Is it possible to capture return value of function used in std::call_once?

user_1856429 0
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Suppose I have a function returning an expensive object and I want it to call exactly once while having an access to return value of the function.
Is this achievable with std::once_flag and std::call_once or I need to go with boolean flags, etc..?

Simple example:

using namespace boost::interprocess;
shared_mempry_object openOrCreate(const char* name)
{
    shared_memory_object shm(open_or_create, name, read_write);
    return shm;
}

I there a way to call this function from some routine and ensure only calling once with mentioned primitives while maintaining return value?

mutex c++
1 Answers

Simply use a static function variable.

using namespace boost::interprocess;
shared_mempry_object& openOrCreate(const char* name)
            ///     ^  return by reference to prevent copy.
{
    static shared_memory_object shm(open_or_create, name, read_write);
 // ^^^^^^ Make it static. 
 //        This means it is initialized exactly once
 //        the first time the function is called.
 //    
 //        Because it is static it lives for the lifetime
 //        of the application and is correctly destroyed
 //        at some point after main() exits.

    return shm; // return a fererence to the object
}

You can add some wrapper to guarantee clean-up at the end of the application.

Martin_York_140650
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