rand function not "defined"

I am currently writing my first game (Snake) in c++ using visual Studio, I am attempting to set the blocks you eat at random areas on the screen however when I use the rand() function it says it is undefined, does anyone know why I have this error?

#include <iostream>
using namespace std;
bool gameOver;
const int WIDTH = 20;
const int HEIGHT = 20;
int x, y, foodX, foodY, score;
enum eDirection { STOP = 0, LEFT, RIGHT, UP, DOWN};
eDirection dir;
void Setup(){
    gameOver = false;
    dir = STOP;
    x = WIDTH /2;
    y = HEIGHT /2;
    //rand function below is not defined??
    //I thought the function was built in

    foodX = rand() % WIDTH;
    foodY = rand() % HEIGHT;

}

void Draw() {

}
void Input() {


}
void Logic() {


}
int main()
{
    Setup();
    while (!gameOver)
    {
        Draw();
        Input();
        Logic();
    }


    return 0;
}