Default constructor with empty brackets

Most vexing parse

This is related to what is known as "C++'s most vexing parse". Basically, anything that can be interpreted by the compiler as a function declaration will be interpreted as a function declaration.

Another instance of the same problem:

std::ifstream ifs("file.txt");
std::vector<T> v(std::istream_iterator<T>(ifs), std::istream_iterator<T>());

v is interpreted as a declaration of function with 2 parameters.

The workaround is to add another pair of parentheses:

std::vector<T> v((std::istream_iterator<T>(ifs)), std::istream_iterator<T>());

Or, if you have C++11 and list-initialization (also known as uniform initialization) available:

std::vector<T> v{std::istream_iterator<T>{ifs}, std::istream_iterator<T>{}};

With this, there is no way it could be interpreted as a function declaration.

Because it is treated as the declaration for a function:

int MyFunction(); // clearly a function
MyObject object(); // also a function declaration

The same syntax is used for function declaration - e.g. the function object, taking no parameters and returning MyObject