struct construction (how?)

bearophile bearophileHUGS at
Mon Dec 28 02:28:31 PST 2009

Ali Çehreli:

> S s = { 1 };
> Not good; because we may forget to provide initializers and the 
> remaining members are not even default initialized.

All fields get initialized, unless explicitly told to not be initialized (and sometimes they get initialized even if you explicitly tell them to be not initialized).

Another simple way to initialize the fields:
struct S {
    int x = 1;
    int y = 2;

>(I wrote about this recently; I hope it's just a bug and will get fixed.)<

A recent change done by Walter:
bugzilla 3476 C-style initializer for structs must be disallowed for structs with a constructor

> S s;
> s.x = 1;
> s.y = 2;
> Not good; because that is default initialization plus assignment.

You can change that to:
S s = void;
s.x = 1;
s.y = 2;
But in practice even without the =void LDC is often able to spot the redundancy and remove it.

> auto s = S(1, 2);
> Not good, because the right hand side is a struct literal and there is a 
> copy involved.

The compiler isn't dumb, here I think it performs only one initialization. You can take a look at the resulting asm when/if you aren't sure. Months ago I have filed a bug to llvm asking for better capabilities in detecting and removing similar double initializations of dynamic arrays.

> Am I giving structs too much 
> attention? Should I just know that they exist but use classes instead?

structs have their purpose, they can be useful to lay memory as you need, or for performance purposes, to avoid heap allocations (this is especially true for small structs, like a Vector3 or Complex, that coupled with the slow D GC and low-tech D compilers produce very slow code if you manage them as objects instead of structs). Structs have some downsides too, so you have to be careful. In some situations I start coding with classes and later when the code works correctly and I have a unittest suite that can be used as a safety net, I convert some of them to structs for performance purposes.


More information about the Digitalmars-d-learn mailing list