Using lambda to create a conditional singleton object

I had to create a conditional singleton object and I tried something but it didn't look so good. And while doing this I've noticed that I colleague did something like this:

static const auto executor = []() -> smt_type
    {
        return smt;
    }();

Which creates a lamba and calls it inplace.

And I liked it so much that I ended up asking him about it and he showed me this post from Herb Sutter. So I ended up doing something like this:

static const auto executor = []() -> std::shared_ptr<Aws::Utils::Threading::PooledThreadExecutor>
    {
        if(!isActive())
        {
            return nullptr;
        }
        size_t workers{0};
        auto const downloadWorkersStr = getenv("env_var");
        if(downloadWorkersStr)
        {
            // Env variable overrides the config
            workers = stoi(downloadWorkersStr);
        }
        else
        {
            workers = getWorkerThreadsFromConfig();
        }

        if(workers == 0)
        {
            workers = boost::thread::hardware_concurrency();
        }
        return Aws::MakeShared<Aws::Utils::Threading::PooledThreadExecutor>(
            "", workers);
    }();
    return executor;

This is called only once and it's also thread safe.