Question

I get a “transfer of control bypasses initialization of:” error when i try to build the following switch:

switch (retrycancel)
{
    case 4:    //The user pressed RETRY
        //Enumerate all visible windows and store handle and caption in "windows"
        std::vector<MainHandles::window_data> windows = MainHandles().enum_windows().get_results(); 
        break;

    case 2: 
        //code
}

It has something to do with my calling my enumerate function. If it is not allowed to call a function from within a switch, is there a workaround for this kind of problem?

Answer 1

section 6.6.4 of the C++ standard:

The goto statement unconditionally
transfers control to the statement
labeled by the identifier. The
identifier shall be a label (6.1)
located in the current function.

section 6.7 of the C++ standard:

It is possible to transfer into a
block, but not in a way that bypasses
declarations with initialization. A
program that jumps from a point
where a local variable with automatic
storage duration is not in scope to a
point where it is in scope is
ill-formed unless the variable has POD
type (3.9) and is declared without an
initializer

Emphasis added by me. Since switch is really goto in disguise, you’re encountering this behavior. To solve this, add braces if you must use a switch

switch (retrycancel)
    {
    case 4:
    {
        const std::vector<MainHandles::window_data> windows(
            MainHandles().enum_windows().get_results()
        );
        break;
    }
    case 2: 
        //code
    }

or refactor into if/else

if (retrycancel == 4) {
    const std::vector<MainHandles::window_data> windows(
        MainHandles().enum_windows().get_results()
    );
} else if (retrycancel == 2)
    // code
} else {
    ...
}

Though it’s not obvious to me what you’re hoping to accomplish with creating the windows vector inside a switch, so you may want to rethink your design. Note I added a const qualifier to windows since it’s not modified in your example.