Calling an instance method only once

Question

I am trying to write a class which has a method that can only be called once per object (well, it can be called multiple times, but it will only do something the first time).

My example code is

#include <iostream>
#include <functional>
#include <mutex>
class OneCallClass
{
    private:
    std::once_flag f_once{};

    public:
    void f() 
    {
        std::call_once(f_once, 
            []() { std::cout << "First call\n"; }
        );
     }
};

int main()
{
    OneCallClass o;
    o.f();
    // o.f();
    // o.f();
}

and it fails on Godbolt with the error message

terminate called after throwing an instance of 'std::system_error'
  what():  Unknown error -1
Program terminated with signal: SIGSEGV

The code looks straight forward to me. I looked up cppreference for std::call_once() and I don't see how I'm doing it wrong.

Well, the documentation says

std::system_error if any condition prevents calls to std::call_once from executing as specified

but which condition should that be?

I am creating an instance of the class on the stack. f_once() should be initialized due to {} and therefore I can use the rule of zero, no constructors, destuctors etc.

Where's the bug and how to fix it?

That's weird, it works for me when running the code in 2 online compilers and compiles fine in my own IDE using g++. Probably some library issues in the Godbolt site? — Ietu, Jul 04 '23 at 08:47
It works like a charm on Qt so Godbolt might be the issue. Adding a `static int` within `f()` as well as try catch around `std::call_once` and print i each time. Calling the 3 `o.f()` in main, only one call is done with i == 0. — Issylin, Jul 04 '23 at 08:48
Indeed, it works in Visual Studio as well. Maybe I should use a real IDE sometimes :-D — Thomas Weller, Jul 04 '23 at 08:51
just a guess. Doing stuff with threads requires `-lpthread` with gcc, if you dont, strange things can happen during runtime. — 463035818_is_not_an_ai, Jul 04 '23 at 08:56
well, a lame answer is better than none. Such duplicates are notoriously hard to find, as the symptoms are various even though the fix is the same — 463035818_is_not_an_ai, Jul 04 '23 at 09:11

score 1 · Accepted Answer · answered Jul 04 '23 at 09:10

1

You need -lpthread when using facilities from the thread support library.

Demo

answered Jul 04 '23 at 09:10

463035818_is_not_an_ai

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I believe `-pthread` should generally be used instead of `-lpthread`. See the following question for further information: [Difference between -pthread and -lpthread while compiling](https://stackoverflow.com/q/23250863/12149471) – Andreas Wenzel Jul 04 '23 at 13:31
@AndreasWenzel I admit its the first time I hear about this difference. I actually believe here `lphtread` is sufficient, because there is only a single thread in OPs code. I'll have to do more reading before I can fix the answer – 463035818_is_not_an_ai Jul 04 '23 at 13:34

Calling an instance method only once

1 Answers1