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digitalmars.D.learn - Turn GC allocated string into a scoped heap allocation

reply Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
How do I turn the GC-allocation in toLower() to a scoped heap 
allocation together with toLowerInPlace() in

void f(const scope const(char)[] expr)
{
     import std.uni : toLower;
     loweredExpr = toLower(expr); // temporary
     // use loweredExpr as key in hash table
}

when `loweredExpr` is used as a temporary inside `f`?
Dec 13 2018
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 12/13/18 4:38 AM, Per Nordlöw wrote:

 How do I turn the GC-allocation in toLower() to a scoped heap allocation 
 together with toLowerInPlace() in
 
 void f(const scope const(char)[] expr)
 {
      import std.uni : toLower;
      loweredExpr = toLower(expr); // temporary
      // use loweredExpr as key in hash table
 }
 
 when `loweredExpr` is used as a temporary inside `f`?


If you use loweredExpr as a key in a builtin AA, then you need to make 
it a heap allocation, because the GC cleans up AAs.

-Steve
Dec 13 2018
parent reply Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Thursday, 13 December 2018 at 13:46:47 UTC, Steven 
Schveighoffer wrote:

 If you use loweredExpr as a key in a builtin AA, then you need 
 to make it a heap allocation, because the GC cleans up AAs.

 -Steve


I only need it for lookup not for storage.
Dec 13 2018
parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 12/13/18 9:06 AM, Per Nordlöw wrote:

 On Thursday, 13 December 2018 at 13:46:47 UTC, Steven Schveighoffer wrote:

 If you use loweredExpr as a key in a builtin AA, then you need to make 
 it a heap allocation, because the GC cleans up AAs.


 I only need it for lookup not for storage.


I guess what I meant is that the GC will clean up the AA. But I forgot 
that you could free the key early, as long as you never use the AA again.

However, if you are using it ONLY to lookup, and not store keys, this 
means you have to take care not to assign values to new keys.

The right way to do this is to check if the key exists when doing the 
lookup, and then idup'ing the key before you store it.

something like:

if(auto v = tmpKey in AA)
{
    // use *v to deal with the value
}
else
{
    // store a new value
    AA[tmpKey.idup] = newValue;
}

If you are never storing, then you can probably use the .get feature of AAs.

If you want to scope destroy, you have to be wary that toLower will 
return the original string if it's already lowercase.

Something like:

auto tmpKey = s.toLower;
auto toFree = s is tmpKey ? null : tmpKey;
scope(exit) GC.free(toFree.ptr);

-Steve
Dec 13 2018