Attempting to create a dynamic array - c++

Question

I have the following piece of code, which is only half on the entire code:
// Declare map elements using an enumeration
enum entity_labels {
EMPTY = 0,
WALL
};
typedef entity_labels ENTITY;
// Define an array of ASCII codes to use for visualising the map
const int TOKEN[2] = {
32, // EMPTY
178 // WALL
};
// create type aliases for console and map array buffers
using GUI_BUFFER = CHAR_INFO[MAP_HEIGHT][MAP_WIDTH];
using MAP_BUFFER = ENTITY[MAP_HEIGHT][MAP_WIDTH];
//Declare application subroutines
void InitConsole(unsigned int, unsigned int);
void ClearConsole(HANDLE hStdOut);
WORD GetKey();
void DrawMap(MAP_BUFFER & rMap);
/**************************************************************************
* Initialise the standard output console
*/
HANDLE hStdOut = GetStdHandle(STD_OUTPUT_HANDLE);
if (hStdOut != INVALID_HANDLE_VALUE)
{
ClearConsole(hStdOut);
// Set window title
SetConsoleTitle(TEXT("Tile Map Demo"));
// Set window size
SMALL_RECT srWindowRect;
srWindowRect.Left = 0;
srWindowRect.Top = 0;
srWindowRect.Bottom = srWindowRect.Top + MAP_HEIGHT;
srWindowRect.Right = srWindowRect.Left + MAP_WIDTH;
SetConsoleWindowInfo(hStdOut, true, &srWindowRect);
// Set screen buffer size
COORD cWindowSize = { MAP_WIDTH, MAP_HEIGHT };
SetConsoleScreenBufferSize(hStdOut, cWindowSize);
}
/*************************************************************************/
/*************************************************************************
* Initialise the tile map with appropriate ENTITY values
*/
MAP_BUFFER tileMap;
for (unsigned int row = 0; row < MAP_HEIGHT; row++)
{
for (unsigned int col = 0; col < MAP_WIDTH; col++)
{
tileMap [row][col] = WALL;
}
}
Essentially the entire code is used to create a tile map and output it to screen but I'm attempting to make tileMap a dynamic array in runtime.
I have tried creating one down where the tileMap is being created.
I've tried creating one just after "entity_lables" are given the typedef "ENTITY".
I've tried creating one after the "MAP_BUFFER" and "GUI_BUFFER" become aliases.
But still I'm at a loss, I have no idea on how to successfully implement a dynamic array to tileMap, and I certainly don't know the best spot to put it.
Any help would be greatly appreciated.

Answer 1

The syntax you are using for defining your array is for a constant sized C array. In general you should shy away from C arrays unless the size of the data is determined at compile time(and never needs to change) and the array never leaves the scope(because a C array does not retain information on its own size.)
In place of constant or dynamically sized C arrays I would suggest to use the Vector container. The Vector is a dynamically sized container that fills up from the back, the last element you have added to
std::vector<std::vector<ENTITY>>
To add the vector container to your project add the line
#include <vector>
To fill the container your loop could look like:
MAP_BUFFER tileMap;
for (unsigned int row = 0; row < MAP_HEIGHT; row++)
{
std::vector<ENTITY> column; // A column of the tile map
for (unsigned int col = 0; col < MAP_WIDTH; col++)
{
column.push_back(WALL); // Add one element to the column
}
tileMap.push_back(column); // Add the column to the tile map
}
or you could initialize the Vector to the size you want at the beginning and use your current loop to assign the tile values:
using TILE_MAP = vector<vector<ENTITY>>;
// MAP_WIDTH x MAP_HEIGHT multidimensional vector
TILE_MAP tileMap(MAP_WIDTH, vector<ENTITY>(MAP_HEIGHT));
for (unsigned int row = 0; row < MAP_HEIGHT; row++)
{
for (unsigned int col = 0; col < MAP_WIDTH; col++)
{
tileMap [row][col] = WALL;
}
}
Calling an element of a vector after it has been filled has the same syntax as an array.
tileMap[2][4]
You can also check the length of the vector:
int rows = tileMap.size();
if( rows > 0 )
int columnsInRow0 = tileMap[0].size()
While you are at it you should look into other containers like Maps and Sets since they make your life easier.
Edit:
Since you want to know how to make a dynamic array not using a vector I will give you an answer: std::vector is the C++ defined dynamically sized array. C arrays will not change size after they are defined, vector will.
However I think you are asking about the ability to define runtime constant sized arrays. So I will explain what they are and why you should not use them.
When you define the C array you are probably getting a warning saying that the expression needs to be constant.
A C array is a pointer to the stack. And the implementation of the compiletime C array is that it needs to be a constant size at compile time.
int compiletimeArray[] = { 1, 2, 3 };
// turns out c arrays are pointers
int* ptr = compiletimeArray;
// prints 2
std::cout << compiletimeArray[1];
// prints 2
std::cout << ptr[1];
// prints 2
std::cout << *(compiletimeArray + 1);
// also prints 2
std::cout << *(ptr + 1); //move pointer 1 element and de-reference
Pointers are like a whiteboard with a telephone number written on it. The same kind of issues occur as with telephone numbers; number on whiteboard has been erased, number on whiteboard has changed, recipient does not exist, recipient changed their number, service provider running out of available numbers to give new users... Keep that in mind.
To get create a runtime constant sized array you need to allocate the array on the heap and assign it to a pointer.
int size = 4;
int* runtimeArray = new int[size]; // this will work
delete[] runtimeArray; // de-allocate
size = 8; // change size
runtimeArray = new int[size]; // allocate a new array
The main difference between the stack and heap is that the stack will de-allocate the memory used by a variable when the program exits the scope the variable was declared in, on the other hand anything declared on the heap will still remain in memory and has to be explicitly de-allocated or you will get a memory leak.
// You must call this when you are never going to use the data at the memory address again
// release the memory from the heap
delete[] runtimeArray; // akin to releasing a phone number to be used by someone else
If you do not release memory from the heap eventually you will run out.
// Try running this
void crashingFunction() {
while(true)
{
// every time new[] is called ptr is assigned a new address, the memory at the old address is not freed
// 90001 ints worth of space(generally 32 or 64 bytes each int) is reserved on the heap
int* ptr = new int[90001]; // new[] eventually crashes because your system runs out of memory space to give
}
}
void okFunction() {
// Try running this
while(true)
{
// every time new[] is called ptr is assigned a new address, the old is not freed
// 90001 ints worth of space is reserved on the heap
int* ptr = new int[90001]; // never crashes
delete[] ptr; // reserved space above is de-allocated
}
}
Why use std::vector? Because std::vector internally manages the runtime array.
// allocates for you
vector(int size) {
// ...
runtimeArray = new runtimeArray[size];
}
// When the vector exits scope the deconstructor is called and it deletes allocated memory
// So you do not have to remember to do it yourself
~vector() {
// ...
delete[] runtimeArray;
}
So if you had the same scenario as last time
void vectorTestFunction() {
// Try running this
while(true)
{
std::vector<int> vec(9001); // internally allocates memory
} // <-- deallocates memory here because ~vector is called
}
If you want to use a runtime constant array I suggest the std:array container. It is like vector in that it manages its internal memory but is optimized for if you never need to add new elements. It is declared just like vector but does not contain resizing functions after its constructor.