Question

Suppose I have a function named caller, which will call a function named callee:

void caller()
{
    callee();
}  

Now caller might be called many times in the application, and you want to make sure callee is only called once. (kind of lazy initialization), you could implement it use a flag:

void caller()
{
    static bool bFirst = true;
    if(bFirst)
    {
        callee();
        bFirst = false;
    }
}

My opinion for this is it needs more code, and it needs one more check in every call of function caller.
A better solution to me is as follow: (suppose callee returns int)

void caller()
{
    static int ret = callee();
}  

But this can’t handle the case if callee returns void, my solution is using the comma expression:

void caller()
{
    static int ret = (callee(), 1);
}  

But the problem with this is that comma expression is not popular used and people may get confused when see this line of code, thus cause problems for maintainance.

Do you have any good idea to make sure a function is only called once?

Answer 1

Thread-safe:

    static boost::once_flag flag = BOOST_ONCE_INIT;
    boost::call_once([]{callee();}, flag);