Re: How to make this exception-safe

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Wed, 19 Nov 2008 14:40:37 CST
On 18 Nov., 19:43, 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() );

Note that this approach would still have
an observable change of v_, if one of the
created C items throws, because the initial
change of the container capacity is not undone.

I just want to notice that, because you did
not specify the degree of exception-safe.

If this is ok, you seem to accept the so-called
basic guarantee.

Which is tedious if you are inserting more objects. Does anyone have a
better solution?

Two approaches come into my mind, both would
use a helper function to do the dirty work
(Generalize the following as templates, if you

1) Combine reserve and C-allocation - no RAII necessary:
void add(std::vector<C*>& v, int arg) {
  v.reserve(v.size() + 1); // Might throw
  v.push_back(new C(arg)); // If construction of C fails,
    // the container size is still the original one.
    // push_back cannot fail here, see [vector.capacity]/6

2) Combine RAII and
void add(std::vector<C*>& v, int arg) {
  std::auto_ptr<C> buf(new C(arg)); // Might throw
  v.push_back(buf.get()); // push_back might fail, but buf is
    // our rescue
  buf.release(); // Cannot fail

In either case your code becomes easy:

struct V {
   V() {
      add(v_, 2);
      add(v_, 1);
      add(v_, 3);

HTH & Greetings from Bremen,

Daniel Kr?gler

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