Is there any good reason that an empty set of round brackets (parentheses) isn't valid for calling the default constructor in C++?
MyObject object; // ok - default ctor
MyObject object(blah); // ok
MyObject object(); // error
I seem to type "()" automatically everytime. Is there a good reason this isn't allowed?
Solution 1
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.
Solution 2
Because it is treated as the declaration for a function:
int MyFunction(); // clearly a function
MyObject object(); // also a function declaration
Solution 3
The same syntax is used for function declaration - e.g. the function object, taking no parameters and returning MyObject
Solution 4
Because the compiler thinks it is a declaration of a function that takes no arguments and returns a MyObject instance.
Solution 5
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.
Solution 6
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);
Solution 7
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.
Solution 8
From n4296 [dcl.init]:
[ Note:
Since()is not permitted by the syntax for initializer,X a();is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X. The form () is permitted in certain other initialization contexts (5.3.4, 5.2.3, 12.6.2).
end note ]
Solution 9
As the others said, it is a function declaration. Since C++11 you can use brace initialization if you need to see the empty something that explicitly tells you that a default constructor is used.
Jedi luke{}; //default constructor
