Most vexing parse [duplicate]

what does this syntax int(x) in the statement Foo f( int(x) ); mean?

The parentheses around x are superfluous and will be ignored. So int(x) is the same as int x here, which means a parameter named x with type int.

Is it the same as Foo f( int x );?

Yes. Foo f( int(x) );, is a function declaration which is named f, returns Foo, takes one parameter named x with type int.

Here’s the explanation from the standard. [dcl.ambig.res]/1:

(emphasis mine)

The ambiguity arising from the similarity between a function-style
cast and a declaration mentioned in [stmt.ambig] can also occur in the
context of a declaration. In that context, the choice is between a
function declaration with a redundant set of parentheses around a
parameter name and an object declaration with a function-style cast as
the initializer. Just as for the ambiguities mentioned in
[stmt.ambig], the resolution is to consider any construct that could
possibly be a declaration.

Note: A declaration can be
explicitly disambiguated by adding parentheses around the argument.
The ambiguity can be avoided by use of copy-initialization or
list-initialization syntax, or by use of a non-function-style cast.

struct S {
  S(int);
};

void foo(double a) {
  S w(int(a));      // function declaration
  S x(int());       // function declaration
  S y((int(a)));    // object declaration
  S y((int)a);      // object declaration
  S z = int(a);     // object declaration
}

So, int(x) will be considered as a declaration (of the parameter) rather than a function style cast.