While composing an answer to When can I use a forward declaration?, I ran into an issue that I cannot explain.
// Forward declared class template:
template <typename T> struct X;
// Two other class templates that use it
template <typename T>
struct Foo {
X<T>* m;
Foo() : m(new X<T>) {}
};
template <typename T>
struct Bar {
X<T> m;
};
// and a function that creates instances of Foo and Bar.
void test1()
{
Foo<int> foo;
Bar<int> bar;
}
// Define the class template after test1. On purpose.
template <typename T> struct X {};
int main()
{
}
When I compile the above program using g++ 4.9.3, I get compiler error for
Bar<int> bar;
but not for
Foo<int> foo;
Both require the definition of X since both fail to compile if omit the definition of X.
However, by just including the definition of X in the translation unit, Foo<int> foo; works while Bar<int> bar; does not work. Is this expected?
