What is RAII in C++?

RAII = Resource Acquisition Is Initialization: tie a resource's lifetime to an object so its destructor releases the resource automatically.

* The constructor acquires, the destructor releases — and because the destructor runs at every scope exit (even during an exception), cleanup can't be forgotten. *

void process() {
    vector<int> v;        // memory acquired
    v.push_back(1);
    // ... use v ...
}   // v goes out of scope -> destructor frees its memory automatically

void readFile() {
    ifstream file("data.txt");   // file opened
    // ... read from file ...
}   // file closed automatically when it's destroyed

Key principle:

• Acquire the resource in the constructor
• Release it in the destructor
• The object's scope then controls the resource's lifetime

Benefits:

• No leaks — cleanup is automatic, even if you forget
• Exception-safe — destructors still run as the stack unwinds during an exception
• No scattered manual cleanup code

Tip: RAII is why smart pointers (unique_ptr, shared_ptr) are preferred over raw new/delete — they're RAII wrappers that delete for you.

Go deeper:

• RAII — cppreference — the idiom defined, with standard RAII types.
• Resource acquisition is initialization — Wikipedia — RAII across languages.