C++ variable has initializer but incomplete type?

I am trying to compile 2 classes in C++ with the following command:

g++ Cat.cpp Cat_main.cpp -o Cat

But I receive the following error:

Cat.cpp:10:10: error: variable ‘Cat Joey’ has initializer but incomplete type

Could someone explain to me what this means? What my files basically do is create a class (Cat.cpp) and create an instance (Cat_main.cpp). Here is my source code:

Cat.cpp:

#include <iostream>
#include <string>

class Cat;

using namespace std;

int main()
{
	Cat Joey("Joey");
	Joey.Meow();

	return 0;
}

Cat_main.cpp:

#include <iostream>
#include <string>

using namespace std;

class Cat
{
	public:
		Cat(string str);
	// Variables
		string name;
	// Functions
		void Meow();
};

Cat::Cat(string str)
{
	this->name = str;
}

void Cat::Meow()
{
	cout << "Meow!" << endl;
	return;
}

You use a forward declaration when you need a complete type.

You must have a full definition of the class in order to use it.

The usual way to go about this is:

1. create a file Cat_main.h

2. move

   #include

   class Cat { public: Cat(std::string str); // Variables std::string name; // Functions void Meow(); };

to Cat_main.h. Note that inside the header I removed using namespace std; and qualified string with std::string.

3. include this file in both Cat_main.cpp and Cat.cpp:

   #include "Cat_main.h"

It's not related to Ken's case directly, but such an error also can occur if you copied .h file and forgot to change #ifndef directive. In this case compiler will just skip definition of the class thinking that it's a duplication.

Sometimes, the same error occurs when you forget to include the corresponding header.

You cannot define a variable of an incomplete type. You need to bring the whole definition of Cat into scope before you can create the local variable in main. I recommend that you move the definition of the type Cat to a header and include it from the translation unit that has main.

I got a similar error and hit this page while searching the solution.

With Qt this error can happen if you forget to add the QT_WRAP_CPP( ... ) step in your build to run meta object compiler (moc). Including the Qt header is not sufficient.