DDSPSMC — Improving usability of EntityQoS API

8. Supporting method chaining for setting qos parameters would improve readability of the API.

For instance,
DataWriterQos dwqos;
dw >> dwqos;
Dwqos.policy<History>().kind(KEEP_ALL).depth(200);
Dwqos.policy<ResourceLimits>().max_samples(p).max_instances(q).max_samples_per_instance(r);
dw << dwqos;

Currently, it needs different function calls for every qos parameter.

Proposed Solution: Change all the normative core qos policy classes (e.g., History, ResourceLimits) to return a reference to itself from the setters. Currently they return nothing.

The problem raised has been addressed by equipping all the Policy classes with fluent setter methods. The example provided by this issue can now written as follows:

DataWriterQos dwqos;
dw >> dwqos;
dwqos << History::KeepAll(200)
<< dwqos.policy<ResourceLimits>()
.max_samples(p)
.max_instances(q)
.max_samples_per_instance(r);

dw << dwqos;

The changes applied to the given policy class P is to have its setter return P&.
See dds-psm-cxx/blob/master/src/hpp/dds/core/policy/TCorePolicy.hpp