Validation

Table of Contents

• Validator
• Rules

As noted in the Architectural Overview, the node's perform as much validation of their configuration as possible, but that typically does not include validation of the relationships with other nodes and policies.

To achieve that we use a validator policy that attaches to the root node and checks the entire parse tree upon construction (which is always after all child nodes and policies).

Validator

The validator that ships with arg_router is policy::validation::validator. It's a complicated bit of machinery but from a user's point of view, it is just a list of rules that apply to each policy and tree_node derived type. Each rule has a predicate that is used to match against the validated policy or node type, and a collection of conditions that all have to be met by the validated type in order for a static_assert not to fire.

The rules, defined by policy::validation::rule and policy::validation::rule_q, are a template parameter pack of policy::validation::validator and so can be modified without having to write your own validator from scratch. The standard configured validator i.e. a validator with the rules defined for all the arg_router types, is policy::validation::default_validator.

Rules

As noted above, each rule starts with a predicate that 'selects' that rule as applying to the validated type. Let's look at an example from policy::validation::default_validator:

rule_q<common_rules::despecialised_any_of_rule<flag_t>,
        must_not_have_policies<policy::multi_stage_value,
                               policy::no_result_value,
                               policy::required_t,
                               policy::validation::validator>>

policy::validation::common_rules::despecialised_any_of_rule is a quoted metafunction that evaluates to true if the validated type matches one of the despecialised types in its argument list. In this case we're just checking that it matches flag_t.

As the name suggests, policy::validation::must_not_have_policies checks that the validated type does not use any of the policies listed in its template parameters.