Error with using keyword with curiously recurring template pattern

Question

Does anyone know why the following code compilation cannot find N::balance_type?

I'm using this example from Stroustrup C++ 4th Ed Page 771. It is the curiously recurring template pattern.

class Bal {};

template<typename N>
struct Node_base : N::balance_type {
};

template<typename Val, typename Balance>
struct Search_node : public Node_base<Search_node<Val,Balance>> {
    using balance_type = Balance;
};

int main()
{
    Search_node<int, Bal> sn;
    return 0;
}

Compilation:

clang++ -std=c++11 -Wall -pedantic test197.cc && ./a.out
test197.cc:4:23: error: no type named 'balance_type' in 'Search_node<int, Bal>'
struct Node_base : N::balance_type {
                   ~~~^~~~~~~~~~~~
test197.cc:8:29: note: in instantiation of template class
      'Node_base<Search_node<int, Bal> >' requested here
struct Search_node : public Node_base<Search_node<Val,Balance>> {
                            ^
test197.cc:14:27: note: in instantiation of template class
      'Search_node<int, Bal>' requested here
    Search_node<int, Bal> sn;
                          ^
1 error generated.

At the point where `N::balance_type` is being used, `N` refers to an **incomplete type**. `balance_type` isn't defined as the compiler hasn't seen it yet — Remy Lebeau, Jul 25 '20 at 17:49
fwiw, you could make `Node` inherit from `N` and then make `Search_node` inherit from `Node`. The example is a bit weird imho — 463035818_is_not_an_ai, Jul 25 '20 at 18:12

score 5 · Accepted Answer · answered Jul 25 '20 at 17:48

5

g++ says SearchNode is an incomplete type when you derive from Node_base<Search_node<Val,Balance>>

test.cpp: In instantiation of ‘struct Node_base<Search_node<int, Bal> >’:
test.cpp:8:8:   required from ‘struct Search_node<int, Bal>’
test.cpp:14:25:   required from here
test.cpp:4:8: error: invalid use of incomplete type ‘struct Search_node<int, Bal>’
 struct Node_base : N::balance_type {
        ^~~~~~~~~
test.cpp:8:8: note: declaration of ‘struct Search_node<int, Bal>’
 struct Search_node : public Node_base<Search_node<Val,Balance>> {

I guess using SearchNode in this place is invalid because you are defining it right now.

answered Jul 25 '20 at 17:48

Łukasz Ślusarczyk

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This is indeed correct. I can't really explain why this example appears in the book. Unless its purpose is to explain pitfalls. – StoryTeller - Unslander Monica Jul 25 '20 at 17:51
So is this possibly Stroustrup Errata? It wouldn't surprise me because I have found other unpublished Errata later in this book. – notaorb Jul 25 '20 at 17:53
@notaorb is it the complete example from the book, no modifications? – 463035818_is_not_an_ai Jul 25 '20 at 18:00
@idclev463035818 I cut and pasted it and added `class Bal` and `main` – notaorb Jul 25 '20 at 18:01

Error with using keyword with curiously recurring template pattern

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