참조 스택오버플로우

Your first usage of Map is inside a function in the combat class. That happens before Map is defined, hence the error.

A forward declaration only says that a particular class will be defined later, so it's ok to reference it or have pointers to objects, etc. However a forward declaration does not say what members a class has, so as far as the compiler is concerned you can't use any of them until Map is fully declared.

The solution is to follow the C++ pattern of the class declaration in a .h file and the function bodies in a .cpp. That way all the declarations appear before the first definitions, and the compiler knows what it's working with.

안돌아간 코드

// Step07_04.cpp
// 자동미분 구현
#include <bits/stdc++.h>
using namespace std;
// 역전파 구현시작
// Function, FunctionConcrete 분리.
// 근본문제: 49번줄 모순 - m_creator를 Variable 에 두지 말고, Function에 두자. => 07_04로 넘어가자.
class Function;
class Variable;
class Square;
class Exp;

class Variable{
    private:
        float m_data;
        float m_grad;
        Function* m_creator; // 이거 순환참조 안뜨나?
    public:
        Variable(float data=0, float grad=0, Function* creator=NULL){
            m_data = data;
            m_grad = grad;
            m_creator = creator;
        }
        void Set_grad(float grad){
            m_grad = grad;
        }
        void Set_creator(Function* creator){
            // cout<<"Before: m_creator = "<<m_creator<<" creator = "<<creator<<endl;
            m_creator = creator;
            // cout<<"After: m_creator = "<<m_creator<<" creator = "<<creator<<endl;
        }
        float Get_data(){
            return m_data;
        }
        float Get_grad(){
            return m_grad;
        }
        Function* Get_creator(){
            // cout<<"Get_creator == "<<m_creator<<endl;
            return m_creator;
        }
        void Backward(){
            Variable& x = m_creator->Get_inputted();
            x.Set_grad(m_creator->Backward(m_grad));
        }
};

class Function{
    private:
        Variable* inputted;
        Variable outputted;
    public:
        Function(){

        }
        Variable& Get_inputted(){ // 여기서 & 안붙이면 복사되서 전달되므로, 아예 새로운 outputted 이므로 null이 출력된다.
            return *inputted;
        }
        Variable& Get_outputted(){
            return outputted;
        }
        void Set_inputted(Variable* input){ // 맞나?
            inputted = input;
        }
        void Set_outputted(Variable input){ // 맞나?
            // delete(outputted) 필요?
            outputted = input;
        }
        virtual Variable& Forward(Variable& x) = 0;
        virtual float Backward(float gy) = 0;
};

class Square : public Function{
    public:
        Square(){
            Set_inputted(0);
            Set_outputted(Variable(0));
        }
        Variable& Forward(Variable& x){
            Set_inputted(&x);
            Set_outputted(Variable(pow(x.Get_data(),2))); // 이줄 빼고 아랫윗줄 전부 부모클래스로 못올리나?
            Get_outputted().Set_creator(this);
            return Get_outputted();
        }
        float Backward(float gy){
            float x = Get_inputted().Get_data();
            float gx = 2 * x * gy;
            return gx;
        }
};

class Exp : public Function{
    public:
        Exp(){
            Set_inputted(0);
            Set_outputted(Variable(0));
        }
        Variable& Forward(Variable& x){
            Set_inputted(&x);
            Set_outputted(Variable(exp(x.Get_data())));
            Get_outputted().Set_creator(this);
            return Get_outputted();
        }
        float Backward(float gy){
            float x = Get_inputted().Get_data();
            float gx = exp(x) * gy;
            return gx;
        }
};

// f 에 Exp, Square 들어가도 될까?
// https://stackoverflow.com/questions/11422070/c-abstract-class-parameter-error-workaround
// Function 같은 가상클래스는 애초에 "인스턴스화" 자체가 안되지만, 참조자나 포인터로 넘겨주는건 허용된다!
float numerical_diff(Function& f, Variable x, float eps=1e-4){ 
    Variable x0 = Variable(x.Get_data()-eps);
    Variable x1 = Variable(x.Get_data()+eps);
    Variable y0 = f.Forward(x0);
    Variable y1 = f.Forward(x1);
    return (y1.Get_data() - y0.Get_data())/(2*eps);
}

int main(){

    Square A = Square();
    Exp B = Exp();
    Square C = Square();

    Variable x = Variable(0.5);
    Variable a = A.Forward(x);
    Variable b = B.Forward(a);
    Variable y = C.Forward(b);
    cout<<"y is "<<y.Get_data()<<endl;

    y.Set_grad(1);
    b.Set_grad(C.Backward(y.Get_grad()));
    cout <<"dy/db is "<<b.Get_grad()<<endl;
    a.Set_grad(B.Backward(b.Get_grad()));
    cout <<"dy/da is "<<a.Get_grad()<<endl;
    x.Set_grad(A.Backward(a.Get_grad()));
    cout <<"dy/dx is "<<x.Get_grad()<<endl;

    
    cout << "y.creator == "<< y.Get_creator() << " C =="<< &C <<endl;
    cout << "y.creator.input == " << &(C.Get_inputted()) << " b ==" << &b <<endl;
    cout << "y.creator.input.creator == "<< b.Get_creator() << " B == "<< &B<<endl;
    cout << "y.creator.input.creator.input == "<< &(B.Get_inputted()) << " a == " << &a <<endl;
    cout << "y.creator.input.creator.input.creator == "<< a.Get_creator() <<" A == " << &A<<endl;
    cout << "y.creator.input.creator.input.creator.input == "<< &(A.Get_inputted()) << " x == " << &x <<endl;    
    
    /********************** Back Prop  77p**********************/
    y.Set_grad(1);
    b.Set_grad(C.Backward(y.Get_grad()));
    a.Set_grad(B.Backward(b.Get_grad()));
    x.Set_grad(A.Backward(a.Get_grad()));
    cout<<(x.Get_grad())<<endl;
    // assert(y.Get_creator() == C);
    return 0;
}

Forward Declaration해서 돌아가는 코드

// Step07_05.hpp
// 자동미분 구현
// 역전파 구현시작
// Function, FunctionConcrete 분리.
// https://stackoverflow.com/questions/20013901/im-getting-an-error-invalid-use-of-incomplete-type-class-map
// 답변에 따라, hpp와 cpp 분리해보자.
class Variable;
class Function;
class Square;
class Exp;

class Variable{
    private:
        float m_data;
        float m_grad;
        Function* m_creator; // 이거 순환참조 안뜨나?
    public:
        Variable(float data=0, float grad=0, Function* creator=NULL);
        void Set_grad(float grad);
        void Set_creator(Function* creator);
        float Get_data();
        float Get_grad();
        Function* Get_creator();
        void Backward();
};

class Function{
    private:
        Variable* inputted;
        Variable outputted;
    public:
        Function();
        Variable& Get_inputted();
        Variable& Get_outputted();
        void Set_inputted(Variable* input);
        void Set_outputted(Variable input);
        virtual Variable& Forward(Variable& x) = 0;
        virtual float Backward(float gy) = 0;
};

class Square : public Function{
    public:
        Square();
        Variable& Forward(Variable& x);
        float Backward(float gy);
};

class Exp : public Function{
    public:
        Exp();
        Variable& Forward(Variable& x);
        float Backward(float gy);
};

// f 에 Exp, Square 들어가도 될까?
// https://stackoverflow.com/questions/11422070/c-abstract-class-parameter-error-workaround
// Function 같은 가상클래스는 애초에 "인스턴스화" 자체가 안되지만, 참조자나 포인터로 넘겨주는건 허용된다!
float numerical_diff(Function& f, Variable x, float eps=1e-4);

// Step07_05.cpp
// 자동미분 구현
#include <bits/stdc++.h>
#include "Step07_05.hpp"
using namespace std;
// 역전파 구현시작
// Function, FunctionConcrete 분리.
// 근본문제: 49번줄 모순 - m_creator를 Variable 에 두지 말고, Function에 두자. => 07_04로 넘어가자.

Variable::Variable(float data/*=0*/, float grad/*=0*/, Function* creator/*=NULL*/){
    m_data = data;
    m_grad = grad;
    m_creator = creator;
}
void Variable::Set_grad(float grad){
    m_grad = grad;
}
void Variable::Set_creator(Function* creator){
    // cout<<"Before: m_creator = "<<m_creator<<" creator = "<<creator<<endl;
    m_creator = creator;
    // cout<<"After: m_creator = "<<m_creator<<" creator = "<<creator<<endl;
}
float Variable::Get_data(){
    return m_data;
}
float Variable::Get_grad(){
    return m_grad;
}
Function* Variable::Get_creator(){
    // cout<<"Get_creator == "<<m_creator<<endl;
    return m_creator;
}
void Variable::Backward(){
    Variable& x = m_creator->Get_inputted();
    x.Set_grad(m_creator->Backward(m_grad));
}

Function::Function(){
}
Variable& Function::Get_inputted(){ // 여기서 & 안붙이면 복사되서 전달되므로, 아예 새로운 outputted 이므로 null이 출력된다.
    return *inputted;
}
Variable& Function::Get_outputted(){
    return outputted;
}
void Function::Set_inputted(Variable* input){ // 맞나?
    inputted = input;
};
void Function::Set_outputted(Variable input){ // 맞나?
    // delete(outputted) 필요?
    outputted = input;
}

Square::Square(){
    Set_inputted(0);
    Set_outputted(Variable(0));
}
Variable& Square::Forward(Variable& x){
    Set_inputted(&x);
    Set_outputted(Variable(pow(x.Get_data(),2))); // 이줄 빼고 아랫윗줄 전부 부모클래스로 못올리나?
    Get_outputted().Set_creator(this);
    return Get_outputted();
}
float Square::Backward(float gy){
    float x = Get_inputted().Get_data();
    float gx = 2 * x * gy;
    return gx;
}

Exp::Exp(){
    Set_inputted(0);
    Set_outputted(Variable(0));
}
Variable& Exp::Forward(Variable& x){
    Set_inputted(&x);
    Set_outputted(Variable(exp(x.Get_data())));
    Get_outputted().Set_creator(this);
    return Get_outputted();
}
float Exp::Backward(float gy){
    float x = Get_inputted().Get_data();
    float gx = exp(x) * gy;
    return gx;
}

// f 에 Exp, Square 들어가도 될까?
// https://stackoverflow.com/questions/11422070/c-abstract-class-parameter-error-workaround
// Function 같은 가상클래스는 애초에 "인스턴스화" 자체가 안되지만, 참조자나 포인터로 넘겨주는건 허용된다!
float numerical_diff(Function& f, Variable x, float eps/*=1e-4*/){ 
    Variable x0 = Variable(x.Get_data()-eps);
    Variable x1 = Variable(x.Get_data()+eps);
    Variable y0 = f.Forward(x0);
    Variable y1 = f.Forward(x1);
    return (y1.Get_data() - y0.Get_data())/(2*eps);
}

int main(){

    Square A = Square();
    Exp B = Exp();
    Square C = Square();

    Variable x = Variable(0.5);
    Variable a = A.Forward(x);
    Variable b = B.Forward(a);
    Variable y = C.Forward(b);
    cout<<"y is "<<y.Get_data()<<endl;

    y.Set_grad(1);
    b.Set_grad(C.Backward(y.Get_grad()));
    cout <<"dy/db is "<<b.Get_grad()<<endl;
    a.Set_grad(B.Backward(b.Get_grad()));
    cout <<"dy/da is "<<a.Get_grad()<<endl;
    x.Set_grad(A.Backward(a.Get_grad()));
    cout <<"dy/dx is "<<x.Get_grad()<<endl;

    
    cout << "y.creator == "<< y.Get_creator() << " C =="<< &C <<endl;
    cout << "y.creator.input == " << &(C.Get_inputted()) << " b ==" << &b <<endl;
    cout << "y.creator.input.creator == "<< b.Get_creator() << " B == "<< &B<<endl;
    cout << "y.creator.input.creator.input == "<< &(B.Get_inputted()) << " a == " << &a <<endl;
    cout << "y.creator.input.creator.input.creator == "<< a.Get_creator() <<" A == " << &A<<endl;
    cout << "y.creator.input.creator.input.creator.input == "<< &(A.Get_inputted()) << " x == " << &x <<endl;    
    
    /********************** Back Prop  77p**********************/
    y.Set_grad(1);
    b.Set_grad(C.Backward(y.Get_grad()));
    a.Set_grad(B.Backward(b.Get_grad()));
    x.Set_grad(A.Backward(a.Get_grad()));
    cout<<(x.Get_grad())<<endl;
    // assert(y.Get_creator() == C);
    return 0;
}

이유 설명

좀 유식한(?) 말로 이 현상을
Cyclic Reference 를 Break하는데 Forward Declaration 쓴다고 하는구나..
웰논이었네

// car.h
#include whl.h
class Car
{
    std::vector<Wheel> wheels;
};

// whl.h
class Car;     // forward declaration
class Wheel
{
    Car* car;
};