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From: Paavo Helde <eesnimi@osa.pri.ee>
Newsgroups: comp.lang.c++
Subject: Re: Pre-main construction order in modules
Date: Mon, 31 Mar 2025 15:26:30 +0300
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On 31.03.2025 12:11, Bonita Montero wrote:
> Am 31.03.2025 um 10:56 schrieb Paavo Helde:
> 
>> Before C++11 one had to just add their own synchronization for thread 
>> safety (assuming multi-threaded access was needed, which was not so 
>> often in the past).
>> C++11 just made coding of a Meyer's singleton easier, that's all.
> 
> That's not that easy since it is undefined when a static local variable
> is initialized before C++11. You'd have to wrap it in a union and leave
> only member which is the object uninitialized. That's possible, but
> that's not Meyer's singleton.

Thread synchronization has been possible since the introduction of 
threads. Otherwise they would not have been usable.

An example of pre-C++11 thread-safe Meyer singleton:

std::map<std::string, std::string>& GetGlobalMap() {

     // No dynamic initialization, so this is safe:
     static std::map<std::string, std::string>* pGlobal = NULL;

     // No dynamic initialization, so this is safe as well:
     static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

     // Initialize the global static if not yet initialized.
     pthread_mutex_lock(&mutex);
     if (!pGlobal) {
         pGlobal = new std::map<std::string, std::string>();
     }
     pthread_mutex_unlock(&mutex);
     return *pGlobal;
}