Re: const gsl_vector *

[John D Lamb <J.D.Lamb@btinternet.com>]

On 19/08/14 23:28, Patrick Alken wrote:
>> One issue would be that if the code were rewritten any code that passed
>> a const gsl_vector* argument would cause a compilation failure. The
>> workaround might be to use a
>>
>> gsl_vector const* const v
>>
>> argument. But I donÂ’t know if that would cause problems, for example
>> with older compilers.
>>
> I don't have a solution to this problem, but with my gcc (4.4.7),
> changing to gsl_vector* const doesn't fix the issue. Compiling with
> -Wall -W produces no warnings and the program runs as before.

Actually my suggested solution wonÂ’t work. A function declaration like

void f( gsl_vector *const v );

is treated by the compiler as  equivalent to

void f( gsl_vector * v );

and itÂ’s possible to use the second form in a declaration and the first 
in a definition.

The only compiler advantage of the first form is in a function 
definition: you guarantee that v (though not what it points to) wonÂ’t be 
changed. In a declaration it does no more than hint to the user of the 
function that the data wonÂ’t be changed.

Although the gsl_vector functions canÂ’t offer a guarantee not to change 
a const gsl_vector * argument, IÂ’m not sure this is a problem. In 
practice they donÂ’t; so there are no surprises for the user.

I think the root problem is this. Neither C nor C++ has a way to declare 
a struct const so that the data pointed to by any pointer in the struct 
is also const. There are workarounds. But I donÂ’t know of any that donÂ’t 
involve copying data, which is an unnecessary overhead when the only 
real requirement is to reassure a function user that nothing pointed to 
by a function parameter will be changed.

-- 
John D Lamb