CODEWITHSUNDEEP
c++usingsubclassing
FlavorScape
FlavorScape

Reputation: 14299

Why must a subclass' using statements be duplicated?

Relatively new to cpp. When you subclass a baseclass, the #include cascade to the subclass, why don't the class-wide usings in the cpp file also cover the scope of the subclass? Is this a historical or pragmatic reason? Err.. What is the reason?

//available to subclass
#include <cinder/app/appBasic.h>

// have to duplicate in subclass
using namespace ci;
using namespace ci::app;
using namespace std;

Upvotes: 0

Views: 125

Answers (2)

Steve Jessop
Steve Jessop

Reputation: 279285

The main reason is that the compiler only looks at one .cpp file plus whatever it #includes. It has no idea what using statements there might be in the .cpp file for your base class. For that matter, it has no idea whether you've even written the .cpp file for the base class yet when you compile the .cpp file for the derived class. And it's not going to go rooting around the filesystem to find out, unlike for example javac.

Furthermore, I guess you're writing one .cpp file per class, and giving the file a name that has something to do with the class name. But C++ doesn't require that. You can have more than one class in a file, or for that matter you could split a class across multiple files if you want to.

So, you know that this .cpp file is the file for your derived class, and that the other .cpp file is the file for the base class, and therefore you think it might be convenient if some stuff from the other .cpp file was lifted into this .cpp file. But the compiler knows no such thing. It wouldn't make sense to the C++ compiler to talk about what's in the file for the base class.

Finally, a reason that is principled rather than pragmatic: just because it is convenient for the implementer of the base class to bring the names from certain namespaces into global scope doesn't mean the same will be true of the implementer of the derived class. The derived class might not use ci::app at all, let alone use it so much that the person writing the derived class is sick of typing it. So even if C++ could require the compiler to fetch those using statements (which, given the compilation model, it can't), I'm pretty sure the language's designers wouldn't want it to.

the #include cascade to the subclass

No they don't. Any #include in the base.h will be included into derived.cpp if (for example) derived.cpp includes derived.h which includes base.h. But any includes in base.cpp have no effect on derived.cpp.

This is all assuming that you follow the usual naming convention. There's nothing other than convention to stop you including a .cpp file from derived.cpp, in which case (1) any includes or using statements would apply in derived.cpp, but unfortunately (2) you'll probably break your build system, because most likely you won't be able to link base.o and derived.o together any more on account of them containing duplicate definitions for code entities subject to the One Defintion Rule. That is, functions.

Upvotes: 1

CS Pei
CS Pei

Reputation: 11047

using directive and using declaration are only valid for the current translation unit. you can put these in the header file which is not a good practice.

Upvotes: 1

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