Question

connector.hpp

#ifndef __CONNECTOR_HPP_
#define __CONNECTOR_HPP_

#include <a/b/c/connection.hpp>

namespace a {
    namespace b {
        namespace c {

        class connector {
        public:
            explicit connector(const int port);
            void run();
            void stop();
        };

    }
}
}
#endif

connection.hpp

#ifndef __CONNECTION_HPP_
#define __CONNECTION_HPP_

#include <a/b/c/connector.hpp>

namespace a {
    namespace b {
        namespace c {
                
            class connection {
            private:
              connector owner_; //line 42

            };

        }
    }
}

#endif

These simple classes have some trivial (empty) implementations in cpp files.

VS2012 says this when I try to compile this:

Error 1 error C2146: syntax error : missing ‘;’ before identifier ‘owner_’ c:\a\b\c\connection.hpp 42 1 test

Error 2 error C4430: missing type specifier – int assumed. Note: C++ does not support default-int c:\a\b\c\connection.hpp 42 1 test

The thing is that VS editor sees no problems – no red underlines etc.
Include paths are set correctly, I’ve added $(SolutionDir)\..\ to includes so that I can use full paths like a/b/c instead of relative ones.

Still, this doesnt compile.. but it did once.

Q: WHY?

Answer 1

Oftentimes those circular #include directives are deadly. That’s certainly what’s happening here. Suppose you #include "connector.hpp" in some other file. What’s going to happen is that connector.hpp will #include "connection.hpp". This file will in turn #include "connector.hpp", but this is now a no-op because the include guards for connector.hpp are already active. What you end up with is class connection being defined before class connector. That’s a problem because a data member in class connection is an instance of class connector.

Why is connector.hpp including connection.hpp?