Re: Passing Parameter as Variable vs as address of variable

Hi Patrik,

Von meinem iPhone gesendet

Am 26.08.2023 um 17:51 schrieb Patrik Schindler <poc@xxxxxxxxxx>:

Am 26.08.2023 um 00:14 schrieb Barbara Morris <bmorris@xxxxxxxxxx>:

If you're asking about passing the address of a variable by value vs passing the variable by reference ...

They both behave the same, with the same performance.

When looking at your assertion from a "modern hardware" point of view, counting wall clock time, I agree. The difference is negligible. Double the amount of some µs is still some µs. Too fast for a human being to be detected.

If I get it right - both passing a pointer as a value or passing a variable by reference is the exact same thing - from the machine code view.

In both cases a pointer to the original variable/memory space is passed on the stack to the called procedure.

But it’s different from the compiler view - for a parameter passed by reference the compiler can do a type check. But it cannot do that for an arbitrary pointer. That’s why C has „typed pointers“.

But passing a variable by reference is generally good and passing its address by value is generally bad.

If this was the case, why is there the possibility to pass by reference at all?

Passing by reference is a pointer „In disguise“ - and that is generally fast, because it only pushes pointers to the stack - all with a fixed address length.

A use case for me to "pass by value" is to "write protect" a variable. The called application now can alter the variables' contents at will. It won't get changed for the calling application.

Altering parameters that a passed to a procedure is generally „bad style“ - only if passed by reference and with full intention.

If a procedure receives a parameter by value and you modify that, this is very convenient - but also confusing.

But maybe there are other ways to get the described "readonly" feature.

Yes there is the „const“ keyword for parameters. Under normal circumstances the parameter is read only but passed as reference if the parameter type and the passed variable type are fitting. If not, a temporary copy with the right type is generated and passed by reference.

Anyway - with „const“ you have write protected parameters which are past fast by reference - and the compiler deals with the read only thing.

Yes - you can trick and modify the „const“ parameter if you like (e.g. using a pointer) - but then you intentionally go around the „stop-read-only“ sign.

Kind regards,
Daniel