Re: member variable :1 syntax

"Alf P. Steinbach" <>
Thu, 07 May 2009 13:30:14 +0200
* Angus:

On 7 May, 12:14, "Alf P. Steinbach" <> wrote:

* Angus:

This sort of idea is used a lot in the code I am looking at:
  bool m_setting1 :1;
  bool m_setting2 :1;
  bool m_setting3 :1;
  bool m_setting4 :1;
etc etc.
I presume it is for efficiency.

It's unlikely to improve efficiency, but it can be a good way to conform to some
externally imposed memory layout.

Cheers & hth.,

- Alf

This example demonstrates a memory advantage:

class myclass
  bool getboolval() { return m_setting1; }

  bool m_setting1:1;
  bool m_setting2:1;
  bool m_setting3:1;
  bool m_setting4:1;
  bool m_setting5:1;
  bool m_setting6:1;
  bool m_setting7:1;
  bool m_setting8:1;

int main(){
    int size = sizeof(myclass);
    std::cout << size; //reports 1

If however, the :1 syntax above is removed, then size is reported as

My test was using MSVC++v6 on Windows XP (32 bit).

You really should upgrade compiler. You'd get the same result, but MSVC 6 is a
pre-standard compiler and has trouble with modern template code, forces you to
write non-standard code, plus it has a host of bugs in floating point support.
AFAIK the latest MSVC compiler is bundled with the SDK, so it's free even
without installing the lobotimized free version of Visual Studio.

Yes, it's a memory advantage, but have you tested efficiency? :-)

Cheers, & hth.,

- Alf

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