Most vexing parse

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The most vexing parse is a counterintuitive form of syntactic ambiguity resolution in the C++ programming language. In certain situations, the C++ grammar cannot distinguish between the creation of an object parameter and specification of a function's type. In those situations, the compiler is required to interpret the line as a function type specification.

The term "most vexing parse" was first used by Scott Meyers in his 2001 book Effective STL.^[1] While unusual in C, the phenomenon was quite common in C++ until the introduction of uniform initialization in C++11.^[2]

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:

// A function named i 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 the subsequent line 10 above. For example, Clang++ warns that the most vexing parse has been applied on line 9 and errors on the subsequent line 10:^[3]

$ clang++ time_keeper.cc timekeeper.cc:9:25: warning: parentheses were disambiguated as a function declaration       [-Wvexing-parse]   TimeKeeper time_keeper(Timer());                         ^~~~~~~~~ timekeeper.cc:9:26: note: add a pair of parentheses to declare a variable   TimeKeeper time_keeper(Timer());                          ^                          (      ) timekeeper.cc:10:21: error: member reference base type 'TimeKeeper (Timer (*)())' is not a       structure or union   return time_keeper.get_time();          ~~~~~~~~~~~^~~~~~~~~ 

The required interpretation of these ambiguous declarations is rarely the intended one.^[4][5] Function types in C++ are usually hidden behind typedefs and typically have an explicit reference or pointer qualifier. To force the alternate interpretation, the typical technique is a different object creation or conversion syntax.

In the type conversion example, there are two alternate syntaxes available for casts: the "C-style cast"

// declares a variable of type int int i((int)my_dbl); 

or a named cast:

int i(static_cast<int>(my_dbl)); 

In the variable declaration example, the preferred method (since C++11) is uniform (brace) initialization.^[6] This also allows limited omission of the type name entirely:

//Any of the following work: TimeKeeper time_keeper(Timer{}); TimeKeeper time_keeper{Timer()}; TimeKeeper time_keeper{Timer{}}; TimeKeeper time_keeper(     {}); TimeKeeper time_keeper{     {}}; 

Prior to C++11, the common techniques to force the intended interpretation were use of an extra parenthesis or copy-initialization:^[5]

TimeKeeper time_keeper( /*Avoid MVP*/ (Timer()) ); TimeKeeper time_keeper = TimeKeeper(Timer()); 

In the latter syntax, the copy-initialization is likely to be optimized out by the compiler.^[7] Since C++17, this optimization is guaranteed.^[8]

• Discussion in the C++03 standard final draft (see §8.2 Ambiguity resolution [dcl.ambig.res]): https://web.archive.org/web/20141113085328/https://cs.nyu.edu/courses/fall11/CSCI-GA.2110-003/documents/c++2003std.pdf
• CppReference on direct initialization (the sort vulnerable to the most vexing parse): https://en.cppreference.com/w/cpp/language/direct_initialization Archived 2021-12-13 at the Wayback Machine