Question

While browsing the code of my friend I came to notice this:

switch(State &state = getState()) {
case Begin: state = Search; break;
// other stuff similar
}

What is with the variable in the switch header? He is using GCC so I think this might be a GCC extension. Any idea?

Answer 1

It’s not a secret or a GCC extension. Variables are allowed to be declared in the conditions of things like ifs, whiles, and switches. For example:

while (char c = cin.get()) { ... }

or

if (int* something = (int*)malloc(4)) { // but don't use malloc in C++
    // ...
}

After they are declared an initialised, they are converted to a bool value and if they evaluate to true the block is executed, and the block is skipped otherwise. Their scope is that of the construct whose condition they are declared in (and in the case of if, the scope is also over all the else if and else blocks too).

In §6.4.1 of the C++03 standard, it says

Selection statements choose one of several flows of control.

selection-statement:

    if ( condition ) statement
    if ( condition ) statement else statement
    switch ( condition ) statement

condition:

    expression
    type-specifier-seq declarator = assignment-expression

So as you can see, it allows type-specifier-seq declarator = assignment-expression in the condition of an if or switch. And you’d find something similar in the section for the "repetition constructs".

Also, switches work on integral or enum types or instances of classes that can be implicitly converted to an integral or enum type (§6.4.4):

The value of a condition that is an initialized declaration in a
switch statement is the value of the declared variable if it has
integral or enumeration type, or of that variable implicitly converted
to integral or enumeration type otherwise.

I actually learned of this FROM AN ANSWER YOU POSTED on the "Hidden Features of C++" question. So I’m glad I could remind you of it 🙂