C++ Multiply Defined constructor error message seems wrong - c++

Question

gcc version 5.4.0 (GCC)
> g++ -std=c++11
During a build I get a multiply defined error message for a class constructor. When I delete the constructor, I get an undefined symbol error message. I'm stumped.
NodeClass::NodeClass( ... ) is marked as
Error Message:
build/Debug/Cygwin64-Windows/nodeClass.o: In function `__gnu_cxx::new_allocator<std::string*>::~new_allocator()':
/c/home/skidmarks/Projects/MPHASH/mphash/NodeClass.cpp:36: multiple definition of `NodeClass::NodeClass(std::vector<std::string*, std::allocator<std::string*> >&)'
build/Debug/Cygwin64-Windows/NodeClass.o:/c/home/skidmarks/Projects/MPHASH/mphash/NodeClass.cpp:36: first defined here
Header file:
# include <vector>
# include <gslip/SlipPointer.h>
class NodeClass : public SlipPointer
{
private:
vector<string*> vec;
public:
NodeClass(vector<string*>& vec);
virtual ~NodeClass() { };
private:
NodeClass(const NodeClass& orig) { };
};
Source Code:
# include <vector>
# include "NodeClass.h"
using namespace std;
using namespace slip;
NodeClass::NodeClass(vector<string*>& vec) :
SlipPointer(new string("Cluster Node")), vec(vec) {}

Answer 1

You have not provided sufficient info to confidently identify your compile error. I agree it is possible that a double definition might happen without sufficient and appropriate include guards. You have shown no include guards, and your error is not evident in the above code,
There are 2 types of include guards. I have seen both used, but less often at the same time. All are examples of conditional compile commands.
In the header file with name "NodeClass.h", my version of the header guard would have the first 2 and last line (examples of conditional compilation) of the following:
#ifndef NODECLASS_H // <--- header include guard
#define NODECLASS_H // <---
#include <vector>
#include <gslip/SlipPointer.h>
class NodeClass : public SlipPointer
{
private:
vector<string*> vec;
public:
NodeClass(vector<string*>& vec);
virtual ~NodeClass() { };
private:
NodeClass(const NodeClass& orig) { };
};
#endif // <---
In the code file, name "NodeClass.cc", my 2nd version of the header guard would include similar conditional compilation statements. This is less used in most environments, and I prefer the above.
#ifndef NODECLASS_H
#include "NodeClass.h"
#endif
#include <vector> // <-- redundant since also in header
using namespace std;
using namespace slip;
NodeClass::NodeClass(vector<string*>& vec)
: SlipPointer(new string("Cluster Node"))
, vec(vec)
{
}

Answer 2

I suspect you have out-of-line definitions of these functions in you header.
Initialization list is part of constructor's definition so you need to define it at the same place you define constructor's body. This means that you should have to do it in your header file:
NodeClass(vector<string*>& vec) :
SlipPointer(new string("Cluster Node")), vec(vec) {}
Since the definition of the constructor is not inline in the class definition, the compiler does not make it implicitly inline. Now, if you include the same file in multiple translation units (i.e. *.cpp files), the linker will produce exactly the error you see, because each of the *.cpp files will contain its own definition of the constructor without them being marked as inline functions.
Alternate easy solution is just to put a inline in front of the constructor declaration:
public:
inline NodeClass(vector<string*>& vec);