1. 05 | Pointers and RAII
Kate Gregory | Gregory Consulting
James McNellis | Senior Engineer, Visual C++

2. Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers

3. Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers

4. Lesson 1: Pointers
void f()
{
int x{1};
int y{2};
}
…
x
y
…

5. Lesson 1: Pointers
void f()
{
int x{1};
int y{2};
}
&x
…
&y
x
y
…

6. Lesson 1: Pointers
void f()
{
int x{1};
int y{2};
int* pointer_to_x{&x};
int* pointer_to_y{&y};
}

7. Lesson 1: Pointers
void f()
{
int x{1};
int* pointer_to_x{&x};
int y{*pointer_to_x};
}

8. Lesson 1: Pointers
void f()
{
int x{1};
int* pointer_to_x{&x};
*pointer_to_x = 2;
}

9. Lesson 1: Pointers
void f()
{
int x{1};
int y{2};
int* p{&x};
p = &y;
}

10. Lesson 1: Pointers
void f()
{
int p{nullptr};
}

11. DEMO
Pointers

12. Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers

13. Lesson 2: Dynamic Allocation
• Objects have scope, and go away when execution reaches
the end of that scope
void f()
{
NamedRectangle fred_rectangle{ "Fred", 3, 4 };
// . . .
}
• What if other code needed that object still?

14. The Free Store
• Separate area of memory
• Objects created there stick around until deliberately cleaned
up
• To create one, use new – returns a pointer
• To clean up, use delete

15. The Free Store
void f()
{
int* p{new int{1}};
*p = 2;
delete p;
}

16. DEMO
Pointers

17. Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers

18. Exceptions
• Sometimes what you want can’t be done
– That file doesn’t exist any more
– You don’t have permission to do that to the system
• Checking in advance is best, but not always possible
• Code that calls a function doesn’t need to check return value
• C++ provides a non ignorable signal that something awful
has happened
• Will return from any nested level of function calls to a place
that can handle the error

19. Exception syntax
try
{
// anything, including function calls
}
catch (const std::exception& e)
{
std::cout << "Exception caught: " << e.what();
}

20. DEMO
Exceptions

21. Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers

22. Copying, Assignment, and Destruction
void f()
{
int x{1};
int y{x}; // Copying
y = x;
}
// Assignment

23. Copying, Assignment, and Destruction
void f()
{
Rectangle x{3, 4};
Rectangle y{x}; // Copying
y = x;
}
// Assignment

24. Copying, Assignment, and Destruction
• Copying a class-type object invokes a special constructor
– Called the copy constructor
– Rectangle(Rectangle const&);
– If you don’t define one, the compiler provides one by default
• The default behaviour is to copy each base and data
member

25. Copying, Assignment, and Destruction
• Copying a class-type object invokes a special constructor
– Called the copy constructor
– Rectangle(Rectangle const&);
– If you don’t define one, the compiler provides one by default
• The default behaviour is to copy each base and data
member
Rectangle(Rectangle const& other)
: _width{other._width}, _height{other._height}

26. Copying, Assignment, and Destruction
• Assigning to a class-type object invokes a special operator
– Called the assignment operator
– Rectangle& operator=(Rectangle const&);
– If you don’t define one, the compiler provides one by default
• The default behaviour is to assign each base and data
member

27. Copying, Assignment, and Destruction
• Assigning to a class-type object invokes a special operator
– Called the assignment operator
– Rectangle& operator=(Rectangle const&);
– If you don’t define one, the compiler provides one by default
• The default behaviour is to assign each base and data member
Rectangle& operator=(Rectangle const& other)
{
_width = other._width;
_height = other._height;
return *this;
}

28. Copying, Assignment, and Destruction
void f()
{
int x{1}; // x comes into existence
int y{2}; // y comes into existence
}
// y goes out of existence
// x goes out of existence

29. Destructor
• Name is always ~ plus the name of the class
– E.g., ~Rectangle()
• Returns nothing, takes no parameters
• Place to cleanup resources used by an object
– Close a file
– Release a lock
– Etc
• When exceptions transfer execution out of a scope,
everything declared in that scope is cleaned up
– Destructor runs

30. DEMO
RAII

31. Smart pointers
• unique_ptr if only one object needs access to the underlying
pointer
• shared_ptr if several want to use the same underlying pointer
– Cleaned up when the last copy goes out of scope
• In <memory> header file
• If you’re using new or delete, you’re doing it wrong

32. RAII in General
• Constructor acquires resource
– Eg opens file
• All other member functions know resource is acquired
– Do not need to test and make sure
• Destructor releases resource
– Works even in the presence of exceptions

33. Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers

34. DEMO
Smart Pointers

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