19. Static assertions

The P4 core library contains two overloaded declarations for a static_assert function, as follows:

\~ Begin P4Example /// Static assert evaluates a boolean expression /// at compilation time. If the expression evaluates to /// false, compilation is stopped and the corresponding message is printed. extern bool static_assert(bool check, string message);

/// Like the above but using a default message. extern bool static_assert(bool check); \~ End P4Example

These functions both return boolean values. Since the parameters are directionless, these functions require compile-time known values as arguments, thus they can be used to enforce compile-time invariants. Since P4 does not allow statements at the program top-level (outside of apply blocks), these functions can be used at the top-level by assigning their result to a dummy constant, e.g.:

\~ Begin P4Example const bool _check = static_assert(V1MODEL_VERSION > 20180000, “Expected a v1 model version >= 20180000”); \~ End P4Example

As the comment indicates, if static_assert returns false, it causes the program compilation to be terminated immediately with an error.