As control passes from the point where an exception is thrown to a handler, destructors are invoked by a process, specified in this section, called stack unwinding.
The destructor is invoked for each automatic object of class type constructed, but not yet destroyed, since the try block was entered. If an exception is thrown during the destruction of temporaries or local variables for a return statement, the destructor for the returned object (if any) is also invoked. The objects are destroyed in the reverse order of the completion of their construction. [ Example:
struct A { };
struct Y { ~Y() noexcept(false) { throw 0; } };
A f() {
try {
A a;
Y y;
A b;
return {}; // #1
} catch (...) {
}
return {}; // #2
}At #1, the returned object of type A is constructed. Then, the local variable b is destroyed ([stmt.jump]). Next, the local variable y is destroyed, causing stack unwinding, resulting in the destruction of the returned object, followed by the destruction of the local variable a. Finally, the returned object is constructed again at #2. — end example ]
If the initialization or destruction of an object other than by delegating constructor is terminated by an exception, the destructor is invoked for each of the object's direct subobjects and, for a complete object, virtual base class subobjects, whose initialization has completed ([dcl.init]) and whose destructor has not yet begun execution, except that in the case of destruction, the variant members of a union-like class are not destroyed. The subobjects are destroyed in the reverse order of the completion of their construction. Such destruction is sequenced before entering a handler of the function-try-block of the constructor or destructor, if any.
If the compound-statement of the function-body of a delegating constructor for an object exits via an exception, the object's destructor is invoked. Such destruction is sequenced before entering a handler of the function-try-block of a delegating constructor for that object, if any.
[ Note: If the object was allocated by a new-expression, the matching deallocation function, if any, is called to free the storage occupied by the object. — end note ]