Example code:
1 class A {
2 public:
3 A(int i) {
4 if (i!=1) {
5 std::vector<int> a;
6 a.at(0) = 1;
7 } else
8 std::cout << "ctor ok." << std::endl;
9 }
10 ~A() {
11 std::cout << "A:dtor." << std::endl;
12 }
13 };
14
15 void f()
16 {
17 try {
18 A a(1);
19 a.~A();
20 new (&a) A(2); /* here, throw one exception, but still destroy a again */
21 } catch(
) {
22 std::cout << "catch exception." << std::endl;
23 }
24 }
25
Above code snippet, if in placement new call, constructor of one class throws one exception, that a will still be destroyed again, since a is one automatic object, according to Cpp standard. In this case, one object constructed once, but destroyed twice, undefined/unpredicated behavior may happen.