string concatenation

[dnewbie]

On Sunday, 8 April 2012 at 05:27:50 UTC, Jonathan M Davis wrote:
> On Sunday, April 08, 2012 07:08:09 dnewbie wrote:
>> I have a wchar[] and I want to convert it to UTF8
>> then append a string. This is my code.
>> 
>> import std.c.windows.windows;
>> import std.string;
>> import std.utf;
>> 
>> int main()
>> {
>>    wchar[100] v;
>>    v[0] = 'H';
>>    v[1] = 'e';
>>    v[2] = 'l';
>>    v[3] = 'l';
>>    v[4] = 'o';
>>    v[5] = 0;
>>    string s = toUTF8(v) ~ ", world!";
>>    MessageBoxA(null, s.toStringz, "myapp", MB_OK);
>>    return 0;
>> }
>> 
>> I want "Hello, world!", but the result is "Hello" only. Please 
>> help me.
>
> D strings are not null-terminated, so v[5] = 0 doesn't do 
> anything to
> terminate that string. The whole point of toStringz is to put a 
> null character
> on the end of a string (and allocate a new string if need be) 
> and return a
> pointer to it. C code will then use the null character to 
> indicate the end of
> the string, but D won't.
>
> What you've done with
>
> string s = toUTF8(v) ~ ", world!";
>
> is create a string that is
>
> ['H', 'e', 'l', 'l', 'o', '\0', '\0', '\0', ...] (all of the 
> characters up to
> the v[99] are '\0')
>
> and append
>
> [',', ' ', 'w', 'o', 'r', 'l', 'd', '!']
>
> to it. The result is a string whith is 108 characters long with 
> a ton of null
> characters in the middle of it. When you call toStringz on it, 
> it may or may
> not allocate a new string, but in the end, you have a string 
> which is 109
> characters long with the last character being '\0' (still with 
> all of the null
> characters in the middle) which you get an immutable char* 
> pointing to.
>
> So, when the C code operates on it, it naturally stops 
> processing when it hits
> the first null character, since that's how C determines the end 
> of a string.
> You can embed null characters in a string and expect C to 
> operate on the whole
> thing. And since D strings aren't null-terminated, you can't 
> expect that
> setting any of it their elements to null will do anything to 
> terminate the
> string in D or stop D functions from operating on the whole 
> string. You have
> to slice the static wchar array with the exact elements that 
> you want if you
> intend to terminate it before its end.
>
> - Jonathan M Davis

OK. Thank you.