Why I can't initialize a variable in switch case block

Question

I am writing some Win32 program, and I meet a small problem, why I cannot initialize a Variable in switch case block.

just like it: enter image description here

and when I do like this, it will be ok. enter image description here

And now I want to know why.

Please post the code as text. – David G Dec 19 '13 at 02:10 — David G, Dec 19 '13 at 02:10
You can, you just need to enclose the block in `{}`. – Retired Ninja Dec 19 '13 at 02:10 — Retired Ninja, Dec 19 '13 at 02:10

score 5 · Answer 1 · answered Dec 19 '13 at 02:19

The case statements in a switch() have the same semantics as goto: When you dispatch to a label in a switch-case, you effectively goto the case label. The reason for this is that cases in a switch-case aren't self-contained. You can fall-through from one case to another.

In fact, switch-case is so goto-like that you can even write a monstrosity such as Duff's Device. Study that until you're properly horrified.

In C++, locally defined objects come into scope at their point of definition, and go out of scope at the closing curly brace of the enclosing scope. A break or continue statement that exits that scope is guaranteed to cleanly handle objects going out of scope.

For your switch statement, the enclosing scope is defined by the curly-braces after switch().

Now, in C++, it's illegal to use goto or something like it skip an object initialization. In your example, the case and default labels run afoul of that rule.

Your fix avoids the issue by replacing object initialization with object assignment. That's one way to fix it. Another is to add an additional level of scope:

    case WM_LBUTTONDOWN:
    {
        HBRUSH hBrush = CreateSolidBrush(RGB(0, 0, 0));
        break;
    }

This works because the curly braces provide an extra level of scope clearly defining when hBrush's lifetime ends.

In terms of technical detail: The statement that triggered the error relies on a copy constructor. The statement that did not relies on the copy assignment operator instead.

πάντα ῥεῖ · Answer 2 · 2013-12-19T02:36:06.687

2

Retired Ninja: You can, you just need to enclose the block in {}

user3116182: yeah,I know that way

@user3116182: And why you're bothering about it??

Without scope declared variables, falling through case blocks will get into troubles to disambiguate them, or skipping variable initializations. That's all it's about.

edited Dec 19 '13 at 02:36

answered Dec 19 '13 at 02:20

πάντα ῥεῖ

1
13
116
190

Why I can't initialize a variable in switch case block

2 Answers2