Local static variable shared between all instantiations of a generic lambda expression

Question

I have some code where a non-generic lamba expression has a local static variable: a mutex for a critical section. Something that can be simplified to:

int i = 0;
auto lambda = [&i](int &v)
{
    static std::mutex mutex;
    std::lock_guard<std::mutex> lock(mutex);
    /* critical section with v and i */};
}

Now, the critical section implemented in this lambda expression could be reused verbatim for other types than int & and I would have liked a change as simple as replacing int with auto in the declarator, like this:

auto lambda = [&i](auto &v)

Unfortunately, if I do this, lambda(int&) and lambda(float&) won't share the same local static variable anymore, which will defeat the lock in the critical section.

What would be the simplest change in the code that would meet all these requirements:

• provide the genericity that I need; and
• ensure that I have a single mutex shared across all instantiations of the critical section; and
• does not expose the mutex to public view

A solution that would work would be to replace the lambda expression with a class with a template method, something like this:

class Lambda
{
public:
    Lambda(int &i) : i_(i) {}
    template<class V>
    void operator()(V &v)
    {
        std::lock_guard<std::mutex> lock(mutex_);
        /* critical section with v and i_ */
    };
private:
    static std::mutex mutex_;
    int &i_;
};
std::mutex Lambda::mutex_;

int i = 0;
Lambda lambda(i);

This would work (with caveats like the private reference i_), but it looks very cumbersome compared to the initial lambda expression. Would there be anything more straightforward?

Answer 1

You can init-capture a shared_ptr, which will be similar to having it defined as a member variable of the closure type, and so all copies of the closure object will share the same mutex:

  auto lambda = [&i, m = std::make_shared<std::mutex>()](auto &v)
  {
      std::lock_guard<std::mutex> lock(*m);
      /* critical section with v and i */
  };