tree 2c2efabd2faffef6f5e305f4700f2bb7adf7e025
parent c7337154e97baa02015591d9ca143fa24cb45d28
author Darius Rueckert <darius.rueckert@fau.de> 1575462723 +0100
committer Darius Rueckert <darius.rueckert@fau.de> 1575635721 +0100

Rework Expression creation and insertion

Objects of 'Expression' can now be freely created and copied around.
This creation does NOT add them to the active ExpressionGraph any
more. The insertion into the graph is now done by ExpressionRef,
which explicitly call graph.add(...) in each operation.

This change brings the following advantages:

1. 'Expression' is now stand-alone and side-effect free
   - Remove the dependency Expression->ExpressionGraph
   - Expressions can be created by the optimizer

2. Explicit graph insertion
  - Previously CreateCompileTimeConstant not only created an
    expression, but also inserted it into the active graph. Now
    this insertion is done explicitly by ExpressionRef

3. It is now easier to insert new types and members
  - Should be straight forward now, because we got rid of the
    3-way dependency
  - This is a preparation patch for the new return-type member in
    https://ceres-solver-review.googlesource.com/c/ceres-solver/+/16224

Change-Id: Icef2fe529a4db001a10d1fb6816c9dc681b14ff2