difference between declaration of default-constructible object with parentheses and without them

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?

Replay

Most vexing parse

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

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

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

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

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

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.

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.

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);

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 ]

Category: c# Time: 2008-10-07 Views: 1

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