RAII in C++

RAII is one of the patterns in C++ to manage resources.

Sometimes at some part of the program, we need to allocate some memory and later free it.

template <typename T, std::size_t N>
T* allocateMemory()
{
    return new T[N];
}
template <typename T>
void freeMemory(T *p)
{
    delete[] p;
}
int main()
{
    int *p = allocateMemory<int, 10>();
    // use p
    freeMemory(p);
    
    return 0;
}

Disaster can happen when

1. allocateMemory() is not called at all. Then the program will crash.
2. freeMemory() is not called at all. There will be memory leak.
3. freeMemory() is called twice from two different control paths or threads. It will cause double deletion and might crash.

This is a problem in C++, because resource management is developer’s responsibility. RAII or Resource Acquisition is Initialization is a pattern which fixes this problem.

In RAII

• Resource is a class invariant.
• Resource acquisition(memory allocation, file opening, acquiring mutex , database connection) is done when lifetime of the object begins. In C++11, lifetime of object begins after construction.
• Resource is available throughout the object lifetime.
• Resource is released when lifetime of the object object ends.