Why does this call the copy constructor 2 times and the assigment constructor once?

Paul Backus snarwin at gmail.com
Fri Nov 19 14:22:07 UTC 2021

On Friday, 19 November 2021 at 14:05:40 UTC, rempas wrote:
> So, when I assign the value of the variable "name" in the 
> "other_name", first it will call the copy constructor, then it 
> will call the assignment constructor and then it will call the 
> copy constructor again. Why is this happening? I was expecting 
> only the assignment constructor to get called.

When you pass a struct instance to a function by value, or return 
a struct instance from a function by value, a copy is made, and 
the copy constructor is called.

Your `opAssign` takes `rhs` by value, and returns a `str` by 

// Returned by value
// |
// v
   str opAssign(str rhs) {
//              ^
//              |
// Passed by value

So, every call to it will result in two calls to `str`'s copy 

If you want to avoid this, you can change `opAssign` to have the 
following signature:

// Returned by referece
// |
// v
   ref str opAssign()(auto ref str rhs)
//                       ^
//                       |
// Passed by reference (if possible)

Since `auto ref` is only allowed for template functions, I have 
added an empty template argument list (the `()`) to make 
`opAssign` into a function template.

You can read more about [`ref` functions][1] and [`auto ref` 
parameters][2] in the D language specification.

[1]: https://dlang.org/spec/function.html#ref-functions
[2]: https://dlang.org/spec/template.html#auto-ref-parameters

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