C++ vector push_back async object in for loop

Question

I was coding a for loop in C++11, where I needed to push back an async object onto a vector. I wanted to split the object initialization into two steps:

    std::vector<std::future<bool>> asyncThreads;

    for (int i = 0; i < processorCount; i++) {
        auto boundFunc = std::bind(&Foo::foo, this);
        auto asyncThread = std::async(std::launch::async, boundFunc)

        asyncThreads.push_back(asyncThread);
    }

Now I do realize that both boundFunc and asyncThread objects go out of scope at the end of the for loop (the push_back function should copy/move the values though), but then why does directly declaring the objects in the push_back call work? Like so:

    std::vector<std::future<bool>> asyncThreads;

    for (int i = 0; i < processorCount; i++) {
        asyncThreads.push_back(std::async(std::launch::async, std::bind(&Foo::foo, this)));
    }

The difference is `std::async(...)` is an rvalue expression while `asyncThread` is an lvalue expression. The first example tries to copy the `future` while the second moves it. Try using `std::move`. — François Andrieux, Mar 22 '22 at 20:36
What's the difference between rvalue and lvalue? And how would I use the move function here? Sorry, still a bit new to c++ — user2793618, Mar 22 '22 at 20:36
You can take a look at [What is move semantics?](https://stackoverflow.com/questions/3106110/what-is-move-semantics). — François Andrieux, Mar 22 '22 at 20:38
@user2793618 lvalue is simply almost everything that has a name (`int a = 5`), rvalue is value or expression that can occur only on the right side of `=` (literal strings, numbers etc) — complikator, Mar 22 '22 at 20:41
@FrançoisAndrieux Thanks for the link, was informative. But why is it that copying the ```future``` object causes the code to fail, while moving it is ok? — user2793618, Mar 22 '22 at 21:24
@user2793618 A `std::future` is not copyable, but it is moveable. A future represents a resource that cannot be meaningfully shared, so its copy semantics are explicitly prevented to avoid errors. You can see [it's constructors here](https://en.cppreference.com/w/cpp/thread/future/future). The copy constructor (3) is `= delete` (disabled) while the move constructor (2) is provided. — François Andrieux, Mar 22 '22 at 21:29

score 1 · Accepted Answer · edited Mar 22 '22 at 21:31

1

A std::future object is not copyable, but moveable. So therefore, you must call move on the object to push it onto the vector.

edited Mar 22 '22 at 21:31

François Andrieux

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answered Mar 22 '22 at 21:30

user2793618

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C++ vector push_back async object in for loop

1 Answers1