pimpl.cpp

#include "pimpl.h"

struct test::inner{ int x; inner() : x(42) {} };

test::test():pimpl(new inner()){}
test::~test(){}

pimpl.h

#include <boost/shared_ptr.hpp>

struct test{
    test();
    ~test();
    struct inner;
    boost::shared_ptr<inner> pimpl;
};

pimpl_test.cpp

#include "pimpl.h"

int main(){
    test t;
}