Most Vexing Parse

The term “most vexing parse” was first used by Scott Meyers in his 2001 book Effective STL.

1. Introduction

The “most vexing parse” is a famous ambiguity in C++ syntax. It occurs when the compiler interprets a statement as function declaration instead of an object construction.

#include <iostream>

struct MyClass {
    MyClass() { std::cout << "Constructor called!\n"; }
};

int main() {
    MyClass obj();  // This looks like it creates an object, but it is not.
    return 0;
}

Some compilers provide warnings that can help catch potential most vexing parse issues using -Wvexing-parse. The above code will generate compile warnings if this option is enabled.

<source>:8:16: warning: empty parentheses interpreted as a function declaration [-Wvexing-parse]
    MyClass obj();
               ^~
<source>:8:16: note: remove parentheses to declare a variable
    MyClass obj();

C++ grammar allows declarations to look very similar to expressions. When the compiler encounters MyClass obj();, it prioritizes interpreting it as a declaration (because declarations are more fundamental in the language’s parsing rules) rather than an expression creating an object.

Another common example when using std::vector

std::vector<int> v(10, 20); // Correct: Creates a vector with 10 elements of value 20
std::vector<int> v2();  // Most Vexing Parse! Declares a function v2 returning a vector<int>

2. How to fix?

MyClass obj{}; // Use uniform initialization with braces (C++11 and later):
MyClass obj;
MyClass obj = MyClass();
MyClass* obj = new MyClass();

References

• https://en.wikipedia.org/wiki/Most_vexing_parse