Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Programming
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Databases
Mail Systems
openSolaris
Eclipse Documentation
Techotopia.com
Virtuatopia.com

How To Guides
Virtualization
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Windows
Problem Solutions
Privacy Policy

  




 

 

Thinking in C++
Prev Contents / Index Next

Static object destructors

Destructors for static objects (that is, all objects with static storage, not just local static objects as in the example above) are called when main( ) exits or when the Standard C library function exit( ) is explicitly called. In most implementations, main( ) just calls exit( ) when it terminates. This means that it can be dangerous to call exit( ) inside a destructor because you can end up with infinite recursion. Static object destructors are not called if you exit the program using the Standard C library function abort( ).

You can specify actions to take place when leaving main( ) (or calling exit( )) by using the Standard C library function atexit( ). In this case, the functions registered by atexit( ) may be called before the destructors for any objects constructed before leaving main( ) (or calling exit( )).

Like ordinary destruction, destruction of static objects occurs in the reverse order of initialization. However, only objects that have been constructed are destroyed. Fortunately, the C++ development tools keep track of initialization order and the objects that have been constructed. Global objects are always constructed before main( ) is entered and destroyed as main( ) exits, but if a function containing a local static object is never called, the constructor for that object is never executed, so the destructor is also not executed. For example,

//: C10:StaticDestructors.cpp
// Static object destructors
#include <fstream>
using namespace std;
ofstream out("statdest.out"); // Trace file

class Obj {
  char c; // Identifier
public:
  Obj(char cc) : c(cc) {
    out << "Obj::Obj() for " << c << endl;
  }
  ~Obj() {
    out << "Obj::~Obj() for " << c << endl;
  }
};

Obj a('a'); // Global (static storage)
// Constructor & destructor always called

void f() {
  static Obj b('b');
}

void g() {
  static Obj c('c');
}

int main() {
  out << "inside main()" << endl;
  f(); // Calls static constructor for b
  // g() not called
  out << "leaving main()" << endl;
} ///:~

In Obj, the char c acts as an identifier so the constructor and destructor can print out information about the object they’re working on. The Obj a is a global object, so the constructor is always called for it before main( ) is entered, but the constructors for the static Obj b inside f( ) and the static Obj c inside g( ) are called only if those functions are called.

To demonstrate which constructors and destructors are called, only f( ) is called. The output of the program is

Obj::Obj() for a
inside main()
Obj::Obj() for b
leaving main()
Obj::~Obj() for b
Obj::~Obj() for a

The constructor for a is called before main( ) is entered, and the constructor for b is called only because f( ) is called. When main( ) exits, the destructors for the objects that have been constructed are called in reverse order of their construction. This means that if g( ) is called, the order in which the destructors for b and c are called depends on whether f( ) or g( ) is called first.

Notice that the trace file ofstream object out is also a static object – since it is defined outside of all functions, it lives in the static storage area. It is important that its definition (as opposed to an extern declaration) appear at the beginning of the file, before there is any possible use of out. Otherwise, you’ll be using an object before it is properly initialized.

In C++, the constructor for a global static object is called before main( ) is entered, so you now have a simple and portable way to execute code before entering main( ) and to execute code with the destructor after exiting main( ). In C, this was always a trial that required you to root around in the compiler vendor’s assembly-language startup code.

Thinking in C++
Prev Contents / Index Next

 
 
   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire