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Dynamic Memory Allocation in C++
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Motivation
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  • An Observation:
    • We often don't know how much space we will need to store things at "compile time"
    • Dynamic memory allocation is the allocation of memory at "run time"
  • Past Experience:
    • You've seen some of this before in Java, since all objects use memory that is allocated at "run time"
Diferences between Static and Dynamic Memory Allocation
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  • Dynamically allocated memory is kept on the memory heap (also known as the free store)
  • Dynamically allocated memory can't have a "name" it must be referred to
  • Declarations are used to statically allocate memory, the new operator is used to dynamically allocate memory
Pointing to Memory Allocated at Run Time
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  • An Example:
    • int *ptr;
  • Illustration:
    • dynamic-memory1
Pointing to Memory Allocated at Run Time (cont.)
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  • An Example:
    • ptr = new int;
  • Illustration:
    • dynamic-memory2
Using Memory Allocated at Run Time
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  • An Example:
    • *ptr = 7;
  • Illustration:
    • dynamic-memory3
Run Time Allocation of Arrays
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  • An Example:
    • int *a;
  • Illustration:
    • dynamic-memory4
Run Time Allocation of Arrays (cont.)
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  • An Example:
    • a = new int[3];
  • Illustration:
    • dynamic-memory5
Run Time Allocation of Arrays (cont.)
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  • An Example:
    • *a = 300; *(a+1) = 301; *(a+2) = 302;
  • Illustration:
    • dynamic-memory6
Run Time Allocation of Arrays (cont.)
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  • An Observation:
    • *a and a[0] are syntactically different ways to refer to the same value
  • An Explanation:
    • The [] operator performs pointer arithemtic and de-references the resulting pointer.
Returning Memory to the Heap
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  • How Big is the Heap?
    • It can only contain as much physical memory as you have installed or as much virtual memory as your operating system can make available (if it supports virtual memory)
  • Running Out of Memory:
    • Most applications request memory from the heap when they are running
    • It is possible to run out of memory (you may even have gotten a message like "Running Low On Virtual Memory")
    • So, it is important to return memory to the heap when you no longer need it
Returning Memory to the Heap (cont.)
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  • The Opposite of new:
    • In C++: The delete operator in C++
    • In Java: The JVM has a garbage collector that returns unused memory
  • An Example:
    • delete ptr;
  • Illustration:
    • dynamic-memory7
Returning Memory to the Heap (cont.)
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  • Dangling Pointers:
    • The delete operator does not delete the pointer, it takes the memory being pointed to and returns it to the heap
    • It does not even change the contents of the pointer
    • Since the memory being pointed to is no longer available (and may even be given to another application), such a pointer is said to be dangling
  • An Example:
    • ptr = NULL;
  • Illustration:
    • dynamic-memory8
Returning Memory to the Heap (cont.)
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  • Remember:
    • Return memory to the heap before undangling the pointer
  • What's Wrong with the Following:
    • ptr = NULL; delete ptr;
Returning Memory to the Heap (cont.)
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  • What About Arrays?
    • You want to return all of the memory to the heap
    • So, a different form of the delete operator is needed
    • Also, the memory allocator must keep track of the size of the array
  • An Example:
    • delete[] a;
  • Illustration:
    • dynamic-memory9
Returning Memory to the Heap (cont.)
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  • An Example:
    • a = NULL;
  • Illustration:
    • dynamic-memory10
Memory Leaks
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  • An Explanation:
    • Memory leaks when it is allocated from the heap using the new operator but not returned to the heap using the delete operator
  • An Example:
    • int *otherptr;
  • Illustration:
    • dynamic-memory11
Memory Leaks (cont.)
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  • An Example:
    • otherptr = new int;
  • Illustration:
    • dynamic-memory12
Memory Leaks (cont.)
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  • An Example:
    • *otherptr = 4;
  • Illustration:
    • dynamic-memory13
Memory Leaks (cont.)
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  • An Example:
    • otherptr = new int;
  • Illustration:
    • dynamic-memory14
Memory Leaks (cont.)
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  • An Example:
    • *otherptr = 5;
  • Illustration:
    • dynamic-memory15
Dynamic Allocation of Objects
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A Simple Class to Get Us Started

cppexamples\memory\Weight.h
 
cppexamples\memory\Weight.cpp
 
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • An Observation:
    • We can dynamically allocate memory for objects
  • An Example:
    • Weight *w;
  • Illustration:
    • dynamic-memory16
Dynamic Allocation of Objects (cont.)
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  • An Example:
    • w = new Weight(6,11);
  • Illustration:
    • dynamic-memory17
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • De-referencing Pointers to Objects:
    • Works the same as with other pointers
  • An Example:
    • (*w).pounds
  • A "Shorthand" Operator:
    • w->pounds
    • Inside of a class you can use this->
Dynamic Allocation of Objects (cont.)
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A More Complicated Class

cppexamples\memory\Person.h
 
Dynamic Allocation of Objects (cont.)
Back SMYC Forward

A More Complicated Class (cont.)

cppexamples\memory\Person.cpp
 
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • An Example:
    • Person *p;
  • Illustration:
    • dynamic-memory18
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • An Example:
    • p = new Person(); p->personalName = "David"; p->familyName = "Bernstein";
  • Illustration:
    • dynamic-memory19
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • An Example:
    • Person *q; char *first, *last; int age;
  • Illustration:
    • dynamic-memory20
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • An Example:
    • first = new char[6]; last = new char[10]; age = 26;
  • Illustration:
    • dynamic-memory21
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • An Example:
    • strcpy(first, "David"); strcpy(last, "Bernstein");
  • Illustration:
    • dynamic-memory22
Dynamic Allocation of Objects (cont.)
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  • An Example:
    • q = new Person(first, last, age);
  • Illustration:
    • dynamic-memory23
Dynamic Allocation of Objects (cont.)
Back SMYC Forward
  • Returning Memory to the Heap:
    • The memory manager doesn't know how many times the object might have requested memory from the heap in its various methods
    • The object must keep track of this itself
  • Destructor:
    • A method with the job of returning all "supplemental" memory used by an object
    • Has the same name as the class but preceeded by a tilde (i.e., the ~ character)
    • Whenever the delete operator is applied to a pointer to an object the object's destructor is called
Dynamic Allocation of C Strings
Back SMYC Forward
  • Recall:
    • Strings (in C) are just arrays of characters terminated with a '\0'
  • An Example:
    • char *s; s = new char[20];
Dynamic Allocation of C Strings (cont.)
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  • A Static Array of Dynamically Allocated Strings:
    • char *names[4]; int nameLength; nameLength = 80; for (int i=0; i<4; i++) names[i] = new char[nameLength]; strcpy(names[0],"Alice"); strcpy(names[1],"Bob"); strcpy(names[2],"Carol"); strcpy(names[3],"Dan");
Dynamic Allocation of Arrays of Objects
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  • An Example:
    • int numWeights; Weight *w; numWeights = 50; w = new Weight[numWeights];
  • Interpretation:
    • w is a pointer to the array
    • Each element of w is a Weight object (not a reference to a Weight)
    • Each Weight is constructed using the default constructor
An Aside on main
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  • So Far:
    • Our main function has had no parameters
  • Now:
    • We can discuss a version of main that is passed command-line arguments
    • int main(int argc, const char *argv[])
There's Always More to Learn
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