Forward Declarations/Includes with Template Classes - "invalid use of incomplete type"

Question

I am trying to make a template class where there is a function that takes in a specific instance of that template. I have made the following contrived example to illustrate this.

Let's say, I have a world of Individuals marked with a templated (generic) type of Data. I have a specific Individual, called a King. And all Individuals should be able to Kneel before a King. Individuals, in general, can be marked as anything. Kings are marked by numbers (the 1st, 2nd king).

The Error

g++ -g -O2 -Wall -Wno-sign-compare -Iinclude -DHAVE_CONFIG_H    -c -o Individual.o Individual.cpp
g++ -g -O2 -Wall -Wno-sign-compare -Iinclude -DHAVE_CONFIG_H    -c -o King.o King.cpp
In file included from King.h:3,
                 from King.cpp:2:
Individual.h: In member function ‘void Individual<Data>::KneelBeforeTheKing(King*)’:
Individual.h:21: error: invalid use of incomplete type ‘struct King’
Individual.h:2: error: forward declaration of ‘struct King’
make: *** [King.o] Error 1

Individual.h (Individual.cpp is empty)

//Individual.h
#pragma once
class King;
#include "King.h"
#include <cstdlib>
#include <cstdio>

template <typename Data> class Individual
{
protected:
    Data d;
    
public:

    void Breathe()
    {
        printf("Breathing...\n");
    };
    
    void KneelBeforeTheKing(King* king)
    {
        king->CommandToKneel();
        printf("Kneeling...\n");
    };
    
    Individual(Data a_d):d(a_d){};
};

King.h

//King.h
#pragma once
#include "Individual.h"
#include <cstdlib>
#include <cstdio>

class King : public Individual<int>
{
protected:

    void CommandToKneel();
    
public:
    
    King(int a_d):
        Individual<int>(a_d)
    {
        printf("I am the No. %d King\n", d);
    };
};

King.cpp

//King.cpp
#include "King.h"
#include <string>

int main(int argc, char** argv)
{
    Individual<std::string> person("Townsperson");
    King* king = new King(1);
    king->Breathe();
    person.Breathe();
    person.KneelBeforeTheKing(king);
    
}

void King::CommandToKneel()
{
    printf("Kneel before me!\n");
}

Makefile

CXX = g++
CXXFLAGS = -g -O2 -Wall -Wno-sign-compare -Iinclude -DHAVE_CONFIG_H 
OBJS = Individual.o King.o

test: $(OBJS)
    $(CXX) -o $@ $^

clean:
    rm -rf $(OBJS) test
    
all: test

Answer 1

Your two classes King and Individual are very tightly coupled.

Having it in two headers like that won't work because both need each other.

If your classes have to be designed that way then:

1. First define class Individual but do not implement KneelBeforeTheKing, just declare that function.

2. Then define King

3. Then implement the method above.

However your design is probably all wrong. For example your template class has many methods that are not dependent on the tamplated type, including KneelBeforeTheKing, and should be refactored out of the template.