C / C++ / MFC

Proper way to put a C++ object inside a struct initialization?

8-Dec-22 9:43

Alternatively you could use WIA (Windows Image Acquisition (WIA) - Win32 apps | Microsoft Learn)

arnold_w5-Dec-22 1:01

Re: Proper way to put a C++ object inside a struct initialization?

5-Dec-22 1:01

I working in an embedded environment (STM32 microcontroller) and I compile using GCC (STM32CubeIDE). I want to put a C++ object inside a struct and this is the only way I could get it to compile:

C++

namespace MyNamespace {
    class MyObject
    {
        int m_i;
public:
        MyObject(int i):m_i(i){}
        MyObject(const MyObject&) = delete;
    };
}

typedef struct {
    MyNamespace::MyObject*  MyObject;
} MyStruct_s;

static MyStruct_s MyStruct = {new MyNamespace::MyObject(0) };

However, I'm having strange hanging issues in my code and I suspect the code above might be the cause. Is the code above legal or will it corrupt RAM? If so, what's the correct way to do it? Ideally I'd like to have the whole object inside the struct, not just a pointer to it.

modified 5-Dec-22 8:23am.

CPallini5-Dec-22 1:15

Re: Proper way to put a C++ object inside a struct initialization?

5-Dec-22 1:15

As matter of fact, you don't need a pointer. Try

C++

#include <iostream>
using namespace std;

namespace MyNamespace
{
 class MyObject
 {
   int m_i;
public:
  MyObject(int i):m_i(i){}
  int get_i(){return m_i;}
  };
}


struct MyStruct
{
    MyNamespace::MyObject  myObject;
};

static MyStruct s_myStruct = { MyNamespace::MyObject(5) };


// this probably won't run on a microcontroller...
int main()
{
  cout << s_myStruct.myObject.get_i() << endl;
}

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

arnold_w5-Dec-22 2:05

Re: Proper way to put a C++ object inside a struct initialization?

5-Dec-22 2:05

I'm able to compile your code example. However, I just realized that I had forgotten the following line of code (I didn't think it was important) in my first post (I have added it now):

C++

MyObject(const MyObject&) = delete;

I'm not good at programming C++ so I'm not sure what that line does. After I have added this line, I get the following error message with your code example: "use of deleted function 'MyNamespace::MyObject::MyObject(const MyNamespace::MyObject&)"

CPallini5-Dec-22 2:54

Re: Proper way to put a C++ object inside a struct initialization?

5-Dec-22 2:54

That disables the (otherwise automatically generated) copy constructor. Not sure why you need that.
Have a look at https://stackoverflow.com/questions/34291938/why-and-when-delete-copy-constructor-and-operator[^].

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

arnold_w5-Dec-22 3:32

Re: Proper way to put a C++ object inside a struct initialization?

5-Dec-22 3:32

I'm not allowed to make changes inside the class, I'm only supposed to write code that uses that class.

Mircea Neacsu5-Dec-22 2:54

Mircea Neacsu

5-Dec-22 2:54

The line prohibits compiler from copying MyObject objects.

Look at the following code to better understand what's going on:

C++

namespace MyNamespace
{
class MyObject
{
  int m_i;
public:
  MyObject (int i) :m_i (i + 1) { cout << "MyObject constructor i=" << i << endl; }
  MyObject (const MyObject&) = delete; //these objects cannot be copied
  int get_i () const { return m_i; }
};
}


struct MyStruct
{
  MyStruct (const MyNamespace::MyObject& r) //tell compiler how to build a MyStruct
    : myObject (r.get_i ()) //cannot copy myObject so I just make a new one
    { cout << "MyStruct constructor" << endl; }

  MyNamespace::MyObject  myObject;
};

static MyStruct s_myStruct { MyNamespace::MyObject (5) };
//created a MyObject, then created a second one and finally finished construction of myStruct

// this probably won't run on a microcontroller...
int main ()
{
  cout << s_myStruct.myObject.get_i () << endl;
}

The output is:

MyObject constructor i=5
MyObject constructor i=6
MyStruct constructor
7

Mircea

Re: Proper way to put a C++ object inside a struct initialization?

CPallini5-Dec-22 7:44

Re: Proper way to put a C++ object inside a struct initialization?

5-Dec-22 7:44

Fine.

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

Mircea Neacsu5-Dec-22 1:30

Mircea Neacsu

5-Dec-22 1:30

Your code is correct but dangerous. You don’t control the time when myStruct is initialized. There are various methods to control it. My preferred method is the nifty counter[^].

In general, you can include objects in structures. As a matter of fact C++ makes little difference between the two. Without seeing the code, I suspect that your problem is that you don’t have a default constructor for your object. Let me explain: as you don’t have a default constructor for myStruct, the compiler will synthesize one that invokes the default constructor for any included non-trivial members. If your object doesn’t have a default constructor, the compiler complains.

Mircea

Re: Proper way to put a C++ object inside a struct initialization?

arnold_w5-Dec-22 2:07

MFC. How to display a bitmap or a .png file

5-Dec-22 2:07

I edited my initial post to also show the actual class.

BigSteve-O14-Nov-22 8:44

14-Nov-22 8:44

Good Morning.

In my view class in the onDraw() function, I am drawing the board and filling in the squares as follows:

	pDC->SetMapMode(MM_LOENGLISH);
	pDC->SetWindowExt(800, 800);
	//pDC->SetViewportOrg(testrec.Width(), testrec.Height());

	for (int i = 0; i < 8; i++)
	{
		for (int j = 0; j < 8; j++)
		{
			COLORREF color = pBC->getSquare(i, j);
			CBrush brush(color);
			int x1 = (j * 70) + 35;
			int y1 = (i * -70) - 35;
			int x2 = x1 + 70;
			int y2 = y1 - 70;
			CRect rect(x1, y1, x2, y2);
			pDC->FillRect(rect, &brush);
		}
	}
	// Now draw the borders
	for (int x = 35; x <= 595; x += 70)
	{
		pDC->MoveTo(x, -35);
		pDC->LineTo(x, -595);
	}
	for (int y = -35; y >= -595; y -= 70)
	{
		pDC->MoveTo(35, y);
		pDC->LineTo(595, y);
	}

This all works well, and I have my board coming up.

Now I want to draw the chess pieces on the board, and have written some code, just to load 1 bitgmap and display it on the screen. However, nothing gets drawn to the screen. (All I see is just the chessboard)

This is the test code to display my bitmap on the screen:

void CMyChessTestView::drawImage(CDC* pDC, int x, int y)
{

	CPaintDC dc(this); // device context for painting

	// TODO: Add your message handler code here
	CBitmap BmpLady;
	CDC MemDCLady;

	// Load the bitmap from the resource
	BmpLady.LoadBitmap(IDB_BISHOP_WHITE);
	// Create a memory device compatible with the above CPaintDC variable
	MemDCLady.CreateCompatibleDC(/*&dc*/pDC);
	// Select the new bitmap
	CBitmap *BmpPrevious = MemDCLady.SelectObject(&BmpLady);

	// Copy the bits from the memory DC into the current dc
	pDC->BitBlt(20, 10, 436, 364, &MemDCLady, 0, 0, SRCCOPY);

	// Restore the old bitmap
	pDC->SelectObject(BmpPrevious);
	// Do not call CView::OnPaint() for painting messages

The bitmap is not drwan to the screen. I have tried using both the CPaintDC dc or the pDC* but to no avail. I cannot get the bitmap to display.

Any suggestions will be greatly appreciated.

CPallini14-Nov-22 9:25

14-Nov-22 9:25

How do you call the drawImage method?

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

BigSteve-O14-Nov-22 9:57

14-Nov-22 9:57

// Test to see if I can draw the bitmap on the screen.
// Pass the pDC and a bougus x and y coordinate for testing purposes.
drawImage(pDC, 100, 100);

Victor Nijegorodov14-Nov-22 10:52

14-Nov-22 10:52

BigSteve-O wrote:
drawImage(pDC, 100, 100);

And where are you calling this method from?

BigSteve-O14-Nov-22 11:29

14-Nov-22 11:29

From the onDraw() funtion.

CPallini15-Nov-22 1:58

15-Nov-22 1:58

A similar code draws the bitmap, on my system. Why didn't you check the return value of the LoadBitmap call?

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

BigSteve-O16-Nov-22 13:45

16-Nov-22 13:45

It looks like loadBitmap returns NULL. I have no idea as to why though.

Do you have a code snippet you can share?

Victor Nijegorodov16-Nov-22 20:22

16-Nov-22 20:22

BigSteve-O wrote:
It looks like loadBitmap returns NULL. I have no idea as to why though.

According to Docs in CBitmap Class | Microsoft Learn :

Quote:
If the bitmap identified by lpszResourceName does not exist or if there is insufficient memory to load the bitmap, the function returns 0.

Victor Nijegorodov16-Nov-22 20:36

16-Nov-22 20:36

You can try to use LoadImage API instesd: LoadImageA function (winuser.h) - Win32 apps | Microsoft Learn

CPallini17-Nov-22 3:02

17-Nov-22 3:02

I used the sample code of this page: CDC Class - CDC::CreateCompatibleDC | Microsoft Learn[^].

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

BigSteve-O17-Nov-22 11:31

17-Nov-22 11:31

Thank You.

I copied the sample code over, and I have noticed the following:

1) If I draw the bitmap first, and then draw the board afterwards, I can see the board and the test bitmap.
2) If I draw the board first and then the test bitmap, I only see the board. The loadBitmap call is successfull)

CPallini17-Nov-22 19:57

17-Nov-22 19:57

That's rather strange. Do you use the same DC for the board and bitmap?

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

BigSteve-O18-Nov-22 12:36

18-Nov-22 12:36

Yes, I do.