auto & class members

Robert M. Münch robert.muench at
Sun May 20 17:40:39 UTC 2018

On 2018-05-20 14:49:59 +0000, Jonathan M Davis said:

> In cases like this, typeof is your friend. e.g. something like
> typeof(myStream.filter!(a => a == myMessage)) mySubStream;

Hi Jonathan, great! This got me a step further. So I can declare my 
member now. But I get an implict cast error when I try:

class a {
	... myStream;

class b {
	typeof(a.myStream.filter!(x => x == myMessage)) mySubStream;

void myFunc() {
	a myA = new a();
	b myB = new b();

	myB.mySubstream = myA.myStream.filter!(x => x == myMessage);

This gives (unnecessary stuff stripped):

Error: cannot implicitly convert expression filter(...) of type 
app.myFunc.filter!(x => x == myMessage) to app.b.filter!(x => x == 

Why is myFunc now entering the game? I mean it's just the function 
containing the code. It seems that:

typeof(myA.myStream.filter!(x => x == myMessage))


typeof(a.myStream.filter!(x => x == myMessage)) are not the same.

But inside class b I can't use a specific variable instance. And in 
myFunc, I can't use a class type but need the specific instance.

Any further idea?

> though you might have trouble with the lambda being subtly different type
> even if you replace myMessage in it with something that will work in the
> scope that mySubStream is being declared.

Not sure if the above problem is exactly what you mention here. This is 
all pretty tricky.

> However, that could be fixed by
> doing something like replacing the lambda with a free function or just
> creating a function that returns what you want to assign to mySubStream.
> Then you could just do something like
> typeof(myHelperFunc(myStream)) mySubStream;
> The exact solution can get a bit annoying in cases like this (it's arguably
> the biggest downside to auto returns), but typeof does provide a way out if
> you can get the expression to it to work. It is easier with local variables
> than member variables though, since you don't necessarily have everything
> you want to use in the expression that will give the variable its value
> available at the point that the variable is declared - but that's why
> a helper function can help, since it provides a way to encapsulate the
> expression and reuse it between the assignment and the declaration.

Not sure I understand every aspect but it's getting clearer... Thanks so far.

Robert M. Münch
smarter | better | faster

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