RAII

Resource Acquisition Is Initialization

RAII is a design pattern where resource lifetime is bound to object lifetime. When the object is created, it acquires the resource. When the object is destroyed, it releases the resource automatically via the destructor.

Resources Include : 

• heap memory
• file handles
• sockets
• mutex locks
• db connections
• GPU buffers
• threads

File I/O RAII Example

#include <iostream>
#include <fstream>
#include <string>

// ifstream abides by RAII design pattern
// Resource (file) is acquired during scope
// Destructor naturally runs when escaping scope
int main() {
  std::ifstream file{"file.txt"};
  if (file.isOpen()) {
    std::cout << "file is open" << std::endl;
  } else {
    std::cout << "file is close" << std::endl;
  }  
}

Mutex RAII Example

std::mutex m;

// No RAII object.
// if some condition is true, m.unlock never runs -> deadlock
void foo() {
  m.lock();

  if (some condition) {
    return;
  }

  m.unlock();
}

// With RAII object
// destructor runs when leaving scope
void bar() {
  std::lock_guard<std::mutex> lock(m);

  if (some condition) return;
}