08 21 constructor invocations

Several P4 constructs denote resources that are allocated at compilation time:

• extern objects
• parsers
• control blocks
• packages

Allocation of such objects can be performed in two ways:

• Using constructor invocations, which are expressions that return an object of the corresponding type.
• Using instantiations, described in Section Instantiations.

The syntax for a constructor invocation is similar to a function call; constructors can also be called using named arguments. Constructors are evaluated entirely at compilation time (see Section P4 abstract machine: Evaluation). In consequence, all constructor arguments must also be expressions that can be evaluated at compilation time. When performing type inference and overload resolution, constructor invocations are treated similar to methods or functions.

The following example shows a constructor invocation for setting the target-dependent implementation property of a table:

extern ActionProfile {
    ActionProfile(bit<32> size);  // constructor
}
table tbl {
    actions = { /* body omitted */ }
    implementation = ActionProfile(1024);  // constructor invocation
}