Question

I suspect I can’t do this directly using a PIMPL pattern. Is it possible to have a smart pointer to a template class? I have not been able to compile by turning knobs on the shared_ptr declaration.

// ============Foo.h ============
// Forward declare the implementation
template <typename T> class FooImpl;

class Foo
{
  public:
    Foo getInstance(const string& fooType);
    ...
  private:
    shared_ptr< FooImpl<T> > m_impl;
};

// ============FooImpl.h ============
template <typename T>
class FooImpl
{
    ...
};

Under Visual Studio 2008: “error C2065: ‘T’ : undeclared identifier”. I receive a similar error under GCC. If I un-parameterize FooImpl (so that FooTempl inherits from FooImpl), the code will compile.

I suspect I can’t paramaterize the smart pointer, but I could be wrong.

EDIT: The second Visual Studio error is more telling: “error C3203: ‘FooImpl’ : unspecialized class template can’t be used as a template argument for template parameter ‘T’, expected a real type”

Jeff

Answer 1

I’m not entirely certain what you are trying to accomplish, but does this help?

Try 1:

// ============Foo.h ============
// Forward declare the implementation

template <typename T> class FooImpl;

template<class C>
class Foo
{
  public:
    Foo getInstance(const string& fooType);
    ...
  private:
    shared_ptr< FooImpl<C> > m_impl;
};

// ============FooImpl.h ============
template <typename T>
class FooImpl
{
    ...
};

Try 2:

// ============Foo.h ============
// Forward declare the implementation

class FooImplBase;

class Foo
{
  public:
    Foo getInstance(const string& fooType);
    ...
  private:
    shared_ptr< FooImplBase > m_impl;
};

// ============FooImpl.h ============
class FooImplBase {
  public:
    virtual void AnAPI();
    virtual int AnotherAPI();
};
template <typename T>
class FooImpl : public FooImplBase
{
    ...
};