Passing a large object to async task in C++

Question

I have program that receives callback calls from a library when messages are received. The callbacks' signature is void on_message(const MyLargeObject &my_large_object);

My code for reacting to some of the messages is more complex than what I want to run directly in the callbacks. Because of that, I am launching an asynchronous task (std::async) to do the actual processing.

Currently, the asynchronous tasks' signature is void on_message_async_task(MyLargeObject my_large_object);. My reasoning says that I should pass the argument to the async task as a copy because I see no guarantee that the original object lives until the task is finished. Static code analysis is, however, saying that I should use const reference instead to avoid expensive copying. Is that false positive or am I missing something here?

The static code analysis is giving a suggestion. It's suggestion does not seem appropriate in this situation. If you really want to placate the static code analyzer, consider passing void on_message(std::unique_ptr<MyLargeObject> my_large_object_in_a_smart_pointer). — Eljay
– Eljay, Commented Dec 2, 2024 at 15:13
Using a smart pointer will ensure the lifetime of the object, as well as save copying. — wohlstad
– wohlstad, Commented Dec 2, 2024 at 15:15
I can't change the signature of the callback. Because of that, as far as I know, I can't use a smart pointer to avoid copying. — TheAG
– TheAG, Commented Dec 2, 2024 at 15:28
There is a big difference between passing the object and passing a reference. If the signature is MyLargeObject b then you are passing a full copy on the stack. Not good. If it is MyLargeObject & b then you are only passing a single pointer. Good. But... if the object is going to change (or worse yet, get deleted) while running the background task then you must make a copy. It is better is to favor an immutable object style. Can make MyLargeObject read only? Or guarantee it will not change? This feels like a problem that is solved by re-thinking the overall dataflow and context. — Richard Keene
– Richard Keene, Commented Dec 2, 2024 at 15:38
I know that the what the static analyzer says is just a suggestion, but I may need to explain why I am ignoring it so I am looking for confirmation for my assumption. — TheAG
– TheAG, Commented Dec 2, 2024 at 15:39

user7860670 · Accepted Answer · 2024-12-02 17:09:15Z

Task functor accepting an object by reference seems like a reasonable suggestion. Note that it does not imply that task's shared state should store reference to the object from library callback. Task should create a copy so it survives long enough, however functor should accept a reference to avoid yet another copy when it is invoked.

#include <future>
#include <iostream>

struct Object
{
    Object()
    { ::std::cout << static_cast<void *>(this) << " Object{}\n"; }
    Object(Object const & object)
    { ::std::cout << static_cast<void *>(this) << " Object{copy}" << static_cast<void const *>(&object) <<"\n"; }
    ~Object()
    { ::std::cout << static_cast<void *>(this) << " ~Object\n"; }
};

void ByRef(Object const & object)
{ ::std::cout << static_cast<void const *>(&object) << " ByRef\n"; }
void ByCopy(Object const object)
{ ::std::cout << static_cast<void const *>(&object) << " ByCopy\n"; }
void Callback(Object const & object)
{
    ::std::cout << "ByRef Start\n";
    ::std::async(::std::launch::async, ByRef, object).get();
    ::std::cout << "ByCopy Start\n";
    ::std::async(::std::launch::async, ByCopy, object).get();
}

int main()
{
    Object object{};
    Callback(object);
    ::std::cout << "Done\n";
    return 0;
}

online compiler

0x7ffe6a71789f Object{}
ByRef Start
0x14e6308 Object{copy}0x7ffe6a71789f
0x14e6308 ByRef
0x14e6308 ~Object
ByCopy Start
0x14e6308 Object{copy}0x7ffe6a71789f
0x7149b1bfecc8 Object{copy}0x14e6308
0x7149b1bfecc8 ByCopy
0x7149b1bfecc8 ~Object
0x14e6308 ~Object
Done
0x7ffe6a71789f ~Object

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