Re: Is MFC reusing memory?

From:

=?Utf-8?B?QVdpbGxLZXk=?= <AWillKey@discussions.microsoft.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 12 Feb 2007 00:16:00 -0800

Message-ID:

<2BC1DA88-ED63-42D9-A88C-67C11F99ACF9@microsoft.com>

Joseph,

You don't say how you verified that all the memory was released. I am not
familiar with the assembler and in any case you seem to be relying on
~CString to do the job, which is exactly what I am suggesting it doesn't do.

Here's what I did this morning.

I created a brand new MFC .exe project using the wizard (called testproj).
Then I changed the document view file so add some code to OnDraw.

void myDraw(CDC* pDC)
{
    CSize cs = pDC->GetTextExtent("ABCDEFG");
}

/////////////////////////////////////////////////////////////////////////////
// CTestprojView drawing

void CTestprojView::OnDraw(CDC* pDC)
{
    _CrtMemState ms;
    CTestprojDoc* pDoc = GetDocument();
    ASSERT_VALID(pDoc);
    // TODO: add draw code for native data here
    int i;
    for (i=0;i<1000;i++)
    {
        myDraw(pDC);
        _CrtMemCheckpoint(&ms);
        _RPT1(_CRT_WARN,"Total allocated: %i\r\n",ms.lTotalCount);
    }
}

I added #include <crtdbg.h> at the top of the file and stepped through the
code with debugger looking at the messages telling me the total amount
allocated after each call to myDraw(). On my computer this increased by 20
bytes on each call.

"Joseph M. Newcomer" wrote:

Well, I know this works in VS6. In fact, here's the code:

VS6 DEBUG

00401991 68 D4 53 41 00 push offset string "ABCDEF" (004153d4)
00401996 8D 4D C8 lea ecx,[ebp-38h]
00401999 E8 4A 05 00 00 call CString::CString (00401ee8)
0040199E C6 45 FC 02 mov byte ptr [ebp-4],2
004019A2 8D 45 C8 lea eax,[ebp-38h]
004019A5 50 push eax
004019A6 8D 4D C0 lea ecx,[ebp-40h]
004019A9 51 push ecx
004019AA 8D 4D D4 lea ecx,[ebp-2Ch]
004019AD E8 30 05 00 00 call CDC::GetTextExtent (00401ee2)
004019B2 8B 10 mov edx,dword ptr [eax]
004019B4 8B 40 04 mov eax,dword ptr [eax+4]
004019B7 89 55 CC mov dword ptr [ebp-34h],edx
004019BA 89 45 D0 mov dword ptr [ebp-30h],eax
004019BD C6 45 FC 01 mov byte ptr [ebp-4],1
004019C1 8D 4D C8 lea ecx,[ebp-38h]
004019C4 E8 37 05 00 00 call CString::~CString (00401f00)

VS6 RELEASE:

00401379 68 20 30 40 00 push offset string "ABCDEF" (00403020)
0040137E 8D 4C 24 10 lea ecx,[esp+10h]
00401382 C7 44 24 38 01 00 00 mov dword ptr [esp+38h],1
0040138A E8 95 03 00 00 call CString::CString (00401724)
0040138F 8B 44 24 0C mov eax,dword ptr [esp+0Ch]
00401393 8D 54 24 10 lea edx,[esp+10h]
00401397 52 push edx
00401398 8B 48 F8 mov ecx,dword ptr [eax-8]
0040139B 51 push ecx
0040139C 50 push eax
0040139D 8B 44 24 2C mov eax,dword ptr [esp+2Ch]
004013A1 50 push eax
004013A2 FF 15 00 20 40 00 call dword ptr [__imp__GetTextExtentPoint32A@16
(00402000)]
004013A8 8D 4C 24 0C lea ecx,[esp+0Ch]
004013AC E8 7F 03 00 00 call CString::~CString (00401730)
joe

On Sun, 11 Feb 2007 15:25:02 -0800, AWillKey <AWillKey@discussions.microsoft.com> wrote:

Unfortunately I don't have VC2005. I am using Visual Studio C++ Version 6.0.

Alan

"Joseph M. Newcomer" wrote:

I find this asserstion somewhat less than credible. I just tried both the Debug and
Release versions of VS2005, calling GetTextExtent. It created a CString, and deleted it.
In fact, the code is (snapshot from the running code)

DEBUG code:
    CSize sz = dc.GetTextExtent(_T("ABCDEFG"));
004128F5 mov esi,esp
004128F7 push offset string L"ABCDEFG" (41CDE8h)
004128FC lea ecx,[ebp-130h]
00412902 call dword ptr [__imp_ATL::CStringT
                          <wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >::
                  CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >
                           (42185Ch)]
-----
or eliminating the fluff, it just called CString::CString(_T("ABCDEFG"));
-----
00412908 cmp esi,esp
0041290A call @ILT+1370(__RTC_CheckEsp) (41155Fh)
0041290F mov byte ptr [ebp-4],2
00412913 lea eax,[ebp-130h]
00412919 push eax
0041291A lea ecx,[ebp-58h]
0041291D push ecx
0041291E lea ecx,[ebp-48h]
00412921 call CDC::GetTextExtent (411393h)
00412926 mov byte ptr [ebp-4],1
0041292A mov esi,esp
0041292C lea ecx,[ebp-130h]
00412932 call dword ptr
       [__imp_ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >
       ::~CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >
           (421868h)]
-----
or, it just called CString::~CString as the temporary object just left scope. I singled
stepped through both the debug and release code and watched it delete the CString object
and free its associated storage
-----
00412938 cmp esi,esp
0041293A call @ILT+1370(__RTC_CheckEsp) (41155Fh)

RELEASE version:

    CSize sz = dc.GetTextExtent(_T("ABCDEFG"));
004012B5 push offset string L"ABCDEFG" (4035A8h)
004012BA lea ecx,[esp+14h]
004012BE mov dword ptr [esp+3Ch],1
004012C6 call dword ptr
[__imp_ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> >

::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > (4030FCh)]

004012CC mov eax,dword ptr [esp+10h]
004012D0 mov ecx,dword ptr [eax-0Ch]
004012D3 lea edx,[esp+14h]
004012D7 push edx
004012D8 push ecx
004012D9 push eax
004012DA mov eax,dword ptr [esp+30h]
004012DE push eax
004012DF call dword ptr [__imp__GetTextExtentPoint32W@16 (403008h)]
004012E5 lea ecx,[esp+10h]
004012E9 call dword ptr
[__imp_ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> >

::~CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > (4030ECh)]