This happens usually when the compiler has seen a forward declaration but no full definition of this type, while the type is being used somewhere. For example: class A; class B { A a; }; The second line will cause a compiler error and, depending on the compiler, will report an incomplete type (other compilers … Read more
Move the struct declaration to the header: customer.h typedef struct CustomerStruct { … }
(I was waiting to Alf Steinbach to post an answer, but since he is not doing it, I will post the reference that he mentioned in the Lounge C++ chat): The standard indicates that template instantiations are performed after the translation unit has already been translated, so that in time, template instantiations happen after all … Read more
When a class template is instantiated, its members other than non-virtual member functions are instantiated together with it. Non-virtual member functions, however, are only instantiated when odr-used (basically, called or have their address taken). When the compiler encounters class Implementation : public Interface<Implementation>, it needs to instantiate Interface<Implementation>. At this point, Implementation is still an … Read more
Let us look at each of the cases. Case 1 Here we have the statement int a[]; //this is a definition so size must be known This is not valid. Case 2 Here we have the statement: extern int b[];//this is a declaration that is not a definition This is valid. Here the type of … Read more
It’s the array that’s causing trouble in: void print_graph(g_node graph_node[], double weight[][], int nodes); The second and subsequent dimensions must be given: void print_graph(g_node graph_node[], double weight[][32], int nodes); Or you can just give a pointer to pointer: void print_graph(g_node graph_node[], double **weight, int nodes); However, although they look similar, those are very different internally. … Read more
You are correct; the example seems to be missing an explicit template instantiation. When I try to run the example with a constructor and destructor for widget::impl on MSVC 2010 SP1, I get a linker error for pimpl<widget::impl>::pimpl() and pimpl<widget::impl>::~pimpl(). When I add template class pimpl<widget::impl>;, it links fine. In other words, GotW #101 eliminates … Read more
This paper was adopted into C++17 which allows incomplete types to be used in certain STL containers. Prior to that, it was Undefined Behavior. To quote from the paper: Based on the discussion on the Issaquah meeting, we achieved the consensus to proceed* with the approach – “Containers of Incomplete Types”, but limit the scope … Read more
At the time you declare your member, you are still defining the A class, so the type A is still undefined. However, when you write A*, the compiler already knows that A stands for a class name, and so the type “pointer to A” is defined. That’s why you can embed a pointer to the … Read more
Here’s my attempt at an interpretation: The standard simply says you mustn’t do this, even though any given concrete implementation may have no problem supporting such a construction. But imagine for example if someone wanted to write a “small vector” optimization by which a vector always contains space for, say, five elements. Immediately you’d be … Read more