what is constraints ? (C++)

Constraints are functional relationship between entities of or object model. The term entity includes objects,classes,attributes,links and associations. A constraint restrict the values that entities can assume.

Constraint provide one criterion for measuring the quality of an object model, a ‘good’ object model captures many constraints through its structure. It often requires several iterations to get the structure of a model right from the perspective of constraints.

Constraint are delimited by braces and positioned near the constrained entity.

A dotted line connects multiple constrained entity. An arrow must be used to connect a constrained entity to the entity it depends on instantiation is kind of constraint and therefore,uses the same notation.