On Thursday, 12 February 2026 at 12:55:24 UTC, Brother Bill wrote:
Near the top of page 306 in Programming in D book, there is
this note:
*As an optimization, sometimes it makes more sense for*
```opAssign``` *to return* ```const ref``` *for large structs.*
```
import std.stdio : writeln, writefln;
void main()
{
auto mms = ManyMembersStruct();
mms = 42;
}
struct ManyMembersStruct
{
int a;
int b;
long c;
long d;
// this fails to compile as "const" means that no
members can be mutated.
const ref ManyMembersStruct opAssign(int a) {
this.a = a;
return this;
}
}
```
What is the correct way to have opAssign return const ref?
And what does it mean to return const ref?
Please provide a working code sample.
`const` as a function attribute applies to the `this` reference.
If you want a `const` return value, you have to use the type
constructor form:
```d
const int foo(); // `this` is const, returns mutable int
const(int) foo(); // `this` is mutable, returns const int
const const(int) foo(); // `this` is const, returns const int
const(int) foo() const; // same as above, trailing const applies
to `this`
```
As shown in the final form, you can apply attributes to `this`
with a trailing type modifier. You should prefer this form
because it's clearer when reading.
-Steve
