How it works...

The if and switch statements with initializers are basically just syntax sugar. The following two samples are equivalent:

Before C++17:

{
    auto var (init_value);
    if (condition) {
        // branch A. var is accessible
    } else {
        // branch B. var is accessible
    }
    // var is still accessible
}

Since C++17:

if (auto var (init_value); condition) {
    // branch A. var is accessible
} else {
    // branch B. var is accessible
}
// var is not accessible any longer

The same applies to switch statements:

Before C++17:

{
    auto var (init_value);
    switch (var) {
    case 1: ...
    case 2: ...
    ...
    }
    // var is still accessible
}

Since C++17:

switch (auto var (init_value); var) {
case 1: ...
case 2: ...
  ...
}
// var is not accessible any longer

This feature is very useful to keep the scope of a variable as short as necessary. Before C++17, this was only possible using extra braces around the code, as the pre-C++17 examples show. The short lifetimes reduce the number of variables in the scope, which keeps our code tidy and makes it easier to refactor.