Pimpl not working

Question

This is a very noobish mistake, but I dont know whats happening here.

There are loads of pimpl examples but I dont understand why this isn't working (this was one of the examples more or less but I dont see the difference).

I have a very simple Pimpl example, but it wont work.

// Foo.hpp
#include <boost/scoped_ptr.hpp>

class Foo
{
 struct Bar;
 //boost::scoped_ptr<Bar> pImpl;
 Bar* pImpl;

public:
 Foo();
 ~Foo() {}

 int returnValue();

private:

};

and

// Foo.cpp
#include "foo.hpp"

struct Foo::Bar
{ 
 Bar() {}
 ~Bar() {}
 int value;
};

Foo::Foo() : pImpl(new Bar())
{
 pImpl->value = 7;
}

int Foo::returnValue() {
 return *pImpl->value;
}

Compiling this gives me the error. C2100: illegal indirection.

Thanks.

Answer 1

int returnValue() should be a member function:

//  vvvvv
int Foo::returnValue() {
 return pImpl->value; // no need to dereference, value isn't a pointer
}

---

You need to define your constructor, copy-constructor, copy assignment operator, and destructor after the implementation class has been defined. (Otherwise the implicit destructor is dangerous, and scoped_ptr won't let you do that):

// Foo.hpp
#include <boost/scoped_ptr.hpp>

class Foo
{
    struct Bar;
    boost::scoped_ptr<Bar> pImpl;

public:
    Foo();
    ~Foo();

    int returnValue(); // could be const (so should be)

private:
    // just disable copying, like scoped_ptr
    Foo(const Foo&); // not defined
    Foo& operator=(const Foo&); // not defined
};

And:

// Foo.cpp
#include "foo.hpp"

struct Foo::Bar
{ 
    int value;
};

Foo::Foo() :
pImpl(new Bar())
{
    pImpl->value = 7;
}

Foo::~Foo()
{
    // okay, Bar defined at this point; scoped_ptr can work
}

int Foo::returnValue()
{
    return pImpl->value;
}