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.
