Why is it an error to use an empty set of brackets to call a constructor with no arguments?

Question

Is there any good reason that an empty set of brackets isn't valid for calling the default ctor in c++?

MyObject  object;  // ok - default ctor
MyObject  object(blah); // ok

MyObject  object();  // error

I seem to type "()" automatically everytime. I just wondered if there was a good reason this isn't allowed?

Answer 1

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

Answer 2

Because the compiler thinks it is a declaration of a function that takes no arguments and returns a MyObject instance.

Answer 3

Because it is the treated as the declaration for a function:

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

Answer 4

I guess, the compiler would not know if this statement:

MyObject object();

is a constructor call or a function prototype declaring a function named object with return type MyObject and no parameters.

Answer 5

As mentioned many times, it's a declaration. It's that way for backward compatibility. One of the many areas of C++ that are goofy/inconsistent/painful/bogus because of its legacy.

Answer 6

Most vexing parse

This is known as "C++'s most vexing parse". Basically, anything that can be interpreted by compiler as function declaration will be interpreted as function declaration, even if resulting AST doesn't compile.

Another instance of the same problem:

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

v is interpreted as a declaration of function with 2 parameters and fails to compile.

The workaround is to add another pair of parentheses:

std::vector v((std::istream_iterator(ifs)), std::istream_iterator());

Answer 7

You could also use the more verbose way of construction:

MyObject object1 = MyObject();
MyObject object2 = MyObject(object1);

In C++0x this also allows for auto:

auto object1 = MyObject();
auto object2 = MyObject(object1);