Subclassing string class

[Ray]

Hi all,

I am thinking of subclassing the standard string class so I can do
something like:

mystring str;
....
str.toLower ();

A quick search on this newsgroup has found messages by others
suggesting that subclassing a standard class is a bad idea, especially
for beginners...

Can someone suggest what is the recommended way to add functionality
to (say) the string class? Actually, I would like to do other things
to strings, and toLower is just one of them. Is it better or is the
only way to just perform a function call:

toLower (string s)

[Of course, there is a C function with the same name, so something
similar to it.]

Another way is to create a class with a private member variable "data"
of type string I should add that I'm somewhat motivated by efficiency
as these tasks with strings will be performed very often.

Which of these two is recommended? Or is there a third that I have
not thought of?

Thank you!

Ray

[Victor Bazarov]

Ray wrote:

I am thinking of subclassing the standard string class so I can do
something like:

mystring str;
...
str.toLower ();

A quick search on this newsgroup has found messages by others
suggesting that subclassing a standard class is a bad idea, especially
for beginners...

I actually don't even like the term "subclassing"...

Can someone suggest what is the recommended way to add functionality
to (say) the string class? Actually, I would like to do other things
to strings, and toLower is just one of them. Is it better or is the
only way to just perform a function call:

toLower (string s)

string toLower(string const& s);

>
[Of course, there is a C function with the same name, so something
similar to it.]

There is no C function with the same name. It's 'tolower' (no capital
letter).

Another way is to create a class with a private member variable "data"
of type string I should add that I'm somewhat motivated by efficiency
as these tasks with strings will be performed very often.

Which of these two is recommended? Or is there a third that I have
not thought of?

Stand-alone functions are better than members of the class deriving from
the standard class. At least with those you could expect application of
conversions (standard and user-defined).

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Victor Bazarov]

Alf P. Steinbach wrote:

* Victor Bazarov:

>>
Stand-alone functions are better than members of the class deriving
from the standard class. At least with those you could expect
application of conversions (standard and user-defined).

Huh?

If you have a function that can uppercase a string, it's better if you
make it a non-member because a class that defines a conversion to a
string, cannot be used with a member.

Consider

class M {};
void foo(M&);
class L { public: operator M&(); };

versus

class M { public: foo(); };
class L { public: operator M&(); };

With the former you can have

L l;
foo(l);

but with the latter you can't do

L l;
l.foo();

>
Consider

struct M {};
void foo( M& );

versus

struct M { void foo(); };

With the former you can write

M o;
foo( o );

With the latter you can write

M().foo();

If foo is an operator like << this can be of practical significance.

Huh?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Christian Hackl]

Ray wrote:

Can someone suggest what is the recommended way to add functionality
to (say) the string class? Actually, I would like to do other things
to strings, and toLower is just one of them.

Why not use the String Algorithms library in Boost?
http://www.boost.org/doc/libs/1_35_0...ring_algo.html

It may already offer most of the functionality you need, including case
conversion algorithms.

std::string source = "FooBar";
boost::algorithm::to_lower(source);
std::cout << source; //prints "foobar"

Note how the library does *not* derive from std::string but uses
non-member functions that take std::string parameters.
--
Christian Hackl

[Ray]

Hi Victor/Alf,

Thank you both for your replies. Actually, I had gotten a little
further than what I posted and stumbled on exactly what you two are
now discussing...

I derived a class mystring (sorry, I won't say "subclass" :-) ) from
string and was trying to do:

mystring foo
....
foo.length ();

I didn't think about casting...but I thought somehow that should have
worked. Obviously, not... Thanks for the two suggestions and the
pros/cons of each!

Ray

On Jul 16, 6:25 am, "Alf P. Steinbach" <[e-mail]:

* Victor Bazarov:

Alf P. Steinbach wrote:

* Victor Bazarov:

class M {};
void foo(M&);
class L { public: operator M&(); };

versus

class M { public: foo(); };
class L { public: operator M&(); };

With the former you can have

L l;
foo(l);

but with the latter you can't do

L l;
l.foo();

Reference operators are bad 'uns generally, but for latter you could just
static_cast<M&>(l).foo(), and even static_cast<M&>(L()).foo().

But I think the above misses the point, by introducing that conversion to reference.

[Ray]

Hi Christian,

Thanks for this -- I didn't know that the Boost library had string
algorithms. I should have thought that something like "tolower" would
be there. I think I'll take this option; I'm obviously too new to
continue going down the current path. :-) Thank you!

Ray

On Jul 16, 7:13 am, Christian Hackl <[e-mail]:

Ray wrote:

Can someone suggest what is the recommended way to add functionality
to (say) the string class? Actually, I would like to do other things
to strings, and toLower is just one of them.

Why not use the String Algorithms library in Boost?http://www.boost.org/doc/libs/1_35_0...ring_algo.html

It may already offer most of the functionality you need, including case
conversion algorithms.

std::string source = "FooBar";
boost::algorithm::to_lower(source);
std::cout << source; //prints "foobar"

Note how the library does *not* derive from std::string but uses
non-member functions that take std::string parameters.

--
Christian Hackl