The most vexing parse is a specific form of syntactic ambiguity resolution in the C++ programming language.

class Timer {
 public:
  Timer();
};

class TimeKeeper {
 public:
  TimeKeeper(const 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 which returns an object of type TimeKeeper and has a single (unnamed) parameter which is a function returning type Timer (and taking no input).

Most programmers expect the first, but the C++ standard requires it to be interpreted as the second. One way to force the compiler to consider this as a variable definition is to add an extra pair of parentheses:TimeKeeper time_keeper( (Timer()) );

Using the new uniform initialization syntax introduced in C++11 solves this issue (and many more headaches related to initialization styles in C++). The problematic code is then unambiguous when braces are used:

TimeKeeper time_keeper{Timer{}};