Re: inout question

[Steven Schveighoffer via Digitalmars-d-learn]

On 2/12/18 12:33 AM, Norm wrote:

Hi,

I'm new to D so can someone explain to me what is happening here?


void func(const char* s, char** e) {
     import core.stdc.stdlib;
     auto result = strtod(s, e);
}

Error: function core.stdc.stdlib.strtod (scope inout(char)* nptr, scope 
inout(char)** endptr) is not callable using argument types 
(const(char*), char**)


I've found I have to use the following:

void func(inout (char)* s, inout(char)** e)


I thought inout was supposed to take const or non-const variants, so 
expected the original const char* s to work.




A way to think about inout, when you don't understand why you can't call 
it, is to think what inout *should* resolve to, and then see if you can 
assign the existing data to the parameter type.


For example, in this case, you are calling:

strtod(inout(char)* nptr, inout(char)** endptr) with const char *, and 
char **.


Since const char * and char ** vary on mutability, the compiler is going 
to try const in place of inout.


So try it out:

// replace inout with const
const(char)* nptr = s; // ok
const(char)** endptr = e; // Error

I wish the error message was more specific, it would have made things 
obvious. But due to overloading, it's really hard to create errors that 
are good explanations, but not too verbose.


-Steve

Re: inout question

[ketmar via Digitalmars-d-learn]

lobo wrote:

sure, i meant that you have to modify the second parameter accordingly. ;-) 
anyway, it's good that you fixed it.

Re: inout question

[lobo via Digitalmars-d-learn]

On Monday, 12 February 2018 at 05:37:23 UTC, ketmar wrote:

Norm wrote:


Hi,

I'm new to D so can someone explain to me what is happening 
here?



void func(const char* s, char** e) {
 import core.stdc.stdlib;
 auto result = strtod(s, e);
}

Error: function core.stdc.stdlib.strtod (scope inout(char)* 
nptr, scope inout(char)** endptr) is not callable using 
argument types (const(char*), char**)


there is a difference between `const char* s`, and 
`const(char)* s`.
the former means that both `s` and `*s` cannot change, the 
latter means that `s` can be changed, but `*s` cannot. i.e. the 
first form means that you cannot do pointer arithmetic with 
`s`, while with second form you can (only *contents* are const, 
not the pointer itself).


that is, `strtod` wants const *contents*, but not pointer. 
`const(char)* s`.


I tried variations of const (char)* s, ... etc. and then stupidly 
realised the problem was with the second parameter 'e'. inout 
does behave the way I expected, I was just looking at the wrong 
parameter.


My silly mistake but thanks for your reply.

Cheers,
Norm

Re: inout question

[Kagamin via Digitalmars-d-learn]

On Monday, 12 February 2018 at 05:33:16 UTC, Norm wrote:
I thought inout was supposed to take const or non-const 
variants, so expected the original const char* s to work.


The problem is in argument e: it's mutable, and strtod stores 
there a part of s, if s is const you end up with const data 
available for writing through e. Should be const(char)** e.

Re: inout question

[ketmar via Digitalmars-d-learn]

Norm wrote:


Hi,

I'm new to D so can someone explain to me what is happening here?


void func(const char* s, char** e) {
 import core.stdc.stdlib;
 auto result = strtod(s, e);
}

Error: function core.stdc.stdlib.strtod (scope inout(char)* nptr, scope 
inout(char)** endptr) is not callable using argument types (const(char*), 
char**)


there is a difference between `const char* s`, and `const(char)* s`.
the former means that both `s` and `*s` cannot change, the latter means 
that `s` can be changed, but `*s` cannot. i.e. the first form means that 
you cannot do pointer arithmetic with `s`, while with second form you can 
(only *contents* are const, not the pointer itself).


that is, `strtod` wants const *contents*, but not pointer. `const(char)* s`.

inout question

[Norm via Digitalmars-d-learn]

Hi,

I'm new to D so can someone explain to me what is happening here?


void func(const char* s, char** e) {
import core.stdc.stdlib;
auto result = strtod(s, e);
}

Error: function core.stdc.stdlib.strtod (scope inout(char)* nptr, 
scope inout(char)** endptr) is not callable using argument types 
(const(char*), char**)


I've found I have to use the following:

void func(inout (char)* s, inout(char)** e)


I thought inout was supposed to take const or non-const variants, 
so expected the original const char* s to work.


Thanks,
Norm