Function template declaration order affects visibility (sometimes)

Question

I'm trying to create a function:

template <typename T>
void doIt( T*& p )
{
   if ( !p ) { return; }
   T& ref = *p;
   getClassName( ref );
}

where the behavior varies according to the type of p passed in. In particular, the version of getClassName called should depend upon the type of p. In the following example, I can successfully call:

doIt<myClass1>( myClass1*& )
doIt<myClass1<int> >( myClass1*& )
doIt<myClass2>( myClass2*& )
doIt<myClass2<int> >( myClass2*& )

but it fails when I call:

doIt< std::vector<int, std::allocator<int> > >( std::vector<int, std::allocator<int>>*& )

with the error:

a.cxx: In function ‘void doIt(T*&) [with T = std::vector<int, std::allocator<int> >]’:
ba.cxx:87:   instantiated from here
a.cxx:33: error: invalid initialization of reference of type ‘MyClass1&’ from expression of type ‘std::vector<int, std::allocator<int> >’
a.cxx:16: error: in passing argument 1 of ‘const char* getClassName(MyClass1&)’

(gcc 4.2.4).

If I move the declaration of:

template<typename T, typename A>
char const* getClassName( std::vector<T,A>& ) { printf("std::vector<T,A>\n"); return NULL; }

before doIt -- then it compiles. So,

• Why is it required that getClassName( std::vector<T,A>& ) appears before doIt but not getClassName( MyClass2T<T>& )
• What can I do to make doIt independent of std::vector? (I want to be able to place doIt in its own header and not have to know about std::vector, or any of the specializations, which will be user-defined).

.

#include <stdio.h>
#include <assert.h>
#include <vector>

//template<typename T>
//char const* getClassName( T& );

//template<typename T, typename A>
////char const* getClassName( std::vector<T,A>& ) { printf("std::vector<T,A>\n"); return NULL; }

#if 1
// ---------  MyClass2
struct MyClass1
{};

char const* getClassName( MyClass1& ) { printf("MyClass1\n"); return NULL; }

// ---------  MyClass1T
template< typename T>
struct MyClass1T
{};

template<typename T>
char const* getClassName( MyClass1T<T>& ) { printf("MyClass1T<T>\n"); return NULL; }
#endif


template <typename T>
void doIt( T*& p )
{
   if ( !p ) { return; }
   T& ref = *p;
   getClassName( ref );
}


// ---------  MyClass2
struct MyClass2
{};


// declared after doIt, OK.
char const* getClassName( MyClass2& ) { printf("MyClass2\n"); return NULL; }

// ---------  MyClass2T
template< typename T>
struct MyClass2T
{};

// declared after doIt, OK.
template<typename T>
char const* getClassName( MyClass2T<T>& ) { printf("MyClass2T<T>\n"); return NULL; }

template<typename T, typename A>
char const* getClassName( std::vector<T,A>& ) { printf("std::vector<T,A>\n"); return NULL; }



void test()
{
#if 1
   MyClass1 mc1;
   MyClass1* mc1p = &mc1;
   doIt( mc1p );

   MyClass2 mc2;
   MyClass2* mc2p = &mc2;
   doIt( mc2p );

   MyClass1T<int> mc1t;
   MyClass1T<int>* mc1tp = &mc1t;
   doIt( mc1tp );

   MyClass2T<int> mc2t;
   MyClass2T<int>* mc2tp = &mc2t;
   doIt( mc2tp );

   // Nested templates are OK.
   MyClass2T<MyClass1> mc2t2;
   MyClass2T<MyClass1>* mc2tp2 = &mc2t2;
   doIt( mc2tp2 );
#endif

#if 1
   std::vector<int> v;
   std::vector<int>* vp = &v;
   doIt( vp );                   // FAIL!
#endif
}

Answer 1

Why is it required that getClassName( std::vector& ) appear before doIt but not getClassName( MyClass2T& )

A declaration in scope is required for any function. When you instantiate your template function with a vector<int> it expects a function with the signature getClassName(vector<int>&) to be present (at least a prototype) for compilation to succeed.

What can I do do make doIt independent of std::vector? (I want to be able to place doIt in its own header and not have to know about std::vector, or any of the specializations, which will be user-defined)

Read the FAQ on Templates. Try putting the prototype of all of doIt's dependent template functions before first instantiation of doIt.