Re: Trick to prevent conversion

Jack Klein <>
Fri, 11 Jan 2008 21:50:23 -0600
On Fri, 11 Jan 2008 19:18:05 GMT, "Tom1s # hilidhe" <>
wrote in comp.lang.c++:

(I'm simplifying this problem greaty in the following code but you get
the idea)

I have a class something like:

class Processor {
    unsigned GetCurrentProcess(void);
    unsigned GetCurrentThread(void);

    void DoSomethingWithProcess(unsigned process_id);
    void DoSomethingWithThread(unsigned thread_id);

    The function, GetCurrentProcess, returns an unsigned integer value
which represents a process.
    The function, GetCurrentThread, returns an unsigned integer value
which represents a thread.

The problem is, that it's very easy to mistakenly take the value from
GetCurrentProcess and pass it as an argument to DoSomethingWithThread.
(I've done it already!). Therefore, I want two different types for these
identifiers, two types that can't convert to each other. At the moment I
have the following:

class Processor {

    struct ProcessHandle { unsigned i; };
    struct ThreadHandle { unsigned i; };

    ProcessHandle GetCurrentProcess(void);
    ThreadHandle GetCurrentThread(void);

    void DoSomethingWithProcess(ProcessHandle process_id);
    void DoSomethingWithThread(ThreadHandle thread_id);

Of course, this does the trick. But I'm wondering if there's a more
elegant way of doing it? What other methods are there for making types
incompatible with each other?

(My actual code is far more complicated than this -- obviously I wouldn't
be very competant if I made such a stupid mistake in code as simple as

Why not two enumeration types, suitably defined so that hold the value
range you want, i.e.:

enum process_id { process_id_min = 0, process_id_max = UINT_MAX };
enum thread_id { thread_id_min = 0, process_id_max = UINT_MAX };

Modify your functions to accept and receive the enumerated types, with
suitable static_casts to and from unsigned int, as needed, inside the
functions, invisible to the caller. Modify your callers who get the
values to accept the appropriate enumeration type.

A C++ compiler will reject code where a caller retrieves a process_id
from one function and attempts to pass it to another that needs a

I even use this technique in C, where any decent compiler will warn
about mixing different enumeration types even though the language
permits it.

Jack Klein
Home: http://JK-Technology.Com
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