Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 18 Jul 2005 12:55:29 -0500

Kay-Uwe Kasemir wrote:

but I still don't see a way around contiguous char * strings,
because that's what most 'legacy' libraries want.

You can add that as a layer around the next EPICS String class
and copy char by char from the non-contig. String into a char *,
but std::string and MFC::CString instead bite the bullet
and provide a c_str() type of method.

In our EpicsString design, we solved this requirement by including a

buffer type that stores contiguous zero-terminated data (the other

buffer types don't have to guarantee a zero terminator). If you need to

pass a string to an external library then you can create one of these

and copy the string to it, or just use that buffer type in the first

place if it's your string and you're not concerned about reallocating

memory if it ever needs to be extended.

It is possible to design a full implementation of C++ std::string that

uses segmented memory. The assumption is that c_str() would rarely be

used, but when it is the implementation just allocates a contiguous

buffer and copies the segmented string into it. The C++ standard states

 two requirements for c_str(): "The program shall not alter any of the

values stored in the array. Nor shall the program treat the returned

value as a valid pointer after any subsequent call to a non-const member

function" Thus the implementation may release the contiguous buffer it

allocated the next time the user attempts to alter the string.

I am not advocating that we use this approach in our string class, just

illustrating that the C++ standards writers saw that they had to provide

a char* accessor method for interfacing to legacy systems.

- Andrew
--
Podiabombastic: The tendency to shoot oneself in the foot.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System