added initializers to resolve compiler warnings

[simsong]

This PR ends these warnings:

thread-pool/thread_pool.hpp: In constructor ‘synced_stream::synced_stream(std::ostream&)’:
thread-pool/thread_pool.hpp:410:5: warning: ‘synced_stream::stream_mutex’ should be initialized in the member initialization list [-Weffc++]
  410 |     synced_stream(std::ostream &_out_stream = std::cout)

[bshoshany]

Thanks for the pull request! However, I don't see a reason to make this change. These objects are already default-initialized, so this is redundant. I never use the -Weffc++ flag because I don't find these warnings useful; if you don't explicitly use this flag, then you won't see these warnings.

[simsong]

You have decided not to accept a pull request that eliminates a compiler warning because you personally do not use the warning flag? With all due respect, that is somewhat short-sighted if your goal is to create a software component that is widely used within the community.

[bshoshany]

I just don't think there's anything wrong with the code as it is, it may not be compliant with that specific style guide (Scott Meyers’ Effective C++) but it is 100% correct and perfectly compliant with the C++17 standard.

However, I do understand that this is important to you and possibly to others in the community who are using that style guide, so I will add this change in the next revision.

I release my projects in cumulative updates after editing them locally, so my policy is to not accept pull requests. Instead, I will merge this change into the next release, possibly together with some other changes, and along with a version bump and a corresponding note in README.md and a link to this pull request. Thanks for your contribution :)

[bshoshany]

Update: I incorporated this change in v1.8. Thanks again :)

[simsong]

Thank you!