Defining a function with the type of a class defined within another class

Question

class A {
    class B { }
    B somefunction();
}

B A::somefunction() {
   B bla;
   return bla;
}

So I have something similar to this in my code. And I am getting an error saying that 'B' does not name a type. Any help would be appreciated. P.S. I am not allowed to change the interface.

Sorry for the abstraction but this is a part of an assignment so I only needed to clarify some concept. Otherwise it would be immoral. Thank you all for the help. — Pure, Dec 15 '14 at 18:26

score 4 · Accepted Answer · answered Dec 15 '14 at 18:17

4

A leading return type is interpreted outside the scope of the function; so nested types need to be qualified if used there:

A::B A::somefunction() {

Alternatively, since C++11, you could use a trailing return type, which (like function parameter types) is interpreted in the scope of the function:

auto A::somefunction() -> B {

answered Dec 15 '14 at 18:17

Mike Seymour

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I liked that you used trailing return types :D – Nard Dec 15 '14 at 18:18
1

Might be worth referencing [ADL](http://stackoverflow.com/questions/8111677/what-is-argument-dependent-lookup-aka-adl-or-koenig-lookup)? – matsjoyce Dec 15 '14 at 18:20
@matsjoyce: There's no ADL here, since no function is being called. This is qualified lookup in the first example, and unqualified lookup (in a suitable scope) in the second. – Mike Seymour Dec 15 '14 at 18:25

score 0 · Answer 2 · answered Dec 15 '14 at 18:15

0

B is scoped to A, so in order to refer to B you need use the scope resolution operator: A::B.

So something like A::B A::somefunction() for your definition of somefunction should work.

answered Dec 15 '14 at 18:15

sdzivanovich

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score 0 · Answer 3 · answered Dec 15 '14 at 18:15

0

B is in the A class, and behaves kindof as if it is in an A namespace. Try:

class A {
    class B { }
    B somefunction();
}

A::B A::somefunction() {
   B bla;
   return bla;
}

answered Dec 15 '14 at 18:15

IdeaHat

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`A` isn't a _namespace_, it's just `B` being a nested class. – πάντα ῥεῖ Dec 15 '14 at 18:16

πάντα ῥεῖ · Answer 4 · 2014-12-15T18:21:06.250

0

Your definition of somefunction() should look like

 A::B A::somefunction() {
   B bla;
   return bla;
 }

edited Dec 15 '14 at 18:21

answered Dec 15 '14 at 18:15

πάντα ῥεῖ

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While certainly not wrong, I do believe the the A:: is unnecessary before bla. – IdeaHat Dec 15 '14 at 18:16
@Pure THX for the attention, but _@MikeSeymoure_'s answer explains more in depth what's actually going on. – πάντα ῥεῖ Dec 15 '14 at 18:23

Defining a function with the type of a class defined within another class

4 Answers4