how to create a dynamic bitmap array?

Question

How to create a dynamic bitmap array?

I tried it like this:

Bitmap* bit = new int(5);

But it didn't work out.

After, I tried like this:

Bitmap* bit = (Bitmap*)malloc(6);
bit[5] = new Bitmap(sFileName);

But it didn't work, too.

EDIT

In .h, I have:

std::shared_ptr<CachedBitmap> *CachedBitmap;

In .cpp (WM_PAINT):

Bitmap* bit = new Bitmap(L"file");
Graphics graphics(hdc);
(CachedBitmap[0]) = PNGLoader::createCachedBitmap(bit, hdc);
graphics.DrawCachedBitmap(CachedBitmap[0].get(), position.x, position.y);

std::shared_ptr<CachedBitmap> PNGLoader::createCachedBitmap(Bitmap* originalBitmap, HDC hdc)
{

    Graphics graphics(hdc);
    return std::make_shared<CachedBitmap>(originalBitmap, &graphics);
}

When I run the program, I have an exception error because CachedBitmap[0] is nullptr.

I tried:

(**CachedBitmap) = std::make_shared<::CachedBitmap>((new ::CachedBitmap(bit, &graphics))[5]);
(*CachedBitmap[0]) = PNGLoader::createCachedBitmap(bit, hdc);
graphics.DrawCachedBitmap(CachedBitmap[0]->get(), position.x, position.y);

But I have error:

Gdiplus::CachedBitmap::CachedBitmap: unable to access private member declared in "Gdiplus" class::CachedBitmap"

Remy Lebeau · Accepted Answer · 2022-03-11 16:02:29Z

1

Bitmap* bit = new int(5);

This doesn't work because you are dynamically allocating a single int whose value is 5, and then trying to assign the address of that int to a Bitmap* pointer. Obviously, an int is not a Bitmap, so the compiler does not allow this assignment.

You would need this instead:

Bitmap* bit = new Bitmap[5];

However, that doesn't work either, because GDI+'s Bitmap class does not have a default constructor. So, you need an array of Bitmap* pointers, not an array of Bitmap objects. Which leads us to the next example...

Bitmap* bit = (Bitmap*)malloc(6);
bit[5] = new Bitmap(sFileName);

This doesn't work because you are allocating only 6 bytes total, but you would need to allocate enough bytes for 6 Bitmap* pointers in order for bit[5] to hold a valid Bitmap* pointer, eg:

Bitmap** bit = (Bitmap**) malloc(sizeof(Bitmap*) * 6);
bit[0] = new Bitmap(sFileName);
bit[1] = ...;
...
bit[5] = ...;

But, don't use malloc() in C++, use new[] instead:

Bitmap** bit = new Bitmap*[6];
bit[0] = new Bitmap(sFileName);
bit[1] = ...;
...
bit[5] = ...;

And don't forget to delete whatever you new when you are done using it:

delete bit[0];
delete bit[1];
...
delete bit[5];
delete[] bit;

But, do you really need the array itself to be allocated dynamically?

Bitmap* bit[6];
...
bit[0] = new Bitmap(sFileName);
bit[1] = ...;
...
bit[5] = ...;
...
delete bit[0];
delete bit[1];
...
delete bit[5];

However, in modern C++ coding, avoid new/delete manually, use standard containers and smart pointers to handle all of the memory management for you, eg:

std::vector<std::unique_ptr<Bitmap>> bit;
...
bit.resize(5);
bit[0] = std::make_unique<Bitmap>(sFileName);
bit[1] = ...;
...

Or:

std::unique_ptr<Bitmap> bit[5];
...
bit[0] = std::make_unique<Bitmap>(sFileName);
bit[1] = ...;
...

UPDATE:

std::shared_ptr<CachedBitmap> *CachedBitmap;

There is no good reason to have a pointer to a smart pointer. Drop the * from the declaration:

std::shared_ptr<CachedBitmap> CachedBitmap;
...
Bitmap bit(L"file");
Graphics graphics(hdc);
CachedBitmap = PNGLoader::createCachedBitmap(&bit, hdc);
graphics.DrawCachedBitmap(CachedBitmap.get(), position.x, position.y);

Or, if you really need an array of smart pointers:

std::vector<std::shared_ptr<CachedBitmap>> CachedBitmap;
...
CachedBitmap.resize(5);
Bitmap bit(L"file");
Graphics graphics(hdc);
CachedBitmap[0] = PNGLoader::createCachedBitmap(&bit, hdc);
graphics.DrawCachedBitmap(CachedBitmap[0].get(), position.x, position.y);

Or:

std::shared_ptr<CachedBitmap> CachedBitmap[5];
...
Bitmap bit(L"file");
Graphics graphics(hdc);
CachedBitmap[0] = PNGLoader::createCachedBitmap(&bit, hdc);
graphics.DrawCachedBitmap(CachedBitmap[0].get(), position.x, position.y);

edited Mar 11, 2022 at 16:02

answered Mar 10, 2022 at 20:34

Remy Lebeau

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6 Comments

Y0MMY Over a year ago

thx, but for feature in .h i have: Bitmap* gifBitmap; how init. in .cpp? *gifBitmap = new Bitmap [6]; is not working

Y0MMY Over a year ago

i edit my ques. with new problem

Remy Lebeau Over a year ago

@Y0MMY I have updated my answer. But it seems you have a fundamental lack of understanding of what pointers actually are and how they work, so I strongly suggest you stop what you are doing and get some decent C++ books that will cover this topic in more detail for you.

Y0MMY Over a year ago

ok, thx for books, but how init. with new? for example, I get some value, and then I insert it to initialize the array. int val = GetValue(); std::shared_ptr<CachedBitmap> CachedBitmap[val];

Remy Lebeau Over a year ago

@Y0MMY std::shared_ptr<CachedBitmap> *CachedBitmap = new std::shared_ptr<CachedBitmap>[val]; but that is trivial if you use std::vector instead, like I showed in my answer.

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