C / C++ / MFC

31-Dec-07 2:50

Hi,

I am working on an element of a program which is performance cruical and I wanted to clarify something with regards to basic C++ inheritance.

Consider the following:

class BaseClass<br />
{<br />
public:<br />
   BaseClass();<br />
<br />
   void foo();<br />
<br />
private:<br />
   int baz;<br />
};<br />
<br />
class AnotherClass : public BaseClass<br />
{<br />
public:<br />
   AnotherClass();<br />
<br />
   void bar();<br />
};

It is important to note that no virtual functions will be used within this element; including no virtual destructor.

My question is this, would there be any performance differences between the above source and the source listed below?

class CombinedClass<br />
{<br />
public:<br />
   CombinedClass();<br />
<br />
   void foo();<br />
   void bar();<br />
<br />
private:<br />
   int baz;<br />
};

The first example would be easier for me to maintain because there will be many variations of 'AnotherClass'. Does anybody know of the nitty-gritty low-level differences between the two and any such differences in performance?

Also do overloaded methods cause performance differences?

Any advice would be greatly appreciated!
Lea Hayes

Florin Crişan31-Dec-07 3:14

31-Dec-07 3:14

Theoretically there should be no performance difference. The calls to foo() and bar() should be non-virtual in either case. But you can try having a look at the generated assembler code just to be sure.

BTW, if it's not a secret, what are you doing so special that you cannot use a virtual function call?

Florin Crisan

Florin Crişan31-Dec-07 3:17

31-Dec-07 3:17

On second thought (assuming your constructors are not trivial and cannot be optimized away), if you use two classes you would get two function calls: first to the constructor of the derived class, then to the constructor of the base class. But the other function calls should be the same.

Florin Crisan

Lea Hayes31-Dec-07 4:23

31-Dec-07 4:23

Hi,

It is a math class which will be used extremely frequently. There are a variety of different math classes, and some of which are derived upon the others...There is no multi-inheritance, and I have ruled out virtual methods here because they would offer no gain, and automatically require additional memory to reference a vtable.

Thanks for your advice!

Re: Simple C++ Inheritance and Performance [modified]

CPallini31-Dec-07 3:25

CPallini

31-Dec-07 3:25

lhayes00 wrote:
My question is this, would there be any performance differences between the above source and the source listed below?

IMHO none noticeable (There are two constructors instead of one, though, if you have a huge number of objects...).

lhayes00 wrote:
The first example would be easier for me to maintain because there will be many variations of 'AnotherClass'.

The above is a good reason to go on with inheritance.

lhayes00 wrote:
Also do overloaded methods cause performance differences?

IMHO no, they are translated by the compiler to different named methods (in fact functions).

BTW C++ language is fairly efficient (is one of its design goals), I think that you should first address your efforts to good, clean design and then on optimization of your algorithms, and finally worry about efficiency of different language constructs.

Please note that I gave you general guidelines, I'm not an expert of that

lhayes00 wrote:
nitty-gritty low-level

things.

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

Florin Crişan31-Dec-07 3:28

What other people say about this:

31-Dec-07 3:28

I should learn to read the whole message Blush | :O

Also do overloaded methods cause performance differences?

Well, a normal non-virtual function call has the normal overhead of a, well, normal function call. That is, the arguments are pushed to the stack, the return address is pushed to the stack, etc.

(As far as I know) If you have a polymorphic object, in most implementations it keeps a (hidden) pointer to a table of function pointers (known as 'vtable'). So, before making the call, the pointer to the vtable and then the one to the function would have to be de-referenced. (I really don't know how expensive that is; it may be significant on some systems, but probably nothing significant for normal use).

However, if the compiler is able to tell the exact type of the object, it won't need to call the function virtually.

For example

<br />
Derived x;<br />
x.bar(); // x is a derived -- call Derived::bar() directly<br />
<br />
Base *x;<br />
x->foo(); // is x a Base or a Derived? use the function pointer to be sure<br />

If you really think it would make a difference, try it both with virtual and with non-virtual and see what happens. (And let us known what the result was Wink | ;-)

)

Florin Crisan

Florin Crişan31-Dec-07 3:35

31-Dec-07 3:35

Googling around for virtual function calls, I found this research paper[^], but I'm too lazy to read it. Hope it helps.

Florin Crisan

Happy New Year!

Florin Crişan31-Dec-07 3:40

31-Dec-07 3:40

What a mess. It took me so many messages to say so little. D'Oh! | :doh:

I'm going to get some sleep before the party. Happy New Year!

Florin Crisan

CPallini31-Dec-07 3:42

CPallini

31-Dec-07 3:42

Florin Crisan wrote:
Also do overloaded methods cause performance differences?

Florin Crisan wrote:
(As far as I know) If you have a polymorphic object, in most implementations it keeps a (hidden) pointer to a table of function pointers (known as 'vtable'). So, before making the call, the pointer to the vtable and then the one to the function would have to be de-referenced. (I really don't know how expensive that is; it may be significant on some systems, but probably nothing significant for normal use).

However, if the compiler is able to tell the exact type of the object, it won't need to call the function virtually.

For example

Derived x;
x.bar(); // x is a derived -- call Derived::bar() directly

Base *x;
x->foo(); // is x a Base or a Derived? use the function pointer to be sure

Method overload and method override are two quite distinct concepts, he asked for the former, you're talking about the latter.

Florin Crisan wrote:
Derived x;
x.bar(); // x is a derived -- call Derived::bar() directly

Base *x;
x->foo(); // is x a Base or a Derived? use the function pointer to be sure

AFAIK method invocation is always done via vtables.

Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

Florin Crişan31-Dec-07 3:46

31-Dec-07 3:46

CPallini wrote:
Method overload and method override are two quite distinct concepts, he asked for the former, you're talking about the latter.

My bad.

CPallini wrote:
AFAIK method invocation is always done via vtables.

I remember reading differently in Stroustrup's book, but it might be just the lack of sleep. I guess it's really up to the compiler's implementer.

Florin Crisan

CPallini31-Dec-07 4:09

CPallini

31-Dec-07 4:09

Florin Crisan wrote:
I remember reading differently in Stroustrup's book

Well, I surely don't debate against C++ founder, maybe you're right.

Happy New Year
Big Grin | :-D

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

Lea Hayes31-Dec-07 4:27

how can i convert a BYTE value into CString

31-Dec-07 4:27

Thank you very much for all of your input!

I am just about to get started on the link you suggested, it looks for interesting reading.

Happy new year!! Big Grin | :-D

rajneshmalik31-Dec-07 2:18

rajneshmalik

31-Dec-07 2:18

hi to all
itook a varible BYTE *bByte and want to convert bByte[1] into a CString type varible,bByte varible has hex value,
i done CString csValue=(CHAR)bByte;
where am i wrong

Florin Crişan31-Dec-07 2:50

31-Dec-07 2:50

Assuming that you want to create a string representation of the number (that is 32 => "32" rather than 32 => " " – space being the character with the ASCII code 32), you can use something like this:

CString x;<br />
BYTE b = 123;<br />
x.Format("%u", (unsigned int) b); // "123"

or
x.Format("%x", (unsigned int) b); // "7b"

You need to convert it to unsigned int because there doesn't seem to be a way to specify a 1-byte integer in the format string.

Or, if you feel adventurous, you can try stringstream[^] or The Boost Format Library[^].

Florin Crisan

Florin Crişan31-Dec-07 3:04

Re: how can i convert a BYTE value into CString [modified]

31-Dec-07 3:04

In your code, bByte is actually a pointer to a BYTE rather than a BYTE. You should have named it better: pbySomethingUseful. p stands for pointer, by stands for byte (b stands for bool, and that one is actually as big as an int). The rest should be a useful name Smile | :)

So your code should look like:
csValue.Format("%x", (unsigned int) (*pbySomethingUseful)).

Florin Crisan

Nishad S31-Dec-07 2:52

Nishad S

31-Dec-07 2:52

You can use CString::Format

csValue.Format( "%x", bByte[1] );

- NS -
[ODBaseBtn]

modified on Monday, December 31, 2007 9:36:06 AM

Lea Hayes31-Dec-07 3:03

31-Dec-07 3:03

Hi,

You are attempting to cast a BYTE* into a CHAR. A BYTE* is a pointer to BYTE data....so the casted CHAR will be a CHAR representation of the pointer address of bByte.

CString csValue = (CHAR)bByte[0]; // for individual character

or

CString csValue = (CHAR*)bByte;     // for entire byte array<br />
// be sure the byte array has a null terminator character '\0' at the end!

If you want to show the hex value of the byte, you can do something like the following:

// for numerical value in hex i.e. "0A" if bByte[0] == 0x0A.<br />
CString csValue;<br />
csValue.Format(_T("%02X"), bByte[0]);

If you wanted to show the entire contents of the byte array as hex then you will need to wrap the above within a loop and concatenate the result each time to the result string.

// For example, the byte array { 0x0A, 0x0B, 0x12 }<br />
// will would create the string "0A0B12".<br />
<br />
CString csValue, csTemp;<br />
BYTE *pActiveByte = bByte;<br />
while(*pActiveByte)<br />
{<br />
   csTemp.Format(_T("%02X"), *pActiveByte++);<br />
   csValue.Append(csTemp);<br />
}

I hope that this helps...
Lea Hayes

Florin Crişan31-Dec-07 3:08