Re: question about linker

On 29/11/2024 07:38, David Brown wrote:

On 29/11/2024 00:05, Bart wrote:

On 28/11/2024 22:38, Keith Thompson wrote:

Bart <bc@freeuk.com> writes:

On 28/11/2024 19:58, Keith Thompson wrote:

Bart <bc@freeuk.com> writes:
[...]

I think 'const' is confusing for similar reasons that VLAs can be both
confusing and awkward to implement.
>
That's because both really apply to /types/, not directly to variables.

Sure.  For example, given
      const int n = 42;
n is of type `const int`, and &n is of type `consts int*`.  Of course
that implies that n itself is const.

>
But that is a separate thing. Suppose T was an alias for 'const int'. Then:
>
   T x;           // defines a readonly variable (which probably needs
                  // initialising)
   T* y;          // defines a variable pointer
>
'const' is out of the picture.

>
You say T is an alias (what, a macro?) for 'const int', you show code
using T, and then you say "'const' is out of the picture".  If you have
a point, it escapes me.

>
Well, can you see 'const' in my example? You can't tell x is readonly by only looking at this.
>
>

Yes, and you seem determines to make it easier to get mixed up.

>
>
C doesn't require any help from me for confusing features. The OP said it was confusing and I tried to point out why it might be.
>
Obviously you as C expert will never be confused. But there are lots of less expert users of the language.
>
I've just several minutes trying to figure why all these assignments are invalid:
>
     typedef int* T;
>
     int  const x;

 That one is really simple - clearly "x" is declared "const", and so you can't assign to it later.
 

     T    const y;

 That one is equally simple - clearly "y" is declared "const", and so you can't assign to it later.
 That shows exactly why it can be a good idea to use "typedef", even for relatively simple things such as adding a pointer or qualifiers to the type.  I would not normally make a typedef just for a pointer, or just to add a "const" qualifier, but if the final type is complicated, it can make things clearer.  Really, it's just like breaking a complicated expression into parts with extra local variables to name them.
 

     int* const z;
>

>
This one requires a little more thought, but it should be well within the capacity of any C programmer to see that "z" is declared "const".

These are similar examples:
     int * const z1;
     int const * z2;
     z1=0;          // invalid
     z2=0;          // valid

     x=0;
     y=0;
     z=0;
>
because I thought would behave differently, with 'const' being the opposite side of '*' to the base-type.
>

 The "const" in each case clearly applies to the type of the declared variable.
 

I forgot that here it would be the right-most 'const' that controls storage attributes of 'z'.

Both 'const' in my new examples are the the right-most one! Yet one makes the immediate storage const and one doesn't. I guess then that it's the right-most possible 'const' if it were to be used. In my example,  that would follow the '*'.

You will of course say that I'm the only person in the world who could make that mistake.
>

 I certainly don't say you are the only person in the world who /could/ make that mistake.  But if we narrow it down to the C programmers who /would/ make such mistakes, you are in a much smaller group.
 There are two types of C programmers - those who like to declare variables "const" on a regular basis, and those who rarely if ever do (reserving "const" for things like "const int *" pointers).  Those that declare const variables, initialise them at the time, and would not be trying to assign them later.  Those who don't have much use of const variables, would not be writing such code in the first place.
 Basically, I'd only expect to see examples like that in the questions section of a beginner's book on C.  And I'd expect anyone who had read the preceding chapter to be able to answer it.

My original point was trying to address why 'const' as used in C might be confusing. I was trying to compare how non-mutable variables are designated in other languages. There it's a keyword that is part of the declaration syntax, not the type syntax.
I suggested that C is confusing because 'const' looks as though it's like the former, but it's part of the latter. Which also means you can have multiple 'const' in a declaration (putting asided repeated 'consts').
So objectively, it IS more complicated than elsewhere with more scope for getting it wrong.
But of course, this group being what it is, people have to turn it round to make it about me: I'm deliberately trying to show confusing examples. Or I'm to thick to understand how const works.