Re: How to make this exception-safe

Lance Diduck <>
Wed, 19 Nov 2008 14:27:01 CST
On Nov 18, 1:43 pm, Triple-DES <> wrote:

Consider the following code:

#include <vector>

struct C {
   explicit C(int) {} // may throw
     C(const C&);
     C& operator=(const C&);


struct V {
   V() {
     // may leak if push_back or C::C(int) throws
      v_.push_back( new C(2) );
      v_.push_back( new C(1) );
      v_.push_back( new C(3) );

   ~V() {} // delete all elems of v

   std::vector<C*> v_;


How would you go about making an exception-safe version of V::V()?
This is what I could come up with:

// 1st attempt
#include <memory>

V::V() {

   std::auto_ptr<C> c1( new C(2) );
   std::auto_ptr<C> c2( new C(1) );
   std::auto_ptr<C> c3( new C(3) );

   v_.push_back( c1.release() );
   v_.push_back( c2.release() );
   v_.push_back( c3.release() );


Which is tedious if you are inserting more objects. Does anyone have a
better solution?
PS! boost/tr1 shared_ptr or similar can not be used.

There are a number of ways:
1. use a sentry class. Mine looks something like
    template<class T>
    struct DeleteObject{
        void operator()(T v){
            delete v;

//for maps use these...
    template<class T>struct DeletePairSecond{
        void operator()(T v){
            delete v.second;
    template<class T>struct DeletePairFirst{
        void operator()(T v){
            delete v.first;
    template<class T>struct DeletePairBoth{
        void operator()(T v){
            delete v.first;
            delete v.second;
template<class Container, class Delete=DeleteObject<typename
Container::value_type> >
struct PointerContainerSentry {
    typedef Container container_type;
        using std::for_each;
    container_type & m_cont;
    void operator=(PointerContainerSentry const&);
    PointerContainerSentry(PointerContainerSentry const&);

Then modify struct V like
struct V{

std::vector<C*> _v;
PointerContainerSentry<std::vector<C*> > sentry;
V(const V&);//not really safe for containers of raw pointers
//in any case

V:V():sentry(_v) {
        _v.push_back( new C(2) );
        _v.push_back( new C(1) );
        _v.push_back( new C(3) );


This has the added benefit of cleaning up _v in ~V automatically.


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