What is stack unwinding?

Question

What is stack unwinding? Searched through but couldn't find enlightening answer!

Answer 1

Stack unwinding is usually talked about in connection with exception handling. Here's an example:

void func( int x )
{
    char* pleak = new char[1024]; // might be lost => memory leak
    std::string s( "hello world" ); // will be properly destructed

    if ( x ) throw std::runtime_error( "boom" );

    delete [] pleak; // will only get here if x == 0. if x!=0, throw exception
}

int main()
{
    try
    {
        func( 10 );
    }
    catch ( const std::exception& e )
    {
        return 1;
    }

    return 0;
}

Here memory allocated for pleak will be lost if an exception is thrown, while memory allocated to s will be properly released by std::string destructor in any case. The objects allocated on the stack are "unwound" when the scope is exited (here the scope is of the function func.) This is done by the compiler inserting calls to destructors of automatic (stack) variables.

Now this is a very powerful concept leading to the technique called RAII, that is Resource Acquisition Is Initialization, that helps us manage resources like memory, database connections, open file descriptors, etc. in C++.

Now that allows us to provide exception safety guarantees.