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digitalmars.D.learn - Does to!(string)(char[]) do any memory allocation on conversion?

reply Marc <jckj33 gmail.com> writes:
Does to!(string)(char[]) do any memory allocation on conversion 
or is this similar to a cast or what else?
Dec 25 2017
next sibling parent Temtaime <temtaime gmail.com> writes:
On Monday, 25 December 2017 at 14:12:32 UTC, Marc wrote:

 Does to!(string)(char[]) do any memory allocation on conversion 
 or is this similar to a cast or what else?


It is translated to idup.
So yes, it allocates memory.
Dec 25 2017
prev sibling next sibling parent reply Mengu <mengukagan gmail.com> writes:
On Monday, 25 December 2017 at 14:12:32 UTC, Marc wrote:

 Does to!(string)(char[]) do any memory allocation on conversion 
 or is this similar to a cast or what else?


yes, it is allocating memory. you can test such cases with  nogc 
[0].

you can get a char[] via .dup of a string and then you can 
cast(string) if you don't want to allocate any memory.

[0] https://dlang.org/spec/attribute.html#nogc
Dec 25 2017
next sibling parent Temtaime <temtaime gmail.com> writes:
On Monday, 25 December 2017 at 14:37:01 UTC, Mengu wrote:

 On Monday, 25 December 2017 at 14:12:32 UTC, Marc wrote:

 Does to!(string)(char[]) do any memory allocation on 
 conversion or is this similar to a cast or what else?


 yes, it is allocating memory. you can test such cases with 
  nogc [0].

 you can get a char[] via .dup of a string and then you can 
 cast(string) if you don't want to allocate any memory.

 [0] https://dlang.org/spec/attribute.html#nogc


dup allocates memory too
Dec 25 2017
prev sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Monday, 25 December 2017 at 14:37:01 UTC, Mengu wrote:

 yes, it is allocating memory. you can test such cases with 
  nogc [0].


nogc is really conservative and thus gives a lot of false 
positives. I'd just compare instr.ptr is outstr.ptr here and see 
if it changes (it will tho)
Dec 25 2017
prev sibling parent reply aliak <something something.com> writes:
On Monday, 25 December 2017 at 14:12:32 UTC, Marc wrote:

 Does to!(string)(char[]) do any memory allocation on conversion 
 or is this similar to a cast or what else?


As said it calls idup, which calls _trustedDup which seems to 
call _dup which does memory allocation -> 
https://github.com/dlang/druntime/blob/v2.077.1/src/object.d#L3863

I think you can use assumeUnique to avoid allocs though. See code 
gen here:

https://godbolt.org/g/44pLpL
Dec 25 2017
parent Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Monday, December 25, 2017 15:00:19 aliak via Digitalmars-d-learn wrote:

 On Monday, 25 December 2017 at 14:12:32 UTC, Marc wrote:

 Does to!(string)(char[]) do any memory allocation on conversion
 or is this similar to a cast or what else?


 As said it calls idup, which calls _trustedDup which seems to
 call _dup which does memory allocation ->
 https://github.com/dlang/druntime/blob/v2.077.1/src/object.d#L3863

 I think you can use assumeUnique to avoid allocs though. See code
 gen here:

 https://godbolt.org/g/44pLpL


assumeUnique just casts to immutable and should be used with extreme care as
casting from mutable or const to immutable runs a serious risk of subtle
bugs if done incorrectly. If you do it, the reference being cast really
needs to be the only reference to that object.

Almost always, the better approach is to construct an object with a pure
function that is able to implicitly convert its return value to immutable,
because the compiler is able to prove that the mutable object was not passed
into the function and therefore that it is unique.

In the case of strings, to!string is good to use, but it will result in a
new string being allocated if it's not given an immutable string.
std.conv.to generally tries to ensure that conversions are done in a safe
manner, and casting mutability is risky business and best to be avoided in
general.

- Jonathan M Davis
Dec 27 2017