Re: Gotchas for returning values from blocks

[jmh530 via Digitalmars-d-learn]

On Monday, 13 June 2016 at 01:41:07 UTC, Mike Parker wrote:

Everything works fine in your example because 'new' always 
allocates on the heap. Anything allocated on the stack is not 
guaranteed to be valid after the scope exits:

struct Foo
{
    int baz;
    ~this() { baz = 1; }
}

void main()
{
    import std.stdio : writeln;

    Foo* foo;
    {
        Foo bar = Foo(10);
        foo = &bar;
    }
    //bar is now out of scope
    assert(foo.baz == 10);
}

Struct constructors are always run when exiting a scope. More 
importantly, the pointer to bar is only valid until the stack 
address where it lives is overwritten by another stack 
allocation. In this example, there's no chance for that to 
happen before I access it, but it could happen at any time.

So returning a reference to something on the stack is a bad idea, 
but copying the value would be fine.