C++ API Gurobi 6.5: Removed the model assignment operator

[Nikos Lappas]

Hi,

I noticed that the capability of using the assignment operator in the model class was removed (we cannot anymore write modelA = modelB). Is there any particular reason behind this choice?

Thanks,

Nick

[Renan Garcia]

The assignment operator in the C++ API was forbidden for the latest 6.5 release. In previous versions, the assignment operator was not explicitly declared, which means a default version of it would be provided by the compiler (see https://en.wikipedia.org/wiki/Assignment_operator_(C%2B%2B) for more details).

However, the default version can lead to a crash because it only performs a shallow copy of the underlying pointer to the model rather than a deep copy of the model itself. Calling the destructor for one of the copies frees the underlying pointer and leaves the other copy with a pointer that is no longer invalid. See https://groups.google.com/forum/#!topic/gurobi/FszoX0-Tx1I for an example of where this was leading to a crash. 

If you need a deep copy, use the copy constructor (see http://www.gurobi.com/documentation/6.5/refman/cpp_grbmodel2.html). For example,

  GRBModel modelB(modelA);

  // or

  GRBModel modelC = modelA;

If you need a shallow copy, use a reference or a pointer. For example,

  GRBModel &modelD = modelA;

  // or

  GRBModel *modelE = &modelA;

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