Archive for May, 2010

Member operator vs Nonmember operator

May 23rd, 2010
Member operators vs nonmember operators
As I was recently reviewing some of the concepts of C++, this question popped up. So what is the difference between member operators and nonmember operators?
Well as it turns out there isn’t much of a difference as far as the language is concered, except the additional argument of the object type in the nonmember function implementation. Given an operation on a user-defined type, either the presence of a member or a nonmember operator would suffice to complete the operation. So then why does the language provide the option of choosing between the two?
The answer turned out to be simple.
A member operator is typically used when
- the operator modifies the value of the object to which it is applied, requring it access to the data type’s internal values.
- or the operator requires access to the object’s internal state, represented by data members which are not otherwise accessible (through a public method).
In all other instances a nonmember operator is to be preferred thereby ensuring that there’s less chance of the object’s data members getting iadvertently modified and also reducing the size of the object.
Examples for operators that explicitly modify the state of an object are +=, +-, <<=, etc. These operators are applied to an object an result of the operation is applied to the object to which the operator is applied.
Examples for the second case where member operators are necessary can be found in the standard library. The basic_ostream template class declares the ‘<<’ operator for all basic datatypes except ‘char’ (actually template parameter ‘Ch’) as member operators. This is because output of the charater representation of these datatypes depend on the various ostream state information as set by the user. Examples for these state include output number system (decimal, octal or hexadecimal), precision, width, display of ‘+’ sign, etc.
However, output of the ‘char’ does not require access to any state information. Consequently, ‘<<’ for various ‘char’ types (signed, unsigned, char*, etc) are achieved through nonmember functions such as:
template<class Ch, class Tr>
basic_ostream<Ch, Tr>& operator<<(basic_ostream<Ch, Tr>&, Ch);
So the golden rule to learn is that if an operator can be implemented as a nonmember function, that is perhaps the best way to go as it would have the least impact on the object concered.

As I was recently reviewing some of the concepts of C++, this question popped up. So what is the difference between member operators and nonmember operators?

Well it turns out there isn’t much of a difference as far as the language is concerned (except the additional argument of the object type in the nonmember function implementation). Given an operation on a user-defined type, either the presence of a member or a nonmember operator would be enough. So then why does the language provide the option of choosing between the two?

The answer turned out to be simple.

A member operator is typically used when

  • the operator modifies the value of the object to which it is applied, requring it access to the object’s data members. Or,
  • the operator requires access to the object’s internal state, represented by data members which are not otherwise accessible (through a public method).

In all other instances a nonmember operator is to be preferred. This ensures that there’s that much less chance of the object’s data members getting inadvertently modified (which is only possible from a member method) and at the same time not increasing the size of the object. Refer to 11.3.1, C++ Programming Language, B.Stroustrup.

Examples for operators that explicitly modify the state of an object are +=, +-, <<=, etc. A real world code example:

class complex {
    double real, imag;
public:
    ...
    complex& operator+=(complex& c);
};

Examples for the second case where member operators are necessary can also be found in the standard library. The basic_ostream template class declares the '<<' operator for all basic datatypes except 'char' (actually template parameter 'Ch') as member operators. This is because output of the character representation of these datatypes (which is what the inserter operator accomplishes) depend on the various state information as set by the user. Examples for these state include output number system (decimal, octal or hexadecimal), precision, width, display of '+' sign, etc.
However, the output of  a 'char' does not require access to any state information. Consequently, '<<' for various 'char' types (signed, unsigned, char*, etc) are achieved through a nonmember function such as:


template<class Ch, class Tr>
       basic_ostream<Ch, Tr>& operator<<(basic_ostream<Ch, Tr>&, Ch);

So the golden rule to learn is that if an operator can be implemented as a nonmember function, that is the best way to go as it would have the least impact on the object concerned.

Beta Qt SDK – Installation Issue

May 16th, 2010

I’m back to Qt again. This time prompted by my need for a simple application that will help me learn Chinese that I wanted to write for my cellphone. I have a Nokia N79 and I tried to venture into this adventure last year, but was put off by the S60 SDK and its flavor of ‘limited C++’. However, the announcement by Nokia of their plans to support Qt as the default application development framework for their mobile devices and plans to ship the runtime with the OS has re-kindled my interest in the framework.

Anyway, this is the issue. While installing the Beta Qt SDK, it fails to create the start menu shortcuts. The screenshot of the error is shown below.QtSDKInstallError1

This may very well be because of my peculiar environment. I’m running Vista on VMWare Fusion running on a Mac. For the first couple of shortcuts, I kept trying Retry to no avail. The Nokia Qt SDK –  beta release and the Symbian subfolder are created correctly though.

When I got the error again, I tried this. I created the shortcuts manually (same name as what is being displayed in the error message) from Windows and then tried the Retry option. And it worked just fine. I have posted a comment to a Nokia developer’s blog about this, let’s hope that it’ll get propagated to the right department.