Another minor trick to keep header inclusions down

gistfile1.cpp

// Assume a header file with a container of items
// Container.h
// -------------------------------------------------
#include <vector>

// Forward-declared to avoid included header file
struct Item;

struct Container
{
    std::vector<Item> items;
};


// When included, this works fine
// When client tries to use it, a compile error occurs
// -------------------------------------------------
#include "Container.h"
Container c;    // error C2036: 'Item *' : unknown size


// Now add a constructor/destructor:
// -------------------------------------------------
struct Container
{
    Container();
    ~Container();
};

Container c;    // unresolved external symbol "public: __thiscall Container::~Container(void)" (??1Container@@QAE@XZ) (...)


// The errors shift from the compiler to the linker. When the constructor/destructor are implemented in a private
// implementation file (i.e. Container.cpp), everything now works.


// They key take-away is that when you don't add your own ctor/dtor the compiler will generate one for you. However,
// these will be generated in the compilation unit that includes the header file, thus requiring std::vector to also
// generate its ctor/dtor. Of course, std::vector stores Item* internally so the forward-declaration is sufficient until
// we realise that the dtor needs to call the Item dtor for each allocated element.