Undefined reference to Classname::Classname() and other errors - c++

Question

This is a noob question, sorry, I'm coming from Java and have no idea why my OO stuff isn't working. I have this main:
#include <iostream>
#include "Foo.h" //changed name
using namespace std;
int main(int argc, char*argv[])
{
int choice;
cin >> choice;
Foo net;
switch(choice)
{
case 1: net.buildNetwork(); break;
}
}
This Foo.h file:
#ifndef FOO_H
#define FOO_H
#include <iostream>
struct City{
std::string cityName;
std::string message;
City *next;
City(){}; // default constructor
City(std::string initName, City *initNext, std::string initMessage)
{
cityName = initName;
next = initNext;
message = initMessage;
}
};
class Foo
{
public:
Foo();
~Foo();
void addCity(std::string, std::string);
void buildNetwork();
void transmitMsg(char *); //this is like a string
void printNetwork();
protected:
private:
City *head;
City *tail;
};
#endif // FOO_H
And this Foo.cpp file, all in the same directory:
#include "Foo.h"
#include <iostream>
using namespace std;
Foo::Foo()
{
head = tail = NULL;
}
Foo::~Foo(){}
void Foo::buildNetwork()
{
cout << "works" << endl;
}
void Foo::transmitMsg(){}
void Foo::printNetwork(){}
void Foo::addCity(){}
When I compile, I get
/tmp/ccNx3fY5.o: In function `main':
main.cpp:(.text+0x38): undefined reference to `Foo::Foo()'
main.cpp:(.text+0x4c): undefined reference to `Foo::buildNetwork()'
main.cpp:(.text+0x59): undefined reference to `Foo::~Foo()'
main.cpp:(.text+0x7e): undefined reference to `Foo::~Foo()'
collect2: error: ld returned 1 exit status
What's wrong? Also, another question: in Foo.cpp, why do I need Foo::Foo() etc? I used namespace std, so why can't I just say Foo()?

Answer 1

Looking at the way you compile, you are only providing only one source file (main.cpp) whereas the correct way is to specify all the source files. In your case, it would be:
g++ main.cpp foo.cpp -o executable
The "undefined reference" error is a error thrown at the linking stage when the linker can't resolve the names correctly because, you didn't link the source files properly like above.
Additionally, make sure that when you declare a function prototype, the implementation of the function should also have the same signature. In your example, you provided the function prototypes as:
void transmitMsg(char *);
void addCity(std::string, std::string);
But your implementation of those functions don't have the correct signature. They should have been:
void Foo::transmitMsg(char *){}
void Foo::addCity(std::string, std::string){}
why do I need Foo::Foo() etc?
Because Foo() is a function of the class Foo.
I used namespace std, so why can't I just say Foo()?
When you make a call to using namespace ; all symbols in that namespace will become visible without adding the namespace prefix. A symbol may be for instance a function, class or a variable.
Foo is not a namespace like "std". It is a user-defined class.

Answer 2

Also, another question: in Foo.cpp, why do I need Foo::Foo() etc? I
used namespace std, so why can't I just say Foo()?
You need to write Foo::Foo() in foo.cpp because you are defining the constructor outside the body of the Foo class which is present in foo.h.
std is the standard namespace and using that in no way absolves you from referring to the Foo class you have created since its not a part of the standard namespace

Answer 3

You have not included foo.cpp in your compile command, So this is why you cant link the functions. You need to use the command line argument:
g++ main.cpp foo.cpp -o main
This allows the compiler to find the functions in foo.cpp.