Most Vexing Parsing

Most vexing parsing is a counterintuitive form of syntactic ambiguity resolution in the C++ programming language. Sometimes C++ grammer(or compiler) can not distinguish between the creation of an object parameter and specification of a function type. ^[1]

A Simple Example

int x();

This line of code may look like a function declaration, but it's actually a declaration of a function pointer named x that takes no arguments and returns an integer. To declare a function that takes no arguments and returns an integer, the correct syntax would be:

int x(void);

Example from WiKi

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

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

In this line:

TimeKeeper time_keeper(Timer());

is ambiguous, since it could be interpreted either as:

• A variable definition for variable time_keeper of class TimeKeeper, initialized with an anonymous instance of class Time.
• 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 function taking no input and returning Time object.

Solutions

Since C++11 introduce uniform initialization that allows limited omission of the type name entirely.

TimeKeeper time_keeper(Timer{});
TimeKeeper time_keeper{Timer()};
TimeKeeper time_keeper{Timer{}};