtime.

For the fixed size array I think you should use std::array:

#define ARR_SIZE 4
class test2
{
    const static std::array<char, ARR_SIZE> array;
};
std::array<char, ARR_SIZE> foo2() {
    std::array<char, ARR_SIZE> arr;
    arr[0] = 'x';
    // other logic
    return arr;
}
const std::array<char, ARR_SIZE> test2::array = foo2();

Finally if you must deal with C-style array, you can try the hack with function calculating each element separately:

class test3
{
    const static char array[ARR_SIZE];
};

char calculateElement(int index) {
    if (index == 0) {
        return 'x';
    } else if (index == 1) {
        return 'y';
    }
    // rest of the logic
    return 0;
}
const char test3::array[ARR_SIZE] = {
    calculateElement(0),
    calculateElement(1),
    calculateElement(2),
    calculateElement(3) };

Nikos C. · Accepted Answer · 2019-08-12 09:16:07Z

0

If all func() does is assignment, then you can convert these assignments to initializer values:

const char test::array[] {
    2 * CONST_VALUE,
    10 * CONST_VALUE
};

But if func() has other logic in it, then you can use an std::array instead:

std::array<char, 3> func()
{
    std::array<char, 3> ar{};
    ar[0]=2*CONST_VALUE;
    ar[1]=10*CONST_VALUE;
    // ...

    return ar;
}

class test {
    static const std::array<char, 3> array;
};

const std::array<char, 3> test::array = func();

answered Aug 12, 2019 at 9:16

Nikos C.

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Comments

darune · Accepted Answer · 2019-08-12 09:21:01Z

0

Using a c-style array

Just initialize it straight away:

// Header file
class test
{
    static inline const char array[]{2 * CONST_VALUE, 10 * CONST_VALUE};
};

Using a `std::array`

// Header file
class test
{
    static inline const std::array<2, char>{2 * CONST_VALUE, 10 * CONST_VALUE};
};

Using a `std::vector`

A vector is just a simpler solution than using an array:

// Header file
class test
{
    static inline const std::vector<char> array{2 * CONST_VALUE, 10 * CONST_VALUE};
};

edited Aug 12, 2019 at 9:21

answered Aug 12, 2019 at 8:54

darune

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3 Comments

463035818_is_not_an_ai Over a year ago

why std::vector instead of std::array ?

darune Over a year ago

@formerlyknownas_463035818 std::array is possible, but vector is just simpler sometimes - no need to worry about constant size etc. - and here it seems irrelevant

463035818_is_not_an_ai Over a year ago

using the right tool for the job is never irrelevant, everything seems to be constant, so std::vector does add unnecessary overhead. just saying...

Generic Name · Accepted Answer · 2019-08-12 11:53:24Z

0

If you use std::array instead of C-style array you can use a constexpr lambda to initialize it. See Compiler Explorer https://godbolt.org/z/izhWcR

class test
{
    constexpr static std::array<char, 2> mArray1 = [](){ 
        char v = const_a + const_c;
        return std::array<char, 2>{ const_a, v }; 
    }();
};

edited Aug 12, 2019 at 11:53

answered Aug 12, 2019 at 11:46

Generic Name

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