struct construction (how?)

Ali Çehreli acehreli at
Mon Dec 28 07:40:58 PST 2009

bearophile wrote:
 > Ali Çehreli:
 >> auto s = S(1, 2);

Doesn't work for structs that have opCall (or maybe an opCall with 
matching parameters to that use).

 > And by the way, that's the idiomatic way to initialize a struct in D.

Excellent! That's the way I have chosen and have been using in my D 
tutorial. :)

I had included a warning against the C-style initializers; good to see 
that they are gone at least for structs with constructors. Since there 
is also '=void', I think the {} should still default initialize the 
remaining members (like C and C++).

One issue remains, which prompted me to open this thread in the first place:

I wanted to experiment with defining opCall for that struct:

struct S
     int x;
     int y;

     const int opCall(int p0, int p1)
         return p0 + p1;

This does not compile anymore:

     auto s = S(1, 2);
     s(3, 4);          // hoping to call opCall

But compiler error instead:

Error: function expected before (), not s of type int

See, the type of 's' is 'int', meaning that S(1,2) is not a constructor 
but a call to opCall. (This behavior documented on the struct spec page.)

Here is a consistent deduction of that behavior:

- S(1,2) is always the opCall
- if the programmer doesn't define an opCall, the automatic one is 
called and the automatic one initializes the members

That is of course my deduction of the current behavior. I don't know 
what part of that is by design. (?)

Also, I have no clue why we would ever want to use the type name as 
function call syntax as in S(1,2). Coming from C++, I can understand 
s(3,4)... :)

Thank you,

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