std::call_once is guaranteed to execute the given callable object
exactly once. This guarantee holds even if the function is called
concurrently from several threads.
Given that, we can replace the mutex and boolean flag with
std::call_once and a std::once_flag to perform the same behavior.
Previously, every entry pair within the map would be copied. The reason
for this is subtle.
A std::map's internal entry type is defined as:
std::pair<const Key, Value>
but the loop was declaring it as:
std::pair<Key, Value>
These two types aren't synonymous with one another and so the compiler
is required to always perform a copy.
Using structured bindings avoids this (as would plain auto or correcting
the explicit type), while also allowing the use of more appropriate
names compared to first and second.
std::function is allowed to heap allocate in order to hold any necessary
bound data in order to execute properly (e.g. lambdas with captures), so
this avoids unnecessary reallocating.
Too much boilerplate that is duplicated if we use curl directly.
Let's add a simple wrapper class that hides the implementation details
and just allows to simply make HTTP requests and get responses.
