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Thursday, June 3, 2010

Stack unwinding (C++ only)

Stack unwinding (C++ only)

When an exception is thrown and control passes from a try block to a handler, the C++ run time calls destructors for all automatic objects constructed since the beginning of the try block. This process is called stack unwinding. The automatic objects are destroyed in reverse order of their construction. (Automatic objects are local objects that have been declared auto or register, or not declared static or extern. An automatic object x is deleted whenever the program exits the block in which x is declared.)

If an exception is thrown during construction of an object consisting of subobjects or array elements, destructors are only called for those subobjects or array elements successfully constructed before the exception was thrown. A destructor for a local static object will only be called if the object was successfully constructed.

If during stack unwinding a destructor throws an exception and that exception is not handled, the terminate() function is called. The following example demonstrates this:

#include <iostream>
using namespace std;

struct E {
  const char* message;
  E(const char* arg) : message(arg) { }

void my_terminate() {
  cout << "Call to my_terminate" << endl;

struct A {
  A() { cout << "In constructor of A" << endl; }
  ~A() {
    cout << "In destructor of A" << endl;
    throw E("Exception thrown in ~A()");

struct B {
  B() { cout << "In constructor of B" << endl; }
  ~B() { cout << "In destructor of B" << endl; }

int main() {

  try {
    cout << "In try block" << endl;
    A a;
    B b;
    throw("Exception thrown in try block of main()");
  catch (const char* e) {
    cout << "Exception: " << e << endl;
  catch (...) {
    cout << "Some exception caught in main()" << endl;

  cout << "Resume execution of main()" << endl;
The following is the output of the above example:

In try block
In constructor of A
In constructor of B
In destructor of B
In destructor of A
Call to my_terminate

In the try block, two automatic objects are created: a and b. The try block throws an exception of type const char*. The handler catch (const char* e) catches this exception. The C++ run time unwinds the stack, calling the destructors for a and b in reverse order of their construction. The destructor for a throws an exception. Since there is no handler in the program that can handle this exception, the C++ run time calls terminate(). (The function terminate() calls the function specified as the argument to set_terminate(). In this example, terminate() has been specified to call my_terminate().)

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