5 raii
Post on 19-Jul-2015
67 Views
Preview:
TRANSCRIPT
05 | Pointers and RAII
Kate Gregory | Gregory Consulting
James McNellis | Senior Engineer, Visual C++
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers
Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers
Module Overview
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers
Module Overview
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?
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
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers
Module Overview
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
Exception syntax
try
{
// anything, including function calls
}
catch (const std::exception& e)
{
std::cout << "Exception caught: " << e.what();
}
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers
Module Overview
void f()
{
int x{1};
int y{x}; // Copying
y = x; // Assignment
}
Copying, Assignment, and Destruction
void f()
{
Rectangle x{3, 4};
Rectangle y{x}; // Copying
y = x; // Assignment
}
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
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}
{}
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
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;
}
Copying, Assignment, and Destruction
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
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
Memory released
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
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
1. Pointers
2. Dynamic Allocation
3. Exceptions
4. Copying, Assignment, and Destruction
5. Resource Acquisition is Initialization (RAII)
6. Smart Pointers
Module Overview
©2013 Microsoft Corporation. All rights reserved. Microsoft, Windows, Office, Azure, System Center, Dynamics and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.
top related