Re: std::map multithreaded access, lock needed?

James Kanze <>
Thu, 10 Jan 2008 02:03:29 -0800 (PST)
    [Sorry for that last posting. Google seems to have screwed

On Jan 10, 2:29 am, "Christopher Pisz" <> wrote:

I have a singleton is going to be accessed by any threads. My
singleton class contains a static std::map that will be read
and written to. I am wondering if I need to lock it when being
read from and written to? The sgi docs lead me to beleive I

Formally, it depends on the guarantees given by your
implementation, but generally, all of the implementations give
the SGI guarantee for std::map (and I'm pretty sure this is what
will be adopted in the next version of the standard). If any
thread may modify the object, all accesses, from all threads,
must be synchronized in some way.

See LoggerWindow::OpenLog below.

// in the .h

#include "LogWindow.h"

#include <map>


class LoggerWindow

* Constructor

Just a nit, but that's really an example of a useless comment.
The sort of thing that you want to avoid. (On the other hand,
it's often useful to document what state the constructor leaves
the object in.)


* Destructor

* Creates child windows for one log
void OpenLog(std::string title);



static HANDLE m_frameCreated; // Event that is signalled when the frame
window has been created

/** Pointers to the LogWindow instances that are the MDI children */
typedef std::map<std::string, LogWindow *> LogWindows;
static LogWindows m_logWindows;



// in the .cpp

#include "LoggerWindow.h"




// Initialize static members of the LoggerWindow class


HANDLE LoggerWindow::m_frameCreated = CreateEvent(NULL, TRUE, FALSE, NUL=


LoggerWindow::LogWindows LoggerWindow::m_logWindows;

The above isn't really what is usually understood by
"singleton". It's just an everyday static variable.





void LoggerWindow::OpenLog(std::string title)
   // This operation cannot execute until the frame window is fully create=


   WaitForSingleObject(m_frameCreated, INFINITE);

   // Is a lock needed here????

   // Check if the LogWindow already exists
   LogWindows::iterator it = m_logWindows.find(title);
   if( it == m_logWindows.end() )
      // Create a Log Window which is an MDI child of this window
      LogWindow * logWindow = new LogWindow(m_hwndClient, title);
      m_logWindows[title] = logWindow;
   // Is an unlock needed here????


If the OpenLog function can be called from different threads,
then you definitely need a lock. (I'd use some sort of scoped
lock.) How could it possibly be otherwise?

Another alternative, of course, would be to organize your code
so that OpenLog is only called from a single thread. (From the
names, it looks like you're dealing with GUI code---it's
generally best if all of the GUI management takes place in a
dedicated thread---other threads would just post an event
requesting the creation of a log window.)

James Kanze (GABI Software)
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