如何定义SPL中可以在文件之间共享的常量？

Question

A Custom operator can define constants in its state clause, but how does one define a constant that can be used across multiple operators, and across multiple files?

Ideally, I'm looking for a way to define constants in a namespace.

Answer 1

You can define a trivial SPL function to return a constant:

namespace my.name.space;
...
float64 MAX_LATITUDE() {return 90.0;}
...
composite MyMainComposite
{
...

This is automatically available throughout the namespace. Other than the parentheses, it works just like any constant wherever you use it. I haven't looked at the generated code in detail but I'm assuming that the SPL or C++ compiler will inline whatever it needs to, ensuring that there is no actual function call overhead at runtime.

Answer 2

There is currently no way to define constants in a namespace, so there is no way to define a constant once and use it in multiple SPL files.

For a single file, here are some options:

• use mixed-mode and define the constants in Perl code
• use composite parameter expressions:

   composite MyMainComposite {
     param
        expression<float64> $TimerInterval : 4.0;  // 4 seconds

Another option is compile-time options/parameters – see getCompileTimeValue() and getCompileTimeListValue() .

For SPL functions that exist in another file, you'll have to pass them as function arguments, or manually keep your code in sync.