template friend of template class fails in Sun Studio C++

Question

I have a problem with giving a template class a template friend in Sun Studio. The code compiles fine with GNU G++ (4.4.1 and 4.4.3), but fails with Sun Studio C++ (5.9 SunOS_sparc Patch 124863-01 2007/07/25).

Here is a minimal example:

// Forward declarations
template<class T> class M;
template<class T> void f(M<T>, M<T>);

// Define M<T>
template<class T>
class M
{
public:
    void f(M<T>) { }

    friend void ::f<>(M<T>, M<T>);
};

// Define global function f
template<class T>
void f(M<T> a, M<T> b)
{
    a.f(b);
}

M<int> a;

When I try to compile it via CC -c -o t3.o t3.cpp, I get the following error messages:

"t3.cpp", line 12: Warning:  A friend function with template-id name must have a template declaration in the nearest namespace.
"t3.cpp", line 22:     Where: While specializing "M<int>".
"t3.cpp", line 22:     Where: Specialized in non-template code.
"t3.cpp", line 12: Error: Global scope has no declaration for "f".
"t3.cpp", line 22:     Where: While specializing "M<int>".
"t3.cpp", line 22:     Where: Specialized in non-template code.
1 Error(s) and 1 Warning(s) detected.

Is this a problem with Sun Studio C++, or is it invalid C++ (which is still accepted by GCC and gives no warnings with -Wall -pedantic)? Is there an elegant way to change the code such that it is standard compliant and compiles both under GCC and Sun Studio?

Thanks a lot in advance!

Its a problem with SunStudio, it is quite bad. Looking at the boost sources reveals many ideas on how to work around things for it. — PlasmaHH, Oct 14 '11 at 15:43
I've been browsing a bit in the boost sources, it seems to me that the most effective way to solve this problem is by using `#ifdef`: on Sun Studio (and other compilers where this does not work), simply make the needed things `public` instead of `protected` or `private`, and remove the `friend` statements. — felix, Oct 16 '11 at 20:51
Since M::f() is public, why do you need a friend declaration at all? — Stephen Gross, Oct 18 '11 at 19:46
My "real" template where this problem appeared had some private data which ::f() was using as well. — felix, Oct 19 '11 at 07:51

score 3 · Accepted Answer · answered Oct 31 '11 at 13:58

Successfully compiled your code using "CC: Sun C++ 5.8 Patch 121017-13 2008/01/02" adding template declaration to a friend:

template<class T>
class M
{
    ...
    template <class A>
    friend void ::f(M<A>, M<A>);
    ...
};

Following is not an answer to original question but those who is looking why a friend template class causes "Error: Multiple declaration for " error compiling using Sun CC compiler, just add forward declaration for the friend class, like this:

template <typename T> class B; //will fail without forward declaration

class A
{
    private:
    template <typename T> friend class B;
};

template <typename T> class B {};

re: your 2nd answer, I've encountered a rather strange situation while attempting to build boost. The code resembles: `template class A; template class A { template class impl; template friend class impl; template class impl { }; /* ... */` ... where that nested `impl` class gets the `Multiple declaration...` error. The only way I've found to resolve it is to remove the forward declaration and move the friend statement to after the class body. — Brian Vandenberg, Sep 23 '13 at 20:59

score 0 · Answer 2 · answered Oct 28 '11 at 14:20

Sun's compiler does tend to have some issues and is certainly updated less frequently than compilers such as g++. In this case, the problem seems to be that the compiler gets confused by the class shadowing the global template function.

I couldn't figure out a way to directly solve your problem here, but there are possible workarounds:

Just don't shadow the global template in your class. Renaming the global f and friend to foo for example allows sun to compile it. This especially makes sense if the functions are unrelated.
Avoid the need for friendship by extending the public interface to M if that seems appropriate.

template friend of template class fails in Sun Studio C++

2 Answers2