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digitalmars.D.learn - Why is it possible to call non-const member functions of rvalues but

reply Johannes Loher <johannes.loher fg4f.de> writes:
Consider the following example:

```
struct A
{

    auto a(int _a)
    {
        return this._a = _a;
    }

    int _a = 0;
}

void main()
{
    static assert(!__traits(compiles, { A()._a = 2; }));
    static assert(__traits(compiles, { A().a(2); }));
}
```
(https://run.dlang.io/is/GkmpA8)

Why is it a compile error to set `_a` directly but calling `a` just
works fine?

If we prevent modifying members of rvalues directly, I would also expect
calling non-const member functions of rvalues to be prevented.
Jun 13 2020
parent reply Stanislav Blinov <stanislav.blinov gmail.com> writes:
On Saturday, 13 June 2020 at 11:26:58 UTC, Johannes Loher wrote:

 Why is it a compile error to set `_a` directly but calling `a` 
 just works fine?

 If we prevent modifying members of rvalues directly, I would 
 also expect calling non-const member functions of rvalues to be 
 prevented.
1) Constructors and destructors are also member functions. Surely you won't suggest making all constructors `const`, and not being able to destruct a temporary? :) 2) The member functions may have useful side effects even on the state of temporary. 3) The member functions may serve as factories. The temporary exists until the end of full expression, or until the end of enclosing statement. It is simply not an lvalue for the caller, but it certainly exists, and so its interface must function. Consider (this code will fail on older compilers, up to 2.067.0, due to a bug): void main() { import std.stdio : File; import std.range : iota; with (File("test.txt", "w")) { foreach (n; iota(1, 100, 5)) writeln(n); } } This will open the file, write numbers [1, 96] on their own lines in that file, and close it. Both the File and the Iota range are temporaries. File can't be `const` as it's a wrapper over FILE*. The range can't be const since the foreach mutates it, and it is effectively a factory of numbers.
Jun 13 2020
parent Johannes Loher <johannes.loher fg4f.de> writes:
On Saturday, 13 June 2020 at 12:47:31 UTC, Stanislav Blinov wrote:
 [...]
 The temporary exists until the end of full expression, or until 
 the end of enclosing statement. It is simply not an lvalue for 
 the caller, but it certainly exists, and so its interface must 
 function.
So public data members are not part of the interface? I understand that it probably doesn't make much sense to mutate data members of an rvalue but this just looks very inconsistent to me, in particular if you consider the fact that you can also call member functions with the = syntax: ``` struct A { auto a(int _a) { return this._a = _a; } int _a = 0; } void main { static assert(__traits(compiles, { A().a = 2; })); // this is ok but looks like it shouldn’t be } ```
Jun 14 2020