simplify c++ code base with copy elision

[anilbey]

Description

Avoids creating and returning unnecessary vectors.

Checklist:

• Unit tests are added to cover the changes (skip if not applicable).
• The changes are mentioned in the documentation (skip if not applicable).
• CHANGELOG file is updated (skip if not applicable).

[anilbey]

Note: this does not change the behaviour it only avoids some redundancies which are nowadays handled by the compilers.