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ClassA a; vs. ClassA a();

Posted on 2014-01-26
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Last Modified: 2014-01-30
What's the difference between
ClassA a;
ClassA a();

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Question by:deleyd
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Expert Comment

by:jkr
ID: 39810485
In short: There is none. Both instantiate an instance of 'ClassA' via the default constructor. The 2nd line could be called more explicit, yet the brackets could as well be called superfluous.
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Expert Comment

by:evilrix
ID: 39810490
The first constructs an object called a.  The second declares a function called a.
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Expert Comment

by:evilrix
ID: 39810496
>>In short: There is none.
The standard disagrees.  Due to the "most vexing parse"  rule, the second is a function declaration.
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Expert Comment

by:evilrix
ID: 39810498
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Expert Comment

by:phoffric
ID: 39810552
ClassA * aptr = new ClassA();

In the above case, the parens are superfluous. Either way, you get the default constructor.
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Expert Comment

by:jkr
ID: 39810957
>> Due to the "most vexing parse"  rule, the second is a function declaration.

Even within a function's body? ;o)
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Assisted Solution

by:phoffric
phoffric earned 50 total points
ID: 39810998
Here is how to (1) remove the error from the "most vexing parse" problem in the link; and (2) how to force the TimeKeeper constructor to be called, rather than treating time_keeper as a function declaration.
class Timer {
 public:
	 Timer() { cout << "Timer" << endl;};
};
 
class TimeKeeper {
 public:
	 TimeKeeper(const Timer& t) {cout << "TimeKeeper" << endl;};
	 int get_time() {cout << "get_time" << endl; return 1;};
};
 
int main() {
  TimeKeeper time_keeper((Timer()));
  return time_keeper.get_time();
}

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Expert Comment

by:phoffric
ID: 39811001
oops - I just saw that the link mentioned this. So, never mind.
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Expert Comment

by:evilrix
ID: 39811159
>Even within a function's body? ;o)

Yes,  anywhere.

NB. You can forward declare a function within the scope of another function. This limits the scope to the function into which the other function is declared.
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Expert Comment

by:eeeabhi
ID: 39811340
ClassA a;//Create an instance of Object type ClassA with the instance name a
ClassA a();//Declare a function which returns ClassA type object and takes in no arguments.
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Expert Comment

by:evilrix
ID: 39811356
eeeabhi, we have already established that.
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Accepted Solution

by:
evilrix earned 50 total points
ID: 39811383
>> ClassA * aptr = new ClassA();
>> In the above case, the parens are superfluous. Either way, you get the default constructor.

Actually, that's also not true. In the case of POD or intrinsic types, not calling the synthesized constructor explicitly leaves the object uninitialised. Calling it explicitly constructs them with default values.

int * p = new int

is not the same as

int * p = new int()

The same is true for user defined PODs.

#include <iostream>

struct foo
{
   int bar;
};

int main()
{
   // aggregate members are not default initialised
   foo const * f1 = new foo;

   // aggregate members are default initialised
   foo const * f2 = new foo();

   std::cout
         << f1->bar << std::endl // output is indeterminate
         << f2->bar << std::endl; // output will be 0 (same as int()).

   delete f1;
   delete f2;
}

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More specifically, the standard says...



To value-initialize an object of type T means:
— if T is a (possibly cv-quali¿ed) class type (Clause 9) with a user-provided constructor (12.1), then the
default constructor for T is called (and the initialization is ill-formed if T has no accessible default
constructor);
— if T is a (possibly cv-quali¿ed) non-union class type without a user-provided constructor, then the object
is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is
called.
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized.

An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.

To default-initialize an object of type T means:
— if T is a (possibly cv-quali¿ed) class type (Clause 9), the default constructor for T is called (and the
initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, no initialization is performed.

If no initializer is speci¿ed for an object, the object is default-initialized; if no initialization is performed, an
object with automatic or dynamic storage duration has indeterminate value.


And with regards to the "most vexing parse" rule, the standard states...


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.
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Author Closing Comment

by:deleyd
ID: 39822749
Most vexing. This C++ syntax will drive me nuts.
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