build main

% g++ -std=c++11 -c main.cpp                                                                                                                                          [0]
main.cpp: In function ‘int main()’:
main.cpp:8:25: error: invalid use of incomplete type ‘class Status’
   std::cout << f.second().str() << '\n';
                         ^
In file included from main.cpp:1:0:
first.h:3:7: error: forward declaration of ‘class Status’
 class Status;

first.cpp

#include "first.h"
#include "status.h"

Status First::second() const {
  return Status{};
}

first.h

#pragma once

class Status;

class First {
public:
  explicit First() = default;
  Status second() const;
};

main.cpp

#include "first.h"
#include <iostream>

int main() {
  auto f = First();
  std::cout << f.second().str() << '\n';
}

status.cpp

#include "status.h"

std::string Second::str() const {
  return std::string{"hello"};
}

status.h

#pragma once

#include <string>

class Status {
public:
  explicit Status() = default;
  std::string str() const;
};