Why does including a declaration resolve a missing definition error? This is related to an earlier question.
In the following set of files I am surprised that a definition is sought for a class that is not used and then when I include only a declaration for the class the program compiles.
I have added comments for the includes that I had to add, but can't understand why they're needed in those particular .cpp files.
A.h
#ifndef A_h
#define A_h
#include <map>
class C;
class A {
public:
C& add();
std::map<int,C> Cmap;
void dosomethingwithC();
};
#endif
B.h
#ifndef B_h
#define B_h
#include <map>
class A;
class B {
public:
A& add();
std::map<int,A> Amap;
};
#endif
C.h
#ifndef C_h
#define C_h
#include <map>
class B;
class C {
public:
B& add();
std::map<int,B> Bmap;
};
#endif
A.cpp
#include "A.h"
#include "B.h"// Required here, but B isn't used in this file.
// Without B.h the error is
// error C2079: 'std::pair<const _Kty,_Ty>::second' uses undefined class 'B'
// But B.h only includes the declaration, not the definition
#include "C.h"
C& A::add()
{
auto emplace_results = Cmap.emplace(std::piecewise_construct,
std::forward_as_tuple(3),
std::forward_as_tuple());
auto pair_iterator = emplace_results.first;
auto& emplaced_pair = *pair_iterator;
auto& map_value = emplaced_pair.second;
return map_value;
}
void A::dosomethingwithC()
{
Cmap[3].add();
}
B.cpp
#include "A.h"
#include "B.h"
#include "C.h" // also required here
A& B::add()
{
auto emplace_results = Amap.emplace(std::piecewise_construct,
std::forward_as_tuple(3),
std::forward_as_tuple());
auto pair_iterator = emplace_results.first;
auto& emplaced_pair = *pair_iterator;
auto& map_value = emplaced_pair.second;
return map_value;;
}
C.cpp
#include "A.h" // also required here
#include "B.h"
#include "C.h"
B& C::add()
{
auto emplace_results = Bmap.emplace(std::piecewise_construct,
std::forward_as_tuple(3),
std::forward_as_tuple());
auto pair_iterator = emplace_results.first;
//auto& emplaced_pair = *pair_iterator;
std::pair<const int,B>& emplaced_pair = *pair_iterator;
auto& map_value = emplaced_pair.second;
return map_value;
}
Main.cpp
#include "A.h"
#include "B.h"
#include "C.h"
int main()
{
C c;
auto& emplacedB = c.add();
auto& emplacedA = emplacedB.add();
auto& emplacedC = emplacedA.add();
emplacedC.add();
return 0;
}
