stasher::process_request() template function
The example in previous chapter,
updatethread.C uses similar code in two cases of
setting up a destructor callback for an asynchronous request's mcguffin.
Each case, a separate destructor callback class gets defined, instantiated,
and attached as a destructor callback for an asynchronous request's
mcguffin.
updatethread2.C
is a modified version of updatethread.C.
It does the same thing, but uses
stasher::process_request():
stasher::process_request(client->get_request(req), [] (const stasher::getresults &res) { // functor });
The stasher::process_request() template function takes
two parameters.
The first parameter is a
return value from an asynchronous request such as
stasher::get_request() or
stasher::put_request().
The second parameter is a functor
with a single parameter that's consistent with the final result of the
request.
For stasher::get_request() it's
stasher::getresults,
for stasher::put_request() it's
stasher::putresults.
stasher::process_request() installs a destructor
callback for the request's mcguffin that invokes the functor, with the
results of the request. stasher::process_request()
uses LIBCXX's x::destroyCallback::create(), which
logs any exceptions thrown in the callback.
Quite a bit of code from updatethread.C gets
replaced with just a short functor in
updatethread2.C.
The functor can capture variables, but in most cases they must be
captured by value; the functor works the same way that a manually-installed
destructor callback does, here; and it usually ends up getting invoked by
the client connection thread, so variables need to be captured by value.
The limitations of destructor callback described in
the section called “What asynchronous C++ API methods can and cannot do” apply here too.