When using smart pointers with the pImpl idiom, as in
struct Foo
{
private:
struct Impl;
boost::scoped_ptr<Impl> pImpl;
};
the obvious problem is that Foo::Impl
is incomplete at the point where the destructor of Foo
is generated.
Compilers usually emit a warning there, and boost::checked_delete
, which is used internally by Boost smart pointers, statically asserts that the class Foo::Impl
is complete and triggers an error if it is not the case.
For the above example to compile, one therefore must write
struct Foo
{
~Foo();
private:
struct Impl;
boost::scoped_ptr<Impl> pImpl;
};
and implement an empty Foo::~Foo
in the implementation file, where Foo::Impl
is complete. This is an advantage of smart pointers over bare pointers, because we can't fail to implement the destructor.
So far, so good. But I came across a weird behaviour when I try to introduce a template constructor in a similar Bar
class (full code, please try it yourself):
// File Bar.h
#ifndef BAR_H
#define BAR_H 1
#include <vector>
#include <boost/scoped_ptr.hpp>
struct Bar
{
template <typename I>
Bar(I begin, I end);
~Bar();
private:
struct Impl;
boost::scoped_ptr<Impl> pImpl;
void buildImpl(std::vector<double>&);
};
template <typename I>
Bar::Bar(I begin, I end)
{
std::vector<double> tmp(begin, end);
this->buildImpl(tmp);
}
#endif // BAR_H
// File Bar.cpp
#include "Bar.h"
struct Bar::Impl
{
std::vector<double> v;
};
void Bar::buildImpl(std::vector<double>& v)
{
pImpl.reset(new Impl);
pImpl->v.swap(v);
}
Bar::~Bar() {}
// File Foo.h
#ifndef FOO_H
#define FOO_H 1
#include <boost/scoped_ptr.hpp>
struct Foo
{
Foo();
~Foo();
private:
struct Impl;
boost::scoped_ptr<Impl> pImpl;
};
#endif // FOO_H
// File Foo.cpp
#include "Foo.h"
struct Foo::Impl
{};
Foo::Foo() : pImpl(new Impl)
{}
Foo::~Foo() {}
// File Main.cpp
#include "Foo.h"
#include "Bar.h"
int main()
{
std::vector<double> v(42);
Foo f;
Bar b(v.begin(), v.end());
}
When compiling this example with Visual Studio 2005 SP1, I get an error with Bar
but not with Foo
:
1>Compiling...
1>main.cpp
1>c:\users\boost_1_45_0\boost\checked_delete.hpp(32) : error C2027: use of undefined type 'Bar::Impl'
1> c:\users\visual studio 2005\projects\checkeddeletetest\checkeddeletetest\bar.h(15) : see declaration of 'Bar::Impl'
1> c:\users\boost_1_45_0\boost\smart_ptr\scoped_ptr.hpp(80) : see reference to function template instantiation 'void boost::checked_delete<T>(T *)' being compiled
1> with
1> [
1> T=Bar::Impl
1> ]
1> c:\users\boost_1_45_0\boost\smart_ptr\scoped_ptr.hpp(76) : while compiling class template member function 'boost::scoped_ptr<T>::~scoped_ptr(void)'
1> with
1> [
1> T=Bar::Impl
1> ]
1> c:\users\visual studio 2005\projects\checkeddeletetest\checkeddeletetest\bar.h(16) : see reference to class template instantiation 'boost::scoped_ptr<T>' being compiled
1> with
1> [
1> T=Bar::Impl
1> ]
1>c:\users\boost_1_45_0\boost\checked_delete.hpp(32) : error C2118: negative subscript
1>c:\users\boost_1_45_0\boost\checked_delete.hpp(34) : warning C4150: deletion of pointer to incomplete type 'Bar::Impl'; no destructor called
1> c:\users\visual studio 2005\projects\checkeddeletetest\checkeddeletetest\bar.h(15) : see declaration of 'Bar::Impl'
I will try this with a recent gcc as soon as I get home.
I don't understand what is going on: at the point where the destructor is defined (ie. in Bar.cpp
), a definition of Bar::Impl
is available, so there should not be any problem. Why does this work with Foo
and not with Bar
?
What am I missing here ?
This is the destructor of
boost::shared_ptr<>
that requires the object to be complete when usingboost::checked_deleter<>
deleter. Because you put the range constructorBar::Bar(I begin, I end)
in the header file the compiler must generate code that destroys already constructed members if your constructor throws, hence it is trying to instantiateboost::scoped_ptr<T>::~scoped_ptr(void)
when instantiating this template constructor.It is less than useful to use smart pointers with pimpl. Since you normally need to provide a destructor anyway, you could as well put
delete pimpl
in that destructor and be done with that.