STM32F4 Problem with #pragma pack(x)

Hello,
Depending on the control #pragma pack(x), the result of the compilation is different.
In C++, Here's the problem, but it is more difficult without picture:

With #pragma pack(2), the result is Ok.

TNetCv::TNetCv(void)
{
  DCvPtrNetPtcl = Nil;
....
Compiling give:
0x0801A95C 2000         MOVS         r0,#0x00
0x0801A95E F8C40026      STR          r0,[r4,#0x26]

void TNetCvCOM::CvDoWhenNetEvent(ENUMNETEVENT TheCvEvent)
{
  TNetCv::CvDoWhenNetEvent(TheCvEvent);         // INHERITED
  switch(TheCvEvent)
  {
    case eNetEventRun:
      if (DCvPtrNetPtcl != Nil) DCvPtrNetPtcl->PtclDoWhenNetEvent(TheCvEvent);
    break;
  }
}
Compiling give:
    97:     if (DCvPtrNetPtcl != Nil) DCvPtrNetPtcl->PtclDoWhenNetEvent(TheCvEvent);
0x0801F782 F8D40026  LDR           r0,[r4,#0x26]
0x0801F786 B128      CBZ           r0,0x0801F794
0x0801F788 F8D40026  LDR           r0,[r4,#0x26]
0x0801F78C 6801      LDR           r1,[r0,#0x00]
0x0801F78E 6A0A      LDR           r2,[r1,#0x20]
0x0801F790 4629      MOV           r1,r5
0x0801F792 4790      BLX           r2

In both examples the offset of the pointer "DCvPtrNetPtcl" is 0x26

With #pragma pack(4), the result is Bad.

TNetCv::TNetCv(void)
{
  DCvPtrNetPtcl = Nil;
....
Compiling give:
0x0801A95C 2000         MOVS         r0,#0x00
0x0801A95E F8C40026      STR          r0,[r4,#0x26]


void TNetCvCOM::CvDoWhenNetEvent(ENUMNETEVENT TheCvEvent)
{
  TNetCv::CvDoWhenNetEvent(TheCvEvent);         // INHERITED
  switch(TheCvEvent)
  {
    case eNetEventRun:
      if (DCvPtrNetPtcl != Nil) DCvPtrNetPtcl->PtclDoWhenNetEvent(TheCvEvent);
    break;
  }
}
Compiling give:
    97:      if (DCvPtrNetPtcl != Nil) DCvPtrNetPtcl->PtclDoWhenNetEvent(TheCvEvent);
0x0801F3FE 6AA0      LDR           r0,[r4,#0x28]
0x0801F400 B120      CBZ           r0,0x0801F40C
0x0801F402 6AA0      LDR           r0,[r4,#0x28]
0x0801F404 6801      LDR           r1,[r0,#0x00]
0x0801F406 6A0A      LDR           r2,[r1,#0x20]
0x0801F408 4629      MOV           r1,r5
0x0801F40A 4790      BLX           r2

Without changing a line of code
Now, in the initialization is always offset 0x26, but in the second function the offset is 0x28. The result is an exception "UsageFault"

thank you for your help

Jean-François