Overview of VISIT

VISIT opens a variant and compiles one region for each payload type. It is useful when the region should use ordinary overload resolution instead of listing separate MATCH arms.

Variable visitation

An attached block temporarily shadows the variant variable with a reference to its active payload:

::number_or_count INLINE_VARIANT [I32 DEFAULT, U32];

VAR value number_or_count := 7 AS I32;

VISIT value
{
  print_number(@value value);
}

Inside the block, value is an I32 reference in one generated specialization and a U32 reference in the other. After the block, value again denotes the variant object.

Ending VISIT with a semicolon specializes the rest of the current block:

VISIT value;
print_number(@value value);

Expression visitation

An AS clause names the payload of an arbitrary expression:

VISIT read_number() AS number
{
  print_number(@value number);
}

The subject is evaluated once. With an attached block, all temporaries made while evaluating it remain alive through the block. The ordinary semicolon form keeps only the final temporary variant:

VISIT read_number() AS number;
print_number(@value number);

The EXTEND modifier preserves the entire evaluation context through the remainder of the current block:

VISIT EXTEND read_number_with_context() AS number;
print_number(@value number);

VOID and valueless variants

A variant containing VOID may be visited only with an attached block. When VOID is active, execution skips that block; no payload binding or VOID specialization is formed.

Visiting a valueless variant is undefined behavior. Check value?? first when the variant's policy permits a valueless state.

Reference

See the VISIT Reference for all five forms, temporary-lifetime rules, qualification, specialization, and control-flow constraints.