Do you need TSystem.h here?
Should probably be TVectorD.h.

Can drop this since it's already included in LauFitObject.hh

No need for the if ( ! conVars_.empty() ).
Suggest instead to replace the for loop with:
for ( LauAbsRValue* par : conVars_ )
then (*iter) becomes par

I wonder, can we have this TVectorD as a member of the StoreNDConstraints struct as well? It should be possible to construct it with the right size when the struct is first created in LauFitObject::addNDConstraint.

To avoid copying the vector

Can we call these something a bit more easily understandable?
Maybe: constraint, parname, and fitPar?

Better to push directly into itervec.conLauPars_ and avoid the need for the params vector.

In fact, as currently written, I think that if you had more than one n-dimensional constraint you'd end up with too many parameters in all but the first one since params is declared outside all loops and is never cleared.

Here you're relying on the fact that checkRepetition aborts the code.
I think it would be better to have checkRepetition return a status boolean and then we can print another error and abort here.

Same comment as above

I would be inclined to allow the checks to continue so that you see all errors rather than one at a time. So you could have an allOK boolean that is initially true, then if an error is encountered it gets set to false. Then at the end of the function you just return that boolean.

Perhaps also try to improve the loop variable names.

Same comments on this function as for the corresponding one in LauAbsFitModel

Same comments on this function as for the corresponding one in LauAbsFitModel