Object Lifetime In C C++

February 28, 2009 Object Lifetime in C/C++ Madhubanti Dasgupta Interra Systems (India) Pvt. Ltd. Current Practices in Object Oriented Software Development Meghnad Saha Institute of Technology

Agenda Object Lifetime Fundamentals Data Memory Areas Types of Objects Static Objects Automatic Objects Dynamic Objects

Object Lifetime Fundamentals Allocation - Deallocation

Object Lifetime Fundamentals Starts with Constructor execution As soon as Initialization ends and control enters Constructor Body Must follow Memory Allocation Ends with Destructor execution As soon as control leaves Destructor Body Must precede Memory De-allocation

Object Lifetime Fundamentals class MyClass; void MyFunc() // Allocation on Stack for myObj { MyClass myObj; // Construction // MyClass::MyClass() ... myObj.myMethod(); // Use ... return; // Destruction - Implicit // myObj.~MyClass() } // De-Allocation from Stack for myObj

Object Lifetime Fundamentals Stages Memory Allocation & Binding Constructor Call & Execution Object Use Destructor Call & Execution Memory De-Allocation & De-Binding Managed Lifetime

Object Lifetime Fundamentals Built-in / Pre-Defined Types No Explicit Constructor / Destructor Compiler optimizes these calls Same pattern for the Creation / Destruction processes

Data Memory Areas System Agnostic View

Data Memory Areas Program Code Text Global / File Scope Objects; Static Class Scope Objects; Function Scope Objects Static Dynamic Objects managed by new / delete & malloc / free Free Store & Heap Automatic Objects Stack Constant literals including strings Const Data Data Area

Types of Objects Static – Automatic – Dynamic

Static Objects Compiler-Managed Objects in the Global Store

Static Objects Objects in Static Memory Constructed only once at Program Startup Function Objects created on first execution Destructed on Normal Program Termination return or exit() Uses atexit() Registration to call Destructors Static Objects are Global and Namespace Objects Class Objects with the static modifier Function Objects with the static modifier Memory requirements handled by the compiler Offsets / Layout statically computed

Static Objects static MyClass obj1; // Global static object MyClass MyClass::obj2; // Class static object void staticFunc() { static MyClass obj3; // Function static object { static MyClass obj4; // Block static object } }

Static Objects Global (or File) / Namespace Scope / Class Scope Built-in Types Has no boundary for translation units – literally “load time” Object created (& initialized) implicitly at program load before first assembly instruction in the program actually before any global object of user-defined type is constructed Object destroyed implicitly before the program unload after last assembly instruction in main() actually after all global objects of user-defined type are destructed User-Defined Types Space allocation is done at “load-time” Object created (and initialized) sequentially within a translation unit arbitrarily across translation units Objects destroyed in the reverse order of creations

Static Objects Local Function / Block Scope Built-in Types Same as static objects in Global Scope User-Defined Types Space allocation is done at “load-time” Object created (and initialized) when the control passes the definition for the first time Destroyed in the reverse order of creations (LIFO) after last assembly instruction in main() before the global objects of user-defined type are destructed Uses atexit() – because the creation order is dynamic

Execution beyond main() static MyClass *obj; // Global Static Object Ptr void clean() { delete obj; } // Wrapper Function void staticFunc() { obj = new MyClass(); // Construction atexit(clean); // atexit() Registration }

Execution beyond main() atexit() signature int atexit(void (*pFunction)(void)); Register any function having the following signature void CallbackFunction(void); Registered function called at runtime after main() exits For Multiple Registrations, calls are made in the reverse order of registration or LIFO Used by Compiler for Local Static Objects Application Programmer for any function call beyond main() return() or exit() invokes all Callbacks abort() bypasses all Callbacks

Automatic Objects Compiler-Managed Objects in Local Store

Automatic Objects Objects in Stack (Local) Memory Objects Managed via Static Scopes Function Scope Block Scope within a Function Scope Block Scope within another Block Scope Objects Laid out in the Stack Frame of the Function Scope Constructed when the control passes the declaration Destructed when (just after) the control leaves the scope Normal Exit: return / break / goto Abnormal Exit: Exception (throw) Objects (within a scope) are destroyed in the reverse order of creations Memory requirements handled by the Compiler

Automatic Objects void automaticFunc() { MyClass obj1; // Function Scoped MyClass *obj2 = new MyClass(); { MyClass obj3; } // Block Scoped if (MyClass obj4()) { delete obj2; } // if Scoped else { } }

Automatic Objects Automatic Objects are Function Scope Objects Formal Parameters Return Values Block Scope Objects Compound Statements (like ‘for’ or ‘switch’ or ‘while’ or ‘do-while’) Non-static Data Member Objects in a class As part of the Parent Object’s Layout Base class Object in a class As part of the Derived Object’s Layout Temporary Objects Unnamed Objects Compiler Generated & Managed

Dynamic Objects User-Managed Objects in the Free Store

Dynamic Objects Objects in Free Store Memory Memory allocated dynamically Constructor called by operator new / placement new Must follow Memory Allocation Destructor called by operator delete Must precede Memory De-allocation Lifetime controlled by the user Transformed to scoped management by Smart Pointers Memory requirements handled by the User Lifetime Managed by the User

Dynamic Objects void dynamicFunc() { MyClass *obj = new MyClass(); // Dynamic Object int offset = sizeof (MyClass); char *plBuf = new char [2 * offset]; // Allocation MyClass *plObj1 = new (plBuf) MyClass(); // Construction MyClass *plObj2 = new (plBuf + offset) MyClass(); plObj1->~MyClass(); // Destruction plObj2->~MyClass(); delete [] plBuf; // Deallocation } // Dynamic Object ‘obj’ Leaks Memory

Object Lifetime In C C++

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