const char* vs. char*

[Miller, James V (CRD)]

Unfortunately it is in general very difficult to go back through a software system and change
parameter lists to take "const" pointers or reference.  This is not simply a matter of changing the
parameter lists.  You have to dig around the code for all uses of the functions.  Usually what
happens is that you end up passing a const into one function which calls another function which calls
another function, etc, etc, and then near the bottom of the call chain you have to call a function
which does not allow a const object to be passed in.  Then you end up riddling the code with extra
copies of functions or copy the data to a non-const, etc., etc.

To retrofit a system to take const parameters is a large bucket of worms. It is much easier to do
this from the beginning of a software system.

Now, your particular case at hand with the SetStringMacro may be easy enough. We'll take a look at it
and see what the ramifications may be.


-----Original Message-----
From: M.Schrijver at el.utwente.nl [mailto:M.Schrijver at el.utwente.nl]
Sent: Tuesday, July 13, 1999 6:11 AM
To: vtkusers at gsao.med.ge.com
Subject: const char* vs. char* 


Hello all,

I find it very inconvenient that some functions use char* instead of
const char* (e.g. the vtkSetStringMacro in vtkSetGet.h, and
vtkImageReader::SetFileName() etc.). This means that the following code
does not compile:

......
......
const char* filename="../filename.bmp";
reader->SetFileName(filename);

I know, I know, I can drop the const here. But that leaves the door wide
open to anyone trying to modify the string (e.g. replacing the '../'
part with the full path, which will definitely blow your system), and
there are other situations in which you simply can't. What, for example,
if you do your string manipulations with the std::string class, and want
to pass the result to vtk. The function std::string::c_str() returns a
const char*, not char*. That would mean you'd have to copy the string to
a temporary buffer in order to pass it to vtk!

In addition, I don't see why anyone would /not/ make the argument to
such functions const. I've tried it with vtk, and it compiles and runs
fine. So, I don't see a problem .... Anyone who does?

Many thanks,

Marc Schrijver


ir. Marc Schrijver
Faculty of Electrical Engineering (S&S NT)
University of Twente
The Netherlands
E-mail: m.schrijver at el.utwente.nl



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