Question

I’m new to c++.
I’d like to have an const array of a certain length. It is a rather long array so the standard method for initializing constant arrays, namely:

const bool some_array[] = {true, false, true};

is impractical. I’d like to initialize a constant array of boolean values. I’ve tried to implement it like this:

bool* sieve_of_eratosthenes(bool* n, n_length);

int main(){
    bool b[500];
    bool primeList[] = {sieve_of_eratosthenes(b, 500)};
}

this almost works, though it isn’t very elegant or space-efficient (i’d be open to comments on how better implement this array). The trouble is, the zeroth element of the returned array, i.e. primeList, is set to true/1, even though I’ve explicitly set the zeroth element in the boolean array returned by my s_of_e() function to false/zero.

primeList[0] == 1

when it should be:

primeList[0] == 0

since 0 isn’t prime…

Can anyone enlighten me as to why it’s doing this?

Answer 1

Your code,

bool* sieve_of_eratosthenes(bool* n, n_length);

int main(){
    bool b[500];
    bool primeList[] = {sieve_of_eratosthenes(b, 500)};
}

doesn’t work because

• the function returns a pointer, not an array, and

• even if the function returned a reference to an array, that would decay into a pointer in its use in the initializer list.

(Also it should not compile because implicit int has never been part of C++.)

The pointer is converted to bool, and that’s the only element your array gets.

Instead, do this:

#include <vector>
using namespace std;

vector<bool> sieve_of_eratosthenes( int const n_length )
{
    // Code here.
}

int main()
{
    std::vector<bool> const primeList = sieve_of_eratosthenes( 500 );
}

Note however that std::vector is designed to optimize the case of bool elements, with an ordinary C++ implementation using just 1 bit per element.

That can be seen as a feature or as a design level bug, depending on one’s view.