Compiler: ARM CGT v5.2.5 Issue with template argument in base class

A template is parsed twice: Once when it is first seen, and another when it is instantiated. In the first pass, T1 and T2 are unknown placeholder types, and in the second, they are known to be types <int,int>.

In the code you've posted, Foo<T1> is a dependent base class of Bar, which means that its contribution to Bar can't be determined until the second time Bar is parsed, when T1 and T2 are known. When dealing with a dependent base class, the default name lookup does not work, because consider what happens when the following specialization of Foo is added:

template <>
class Foo<int>
{
    public:
        virtual void dump() {
            printf("%08X:\n", this);
        }
};

Now, the code may or may not be ill-formed depending on the type of 'T1'. Since the language can't determine this when parsing the template with T1 and T2 unknown, it must defer the check until later, when it knows for sure. Additionally, it must be conservative and assume that since no global 'p1' is available, that it is a reference to an undefined variable.

How do we fix this? We can defer the name lookup later by linking the reference to 'p1' to a dependent entity. Since Bar is dependent due to the inheritance of a dependent base class, we can reference p1 through the 'this' pointer. The following code compiles.

#include <stdio.h>

template <class T1>
class Foo
{
    public:
        int  p1;
        virtual void dump() {
            printf("%08X: %i\n", this, p1);
        }
};

template <class T1, class T2>
class Bar : public Foo<T1>
{
    public:
        T2  p2;
        virtual void dump() {
            printf("%08X: %i, %i\n", this, this->p1, p2); // Reference a dependent member variable through the dependent 'this' pointer
        }
};

void test()
{
        Bar<int, int> bar;
        bar.p1 = 1; // ok !!!
        bar.p2 = 2;
        bar.dump();
}

Hi James,

thanks a lot for the detailed explanation and for your suggestion to fix my problem. It works fine now. :)

BR, Lars