On 6/19/23 2:19 PM, mw wrote:
Hi, I just saw this line:
https://github.com/dlang/dmd/blob/master/druntime/src/core/stdcpp/vector.d#LL66C5-L66C39
```
66: ref inout(T) opIndex(size_t index) inout pure nothrow @safe
@nogc { return as_array[index]; }
```
I'm wondering if the `ref` and `inout` redundant here? They both mean
the same thing? in C++ terms both return the reference of the i-th
element? so only one of them should be enough?
No, they do not both mean the same thing. inout is a form of mutability
that is unique to D. It does *not* mean the same as `ref` like other
languages (or even D1).
What `inout` does is forward the mutability of the parameter to the
return type.
If not, can someone help to explain the difference? the following 4
return types:
1) `ref T` alone
a reference to a T.
2) `inout T` alone
An inout T passed by value. Sorry for the recursive definition, but
inout is kinda unique with D.
3) `ref inout(T)`
A reference to an inout T.
4) `inout ref(T)`
I'm not sure that's valid. `ref` is a storage class, not a type modifier.
BTW, what does the second `inout` before `pure` do? it's also redundant?
This is the qualifier put onto the `this` parameter (i.e. the `vector`
in this case).
Because of this, you get the mutability of the parameter forwarded to
the return type.
```d
const vector!int c;
immutable vector!int i;
vector!int m;
static assert(is(typeof(c[0]) == const(int)));
static assert(is(typeof(i[0]) == immutable(int)));
static assert(is(typeof(m[0]) == int));
```
I gave a presentation on const/inout, which you might find helpful.
https://dconf.org/2016/talks/schveighoffer.html
-Steve
