Originally posted by: Ccl_

Hello,

I am using Concert (12.8) with the C++ API.

I have an IloModel to be solved by Cplex (without any callback).

I notice that when I remove a constraint from my model, the optimal value returned by Cplex is higher than before (which should not happen as this is a minimization problem!).

But if I use Cplex 12.6 instead of Cplex 12.8 to solve this relaxed model I get another optimal value (which is then lower than the optimal value of the original model).

Did I miss something or is this a bug?

Thanks!

What are the actual objective values you observe? Could it be that CPLEX just prematurely because it hits some stopping criterion like a time limit or a gap? Depending on your settings CPLEX may stop with a slightly non-optimal solution and then your situation can certainly happen (and is expected).
Does the problem persist of you set parameter IloCplex::EpGap to 0?