包括 g++ header：仍然找不到定义

Question

Good evening:)

I'm playing around with g++ and makefiles. I've gotten to this point:

foo.h:

#ifndef _FOO_H_
#define _FOO_H_

#include "bar.h"

class foo {
private:
    bar something;
public:
    bool start();
    bool stop();
};

#endif // _FOO_H_

Foo.h is eventually included in my main cpp file so I can set things in motion by calling start/stop.

void somewhere() {
    foo* hihi = new foo;
    hihi->start();
    delete hihi;
}

Then there's bar.h:

#ifndef _BAR_H_
#define _BAR_H_

class bar {

};

#endif // _BAR_H_

g++ doesn't seem to like it however:

g++  (some_flags) -c main.cpp
In file included from main.cpp:2:
foo.h:8: error: ‘bar’ does not name a type

I'm using makefiles, and tried a combination of things like:

main.o: main.cpp foo.h bar.h

Even though I don't think I should have to add bar.h here, shouldn't including it in foo.h be enough?

To clarify, this is roughly how it's set up now (yes I know this can be done in a more efficient manner):

main.o: main.cpp foo.h
    $(CC) $(CFLAGS) -c main.cpp

foo.o: foo.h foo.cpp
    $(CC) $(CFLAGS) -c foo.cpp

bar.o: bar.h bar.cpp
    $(CC) $(CFLAGS) -c bar.cpp

What's going on? I figure it's something I'm missing about g++ and the way it handles header includes, point me in the right direction please!

edit - found the solution:

Doh. I feel dumb right now: Was messing around with boost::asio and kind of forgot I still left this on top of my headers somewhere: using boost::asio::ip:;tcp;

Let's just say there's a boost::asio::ip::tcp::bar function:D

Oh well, thanks anyway!

Answer 1

Double check everything. If you include bar.h into foo.h , the compiler should not raise up an error. Do you include foo.h from bar.h ? Better don't do this because that would cause a circular dependency between headers, which will cause that kind of bugs.

Also check for spelling of header guards. This can be a common source of annoyance:

#ifdef _BAR_H_ // OOPS! we wanted #ifndef
#define _BAR_H_

class bar {

};

#endif // _BAR_H_

In addition, you should avoid putting the underscore before your header guard macro name. These names are reserved to the compiler. Call it INCLUDED_BAR_H or just BAR_H_ instead.

Answer 2

Was messing around with boost::asio and kind of forgot I still left this on top of my headers somewhere: using boost::asio::ip::tcp;

Let's just say there's a boost::asio::ip::tcp::bar function

Dan Saks explains some reasons why you should typedef your class names, even though it might seem redundant.

Well, you've run into real life situation where typedefing a class would have probably helped you find your problem a little easier:

 class bar {
 // ...
 };
 typedef class bar bar;

Generates this more meaningful message if there's a function named bar() already declared:

In file included from C:\temp\foo.h:4,
                 from C:\temp\test.cpp:4:
C:\temp\bar.h:7: error: `typedef class bar bar' redeclared as different kind of symbol
C:\temp\test.cpp:1: error: previous declaration of `void bar(int)'
C:\temp\bar.h:7: error: declaration of `typedef class bar bar'

Answer 3

Tommy Hui and litb have already pointed to two probable causes; here's some background information that will hopefully come in useful.

First off, this has nothing to do with makefiles. A makefile is just a convenience thing that calls g++ for you, nothing more.

This stuff with header files can be a bit tricky to understand at first, especially once you get into circular dependencies. It really helped me to realize these three things:

• A #include is nothing more than a copy/paste operation, inserting the contents of the included file at that point.
• Everything must have been declared before it is used. Sometimes, when circular dependencies crop up, you might need to predeclare a class (eg class bar; ) in another header.
• If, at the point of usage, any more information about the type is needed than just its existence, then the type's definition is needed as well. This goes for eg calling of a method on an object of that type, but also for including a field of that type, (Not for a pointer to an object of that type, though. because pointers are all the same size.)

Answer 4

In the main.cpp, you'll need to add the following line at the top of the file:

#include "foo.h"