Most vexing parse

A simple example appears when a functional cast is intended to convert an expression for initializing a variable

void f(double my_dbl) {
  int i(int(my_dbl));
}

Line 2 above is ambiguous. One possible interpretation is to declare a variable i with initial value produced by converting my_dbl to an int. However, C allows superfluous parentheses around function parameter declarations; in this case, the declaration of i is instead a function declaration equivalent to the following

// takes an integer and returns an integer
int i(int my_dbl);

A more elaborate example is:

struct Timer {};

struct TimeKeeper {
  explicit TimeKeeper(Timer t);
  int get_time();
};

int main() {
  TimeKeeper time_keeper(Timer());
  return time_keeper.get_time();
}

The line

  TimeKeeper time_keeper(Timer());

is ambiguous, since it could be interpreted either as

1. a variable definition for variable time_keeper of class TimeKeeper, initialized with an anonymous instance of class Timer or
2. a function declaration for a function time_keeper that returns an object of type TimeKeeper and has a single (unnamed) parameter, whose type is a (pointer to a) function[Note 1] taking no input and returning Timer objects.

The C++ standard requires the second interpretation, which is inconsistent with line 11 above. For example, Clang++ warns that the most vexing parse has been applied on line 10 and errors on the following line:[3]

$ clang++ time_keeper.cc
timekeeper.cc:10:25: warning: parentheses were disambiguated as a function declaration
      [-Wvexing-parse]
  TimeKeeper time_keeper(Timer());
                        ^~~~~~~~~
timekeeper.cc:10:26: note: add a pair of parentheses to declare a variable
  TimeKeeper time_keeper(Timer());
                         ^
                         (      )
timekeeper.cc:11:21: error: member reference base type 'TimeKeeper (Timer (*)())' is not a
      structure or union
  return time_keeper.get_time();
         ~~~~~~~~~~~^~~~~~~~~