C++ assignment error [closed] - c++

Question

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Closed 9 years ago.
I am new to C++ and I have the code below. I am trying to assign int board[] = create_board();, like I would in java, but it gives me the error
Initializer failed to determine size of board.
Also when I remove that line of code, my print statement is not shown in the command prompt regardless.
Is there something that I am doing wrong in printing the array as well?
Code:
#include <iostream>
using namespace std;
int* create_board() {
int* pointer;
int board[15];
pointer=board;
for(int i=0; i<16; i++)
board[i] = 1;
return pointer;
}
int main () {
int board[] = create_board();
for (int i; i<16; i++)
std::cout << board[i];
return 0;
}

Answer 1

As a beginner, I think you should actually be using std::vector instead of raw pointers. Here is an example of your program that uses it:
#include <iostream>
#include <vector>
std::vector<int> create_board()
{
return std::vector<int>(16, 1); // declare 16 integers whose value is 1
}
int main()
{
std::vector<int> board = create_board();
for (int i = 0; i < 16; ++i)
std::cout << board[i];
}
In C++11, this might look like the following:
#include <iostream>
#include <array>
int main()
{
std::array<int, 16> array{};
array.fill(1);
for (auto var : array) std::cout << var << '\n';
}

Answer 2

Two things wrong really:
You can't use an array as an lvalue, instead you need to use a pointer. E.g.
int* board = create_board();
The bigger issue, however, is that in your create_board() function you are returning a pointer to a stack-based array that will be destroyed as soon as your function returns. You need to either allocate this using malloc or new, or make it static in order to return a pointer to it that can be used outside the function.

Answer 3

Memory management is very tricky. You should read more about it before trying to mess with manual memory. I'd recommend sticking with stl collections and the like for now.
As for the actual question, the array you declared lives on the stack, meaning that it disappears as soon as the function returns. This means that the pointer you returned is now a dangling pointer, meaning that accessing it gives undefined behavior.
You've also got an off by one error. You made an array of size 15, but you're accessing 16 elements of it.
Here's how things might look if you were using a std::array (essentially a type safe, collection like wrapper over raw arrays) and returned by value instead of pointer (keep things simple for now).
For normal arraylike collections, you'd use a std::vector, but for a small fixed size array like this, you might as well just use std::array. Vectors are the C++ equivalent of Java's ArrayList, so they can change size.
#include <iostream>
#include <array>
std::array<int, 16> create_board()
{
std::array<int, 16> board;
for(int i=0; i<16; i++)
board[i] = 1;
return board;
}
int
main ()
{
std::array<int, 16> board = create_board();
for (int i; i<16; i++)
std::cout << board[i];
return 0;
}