1. Resource Acquisition Is
Initialization (RAII)

2. {
int device;
if( . . . ) // Some selection criteria
{
device = open(. . .);
} else {
device = socket(. . .);
. . .
}
close(device);
}

3. {
int device;
if( . . . ) // Some selection criteria
{
device = open(. . .);
} else {
device = socket(. . .);
. . .
}
// If you return, or throw exception, here then device
// remains open:
// if ( . . . ) // close(device) was accidentally forgotten
// return;
close(device);
}

4. struct DeviceCloser {
DeviceCloser() // Initialization
: m_device(NULL)
{}
~DeviceCloser() // Destruction
{
close(m_device); // Close the device handle
}
int m_device;
};

5. {
DeviceCloser dc;
int &device = dc.m_file;
if( . . . ) // Some selection criteria
{
device = open(. . .);
} else {
device = socket(. . .);
. . .
}
// … Whatever happens here, the device will be closed
} // Device closed here

6. struct DeviceCloser {
DeviceCloser(string device_name) // Initialization
//: m_device(NULL)
{
if( . . . ) // Some selection criteria
{
m_device = open(device_name, . . .);
} else {
m_device = socket(. . .);
. . .
}
}
~DeviceCloser() // Destruction
{
close(m_device); // Close the device handle
}
int m_device;
};

7. {
string device_name(“Large Hadron Collider”);
DeviceCloser dc(device_name);
dc.start();
// … Whatever happens here, the device will be closed
// and the world will be saved
} // Device closed here

8. Object lifecycle
Construction Acquire
Resource
Destruction Release

9. // in the the C language, see."cleanup" variable attribute”
#define RAII_VARIABLE(vartype,varname,initval,dtor)
void _dtor_ ## varname (vartype * v) { dtor(*v); }
vartype varname __attribute__((cleanup(_dtor_ ##
varname))) = (initval)
// This macro can then be used as follows:
void example_usage() {
RAII_VARIABLE(FILE*, logfile, fopen("logfile.txt", "w+"),
fclose);
fputs("hello logfile!", logfile);
}

10. Typical Uses
• Synchronization primitives
(Mutex Lock/Unlock)
• File Open/Close
• Memory allocation/deallocation

11. Things to Remember
• To prevent resource leaks, use RAII objects
that acquire resources in their constructors
and release them in their destructors.
• Commonly useful RAII classes are
shared_ptr,auto_ptr, etc[code] (another
seminar)

12. List of references:
• Scott Meyers, Effective C++: 55 Specific Ways
to Improve Your Programs and Designs
• Stephen C. Dewhurst, C++ Common
Knowledge: Essential Intermediate
Programming
• Matthew Wilson, Imperfect C++: Practical
Solutions for Real-Life Programming