Code Compilation Fails on Linux, Succeeds on Windows: Cause/Fix? - c++

Question

I have some c++ code that will compile fine in Visual Studio 2013 but will not compile in linux using g++ (no IDE).
What is causing the difference and how can I make the code compile on linux? Is it because they are different compilers? Do I need a specific compiler setting?
Code:
#include <iostream>
typedef class IApp;
typedef class Component;
class Component
{
public:
protected:
IApp* app;
template<typename T>
void registerEvent()
{
app->logEvent();
}
};
class IApp : protected Component
{
public:
static IApp NULL_APP;
void logEvent()
{
printf("Event Logged\n");
}
protected:
virtual void foo() = 0;
};
int main(int argc, char** argv)
{
printf("Alive\n");
system("pause");
return 0;
}
On windows I get no compiler errors. On linux I get the following compiler errors:
g++ - o res main.cpp - std = c++11
main.cpp:3 : 15 : warning : ‘typedef’ was ignored in this declaration[enabled by default]
typedef class IApp;
^
main.cpp:4 : 15 : warning : ‘typedef’ was ignored in this declaration[enabled by default]
typedef class Component;
^
main.cpp: In member function ‘void Component::registerEvent()’ :
main.cpp : 16 : 6 : error : invalid use of incomplete type ‘class IApp’
app->logEvent();
^
main.cpp:3 : 15 : error : forward declaration of ‘class IApp’
typedef class IApp;
^
main.cpp: At global scope :
main.cpp : 23 : 14 : error : cannot declare variable ‘IApp::NULL_APP’ to be of abstract type ‘IApp’
static IApp NULL_APP;
^
main.cpp:20 : 7 : note : because the following virtual functions are pure within ‘IApp’ :
class IApp : protected Component
^
main.cpp : 31 : 15 : note : virtual void IApp::foo()
virtual void foo() = 0;
^
make: ***[all] Error 1

Answer 1

Here you should simply remove typedef:
typedef class IApp;
Then this template method should be defined out-of-line, below IApp:
template<typename T>
void registerEvent()
{
app->logEvent();
}
Otherwise, it doesn't see the declaration of IApp which it needs to dereference.
Finally, this does not make sense:
virtual void foo() = 0;
Because the same class has a static member of its own class type, so needs to instantiate it. But you've prevented that with a pure virtual function.
GCC is right not to compile this code.

Answer 2

Leave out the typedef keywords before the forward class declarations, they're not necessary. Just class MyClass; is sufficient for a forward declaration. The other issue is that foo() is pure virtual - you need to implement it if you want to instantiate the class. I'm surprised the Microsoft complier doesn't complain but then that's Microsoft.