如何使用函数的输出初始化 const 数组结构字段？

Question

I have a struct that contains a const array, and would like to initialise it to specific values upon construction. Unfortunately, its contents depend on several parameters which are passed into the constructor as parameters, and require a function to compute the contents of the array.

What I'd ideally like to do looks something like this:

struct SomeType {
    const unsigned int listOfValues[32];

    unsigned int[32] processParameters(unsigned int parameter) {
        unsigned int arrayValues[32];
        for(int i = 0; i < 32; i++) {
            arrayValues[i] = i * parameter;
        }
        return arrayValues;
    }

    SomeType(unsigned int parameter) : listOfValues(processParameters(parameter)) {

    }
};

Of course there are several issues here (returning an array from a function is not possible, data type mismatches, etc). However, is there any way this is possible?

I've seen other similar questions suggest using a std::vector for this, but the heap allocation(s) this incurs is something my performance budget can't afford.

Answer 1

As Nathan suggested you should change the raw array with an std::array . This way you still have the benefit of stack allocation but now you can initialize from a copy.

using MyArray = std::array<unsigned int, 32>;

const MyArray listOfValues;

MyArray processParameters(unsigned int parameter) {

    MyArray arrayValues;

    for(int i = 0; i < 32; i++) {
        arrayValues[i] = i * parameter;
    }
    return arrayValues;
}

I removed the const from the array data type since it's not necesary because your array is const already, also with const unsigned int you wouldn't be able to set the values of arrayValues at run time.

Answer 2

Does this serve your purpose? No heap allocations that I can see.

struct SomeType {
    const unsigned int *listOfValues;

    const unsigned int * processParameters(unsigned int parameter) {
        for(int i = 0; i < 32; i++) {
            _listOfValues[i] = i * parameter;
        }
        return _listOfValues;
    }

    SomeType(unsigned int parameter) :
        listOfValues(processParameters(parameter))
    {

    }
    private:
        unsigned int _listOfValues[32];
};