Passing ref through a template chain

[Steve Teale]

To bind variables, the MySQL api wants their addresses - in my tiny example
below, these are represented by the void*[].

If I just use something like setTarget in the example, it works fine, but then
when I try to set a bunch of them in one go I'm hosed because I can't get the
right addresses through the chain.

import std.stdio;

void*[2] vpa;

void setTarget(T)(ref T t, int i)
{
   vpa[i] = t;
}

void setSeveral(T...)(ref T args)
{
   foreach (int i, arg; args)
  setTarget(arg, i);
}

//ref.d|19|Error: variable i cannot be read at compile time|
//ref.d|19|Error: variable i cannot be read at compile time|
//ref.d|19|Error: Integer constant expression expected instead of cast(uint)i|
/*
void setSeveral2(T...)(ref T args)
{
   for (int i = 0; i  args.length; i++)
  setTarget(args[i], i);
}
*/

void main()
{
   bool a;
   int b;
   setSeveral(a, b);
   writefln(orig %x %x, set %x %x, a, b, vpa[0], vpa[1]); // addresses 
differ
}

The problem arises I believe in 'foreach (int i, arg; args)' which makes a
copy for 'arg'. Usually in a foreach I can get round that by making it 'ref
arg'. But in the template function that does not work.

I can't use the setSeveral2() form, because that provokes the errors shown in
the comments.

How can I force the real addresses through the chain?

Thanks
Steve

Re: Passing ref through a template chain

[Jacob Carlborg]

On 2011-10-03 15:08, Steve Teale wrote:

To bind variables, the MySQL api wants their addresses - in my tiny example
below, these are represented by the void*[].

If I just use something like setTarget in the example, it works fine, but then
when I try to set a bunch of them in one go I'm hosed because I can't get the
right addresses through the chain.

import std.stdio;

void*[2] vpa;

void setTarget(T)(ref T t, int i)
{
vpa[i] =t;
}

void setSeveral(T...)(ref T args)
{
foreach (int i, arg; args)
   setTarget(arg, i);
}

//ref.d|19|Error: variable i cannot be read at compile time|
//ref.d|19|Error: variable i cannot be read at compile time|
//ref.d|19|Error: Integer constant expression expected instead of cast(uint)i|
/*
void setSeveral2(T...)(ref T args)
{
for (int i = 0; i  args.length; i++)
   setTarget(args[i], i);
}
*/

void main()
{
bool a;
int b;
setSeveral(a, b);
writefln(orig %x %x, set %x %x,a,b, vpa[0], vpa[1]); // addresses differ
}

The problem arises I believe in 'foreach (int i, arg; args)' which makes a
copy for 'arg'. Usually in a foreach I can get round that by making it 'ref
arg'. But in the template function that does not work.

I can't use the setSeveral2() form, because that provokes the errors shown in
the comments.

How can I force the real addresses through the chain?

Thanks
Steve


Does this work:

void setSeveral(T...)(ref T args)
{
   foreach (i, _; args)
  setTarget(args[i], i);
}

Or you already tried that in setSeveral2.

--
/Jacob Carlborg

Re: Passing ref through a template chain

[Steve Teale]

Works like a charm, and it's so obvious a thing to try that I'm kicking myself.

Thanks
Steve