Automatic Variables

Automatic Variables

• The constructor is called when the declaration is reached
• The destructor is called right before the block exits
• Order of destructor calls is inverse of constructor order

Software View of Resources

• Memory
• File handles (e.g., file descriptors)
• Network connections
• Locks & semaphores
• Database connections

Resource must be…

• Properly allocated
• Properly initialized before use
• Properly validated before use
• Properly deallocated after use

Resource Problems

• Improper initialization
• Use when invalid
• Resource leak (never deallocated)
• Double free (deallocated more than once)

File Descriptor Resource Leak

• File descriptor is allocated each time through the loop
• But file descriptor is not deallocated each time through the loop
• Will hit a limit that depends on your O.S. environment
• To view the limit: ulimit -n
• Note: Some discrepancy on the default limit in Ubuntu (and in srcml/codespaces)
• Even without a resource leak, you can open too many files simultaneously

Effect of Improperly Used Resources

• Exceed resource limits
• Program crash
• Invalid output
• Security vulnerabilities

C++ Pointers

• Pointers to memory
• Limits on the amount of memory available to a program/process
• Good pointer usage is not easy to do

C++ Pointer Problems

Avoiding C++ Pointer Problems

• Behavioral issue
• Requires complex tools to detect, with lots of false positives
• Must have a scenario for each usage pattern
• Good design can prevent the problem from occurring

Programming Idiom

A recurring construct to fix a commonly occurring coding problem

• Not a direct feature of the programming language, but a combination of features to solve the problem
• Typically, this is a design problem
• Related to design patterns which we will discuss soon

RAII Idiom

Resource Acquisition Is Initialization

• Holding a resource is tied to an object's lifetime
• Constructor: Allocate resource
• Destructor: Deallocate resource
• Prevents resource leaks, double-free, use when invalid

RAII Interface

• Constructor - Allocate resource
• Destructor - Deallocate resource
• Access - Direct access to a resource
• Boolean - Check if the resource exists (i.e., has been allocated and is still valid)
• Copy, Assignment - Transfer resource control to new RAII object
• Deallocate - Deallocate resource before Destructor (safely)

RAII: std::ofstream

RAII: std::unique_lock

Smart Pointers

A wrapper type that makes pointers safer

• Automatic initialization with nullptr
• Convertible to bool for validity checking
• Automatic destructor call for deallocation of the contained resource

C++ Smart Pointers

Pointer Use Cases

Use Case: Lazy Initialization

Delaying the construction of an object until (or if) it is needed

• Primarily for resource expensive objects
• Directly, e.g., sizeof(Data), store a large amount of data
• Use a large number of resources
• File handles
• Network connections
• Memory
• Processes
• Threads
• Synchronization primitives (mutexes, semaphores, etc.)
• Kernel handles

Lazy Initialization with Pointers

std::unique_ptr

Comparison

Use Case: Existing library with pointers

• Calculating Complexity
• Uses libxml2

Example: xmlReadFile()

• Concerns
• libxml2 functions, e.g., xmlReadFile
• error handling for srcMLXPathCount()
• deallocate with xmlFreeDoc(), etc. with error handling
• Full Example

Custom Deleter

• Concerns
• libxml2 functions, e.g., xmlReadFile
• error handling for srcMLXPathCount()
• deallocate with xmlFreeDoc(), etc. with error handling
• Full Example
• Example of Full Use

Comparison

Specialization of default_delete<>

Alternative Solutions

• Create custom C++ wrapper for every libxml2 function used
• Use goto and other constructs in the code

Conclusion

• RAII behavior prevents resource errors without any additional burden on the developer
• Ensures proper behavior
• Makes the resource much easier to work with, e.g., don't have to explicitly close()
• All resources should be built with RAII behavior