C++ variable has initializer but incomplete type?

57

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;
}

This question is tagged with c++ class compiler-construction

~ Asked on 2012-03-19 13:22:38

The Best Answer is


53

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 <string>

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"

~ Answered on 2012-03-19 13:24:37


14

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.

~ Answered on 2012-08-30 12:30:09


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