What is the most astounding C++ syntax construct?

[tea-age.solutions]

C++ is a great language and for me personal the greatest programming language so far.

But there are also constructs possible which may look strange or really complicated.

Do you know some?

I wanna share a strange looking but sometimes very useful example:

// imagine having also some struct/class X, Y and Z...
class Foo
{
private:
    // some member mMember1 and mMember2 here...
public:
    Foo( X x, Y y, Z z ) // Constructor
    try : mMember1( x )  // A functional try block around the member initilizer list ...
        , mMember2( y, z )
    {
    } catch( ... ) {     // ... and the complete constructor body ...
        /* insert useful
        code here */
    }                    // ...  catching all execptions thrown from there.
};

There you see a Functional Try Block.
This is useful if in the middle of the member initialization an exception is thrown and the half created class instance need to do some clean up. There is one special behavior in this construct. Do you know which?

The catch block will always re-throw the exception automatically because the class instance was not finally created and stays broken.

One complete example:

class Foo
{
    std::string mStr;
public:
    // Constructor
    explicit Foo( std::string const & s, size_t pos )
    // # 4: Function try block. The try block includes the initializer list!
    try : mStr( s, pos )
    {
        puts( "Foo constructor - normal block." );
    } catch( ... ) {
        // here can only cleanup already initialized/allocated stuff, which must be clean up manually.
        // Constructed members are destructed already.
        // the class instance itself stays broken. this catch block will auto re-throw the exception always!
        puts( "Foo constructor - catch block!." );
    }
};
 
try {
    std::string s = "test";
    Foo foo1( s, 2 );  // not throw
    Foo foo2( s, 10 ); // will throw std::out_of_range
} catch( ... ) {
}

This is one example from my Christmas Special Blog Post:
TOP 5 most astounding C++20 syntax.
The complete top 5 you can read here: TOP 5 most astounding C++20 syntax

Do you know other awesome syntax constructs?

Cheers!