Is a capture captured even if it isn't used?

I'm using an async function that takes an object reference &foo and a callback cb as arguments. I want to prevent destruction of foo until the callback is called.

void async_thing(Foo &foo, function<void()> cb) {
    // do something asynchronously with foo
}

Is it enough to simply capture it in the callback lambda? Or does it need to be actually used in the lambda?

auto foo = make_shared<Foo>();
async_thing(*foo, [foo]() {
    cout << "Callback ran" << endl;
});

Might a compiler optimise the capture out, and delete foo prematurely?