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#include <type_traits>
 
/** This is a base class that implements the watching behvaior */
class Watchable
{
	// Some stuff...
};
 
/** This is a pointer that let's you observe the lifetime of types extending "Watchable" */
template <typename T>
struct WatchPtr final
{
	// Some more stuff...
 
	// Make sure that "T" extends "Watchable"
	static_assert(std::is_base_of<Watchable, T>::value, "...");
 
private:
 
	T* value;
};
 
/** For various reasons, we have chosen to forward-declare this type */
class SomeTypeThatMustBeForwardDeclared;
 
/** This type wants to have a WatchPtr to "SomeType..." */
class SomeOtherType
{
	WatchPtr<SomeTypeThatMustBeForwardDeclared> value; 
	// Here you get some crazy template errors
	// "But why? "SomeType..." is defined right below this! 
	// You don't even really need the definition at this moment anyway!
	// Because fuck you, that's why.
};
 
/** Now we define it */
class SomeTypeThatMustBeForwardDeclared : public Watchable
{
	// Blah blah blah
};
 
int main() {
	// your code goes here
	return 0;
}

In file included from prog.cpp:1:0:
/usr/include/c++/4.9/type_traits: In instantiation of 'struct std::is_base_of<Watchable, SomeTypeThatMustBeForwardDeclared>':
prog.cpp:12:1:   required from 'struct WatchPtr<SomeTypeThatMustBeForwardDeclared>'
prog.cpp:29:46:   required from here
/usr/include/c++/4.9/type_traits:1384:12: error: invalid use of incomplete type 'class SomeTypeThatMustBeForwardDeclared'
     struct is_base_of
            ^
prog.cpp:24:7: error: forward declaration of 'class SomeTypeThatMustBeForwardDeclared'
 class SomeTypeThatMustBeForwardDeclared;
       ^
prog.cpp: In instantiation of 'struct WatchPtr<SomeTypeThatMustBeForwardDeclared>':
prog.cpp:29:46:   required from here
prog.cpp:12:1: error: 'value' is not a member of 'std::is_base_of<Watchable, SomeTypeThatMustBeForwardDeclared>'
 {
 ^

Standard output is empty