Re: C++ style casting - C / C++ / MFC Discussion Boards

Re: C++ style casting

Stephen Hewitt17-Jul-06 14:15

Stephen Hewitt

17-Jul-06 14:15

Yes there is, visability and clarity.

Steve

Re: C++ style casting

David Crow18-Jul-06 2:46

David Crow

18-Jul-06 2:46

I get the same benefit(s) from C-style casts. It's very clear to me what they are doing. If casting errors are something that a person is prone to making, having the compiler warn of impending danger might be a benefit.

"Money talks. When my money starts to talk, I get a bill to shut it up." - Frank

"Judge not by the eye but by the heart." - Native American Proverb

Re: C++ style casting

Stephen Hewitt18-Jul-06 3:58

Stephen Hewitt

18-Jul-06 3:58

C style casts are anything but clear. While C-style casts look simple they are actually very complicated and subtle. In small programs this often goes unnoticed. But as the program gets larger real problems can and do occur. Try this example program. I know it's on the longish side but it's typical of real applications and may suprise you.

The output with the #include is:
Base2

The output without #include is:
Base1

Not many C++ programmers will be able to spot the problem.

--------------------

// Casts.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include "Functions.h"

// C-style casts suck. The program will compile with or without the line below using C-style casts.
// With it all is OK, without it the program misbehaves at runtime. If 'static_cast' is used the
// program not compile without the #include and you don't get any nasty suprises.
#include "Classes.h"

int main(int argc, char* argv[])
{
Derived *pDerived = MakeDerived();
DoStuff((Base2*)pDerived);
//DoStuff(static_cast<Base2*>(pDerived));
KillDerived(pDerived);

return 0;
}

\/\/\/\/\/\/

// Classes.h

#ifndef __CLASSES_H__
#define __CLASSES_H__

class Base1
{
public:
Base1();
void Print() const;

private:
const char *m_pName;
};

class Base2
{
public:
Base2();
void Print() const;

private:
const char *m_pName;
};

class Derived : public Base1, public Base2
{
};

#endif // !__CLASSES_H__

\/\/\/\/\/\/

// Functions.h

#ifndef __FUNCTIONS_H__
#define __FUNCTIONS_H__

class Base1;
class Base2;
class Derived;

Derived* MakeDerived();
void KillDerived(Derived *pDerived);
void DoStuff(Base2 *pB2);

#endif // !__FUNCTIONS_H__

\/\/\/\/\/\/

// Implementation.cpp

#include "stdafx.h"
#include "Functions.h"
#include "Classes.h"
#include <iostream>

Base1::Base1()
{
m_pName = "Base1";
}

void Base1::Print() const
{
using namespace std;;
cout << m_pName << endl;
}

Base2::Base2()
{
m_pName = "Base2";
}

void Base2::Print() const
{
using namespace std;;
cout << m_pName << endl;
}

Derived* MakeDerived()
{
return new Derived;
}

void KillDerived(Derived *pDerived)
{
delete pDerived;
}

void DoStuff(Base2 *pB2)
{
pB2->Print();
}

Steve

Re: C++ style casting

Maximilien17-Jul-06 5:36

Maximilien

17-Jul-06 5:36

in C++ you can (should?) use the static_cast; it takes a little longer to type, but I think it makes casting a lot more clearer.

long aLong = 1234L;
short aShort = static_cast<short>( aLong );

Maximilien Lincourt
Your Head A Splode - Strong Bad

Re: C++ style casting

toxcct17-Jul-06 5:37

toxcct

17-Jul-06 5:37

Maximilien wrote:
short aShort = static_cast( aLong );

you meant short aShort = static_cast<short>( aLong ); i believe

TOXCCT >>> GEII power

[VisualCalc 3.0 updated ][Flags Beginner's Guide new! ]

Re: C++ style casting

Maximilien17-Jul-06 6:31

Maximilien

17-Jul-06 6:31

darn, I forgot the <> get zapped by default.

Maximilien Lincourt
Your Head A Splode - Strong Bad

Re: C++ style casting

David Crow17-Jul-06 6:51

David Crow

17-Jul-06 6:51

Maximilien wrote:
I forgot the <> get zapped by default.

The angle brackets are there, just hidden.

"Money talks. When my money starts to talk, I get a bill to shut it up." - Frank

"Judge not by the eye but by the heart." - Native American Proverb

Re: C++ style casting

Jörgen Sigvardsson17-Jul-06 9:03

Jörgen Sigvardsson

17-Jul-06 9:03

Robert Edward Caldecott wrote:
I can't believe I haven't come across the x = type(y); syntax before. I asked some other C/C++ old-timers here and they hadn't seen it either. Mad. Always something new to learn!

I use it all the time. I use that syntax because it looks like a constructor, rather than a cast. Easier to type than static_cast, and just as easy to search for as static_cast. It's a bit dangerous, as it works like a reinterpret_cast, but hey! I've grown up, and I don't need mommy to hold my hand. Wink | ;)

(Lots of drugs are required before I start casting HANDLEs to BYTEs, etc. Since I don't use drugs, I don't worry much about bad casts. Big Grin | :-D

)

--
80 percent entertainment by volume

Re: C++ style casting

Zac Howland17-Jul-06 9:49

Zac Howland

17-Jul-06 9:49

Jörgen Sigvardsson wrote:
I've grown up, and I don't need mommy to hold my hand. (Lots of drugs are required before I start casting HANDLEs to BYTEs, etc. Since I don't use drugs, I don't worry much about bad casts. )

You may have grown up, but I can almost guarantee that 1) you won't be working at the same place forever 2) some knucklehead will come in and think he can safely cast anything to anything and use your code as an example and 3) you will find out that not using the C++-style casts will create an inconsistent coding style in your developments.

Just an FYI.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: C++ style casting

Stephen Hewitt17-Jul-06 14:16

Stephen Hewitt

17-Jul-06 14:16

Zac Howland wrote:
some knucklehead will come in and think he can safely cast anything to anything and use your code as an example

This is the wisest thing I've heard on CP for some time; I agree completely.

Steve

Re: C++ style casting

Zac Howland17-Jul-06 16:06

Zac Howland

17-Jul-06 16:06

Stephen Hewitt wrote:
This is the wisest thing I've heard on CP for some time; I agree completely.

Thank you. That statement comes from years of watching that exact situation occur time and time again.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: C++ style casting [modified]

Jörgen Sigvardsson17-Jul-06 15:13

Jörgen Sigvardsson

17-Jul-06 15:13

Zac Howland wrote:
2) some knucklehead will come in and think he can safely cast anything to anything and use your code as an example

I think casts would be the least of this knuckleheads's troubles if he doesn't understand how casts work in C++. Unsure | :~

Zac Howland wrote:
you will find out that not using the C++-style casts will create an inconsistent coding style in your developments.

Yes, and no. If I start mixing the casts, then it'd be inconsistent. I use C++ casts to indicate that something worth paying attention to is happening. For instance - downcasts (static_cast), round-ball-into-square-hole-casts (reinterpret_cast), and dynamic casts (dynamic_cast). In essence, I only to constructor like casts when dealing with integral types. In that code, it's obvious what's going on - no need to pollute the visibility with lots of text.

My 2 cents.

--
As Foretold by Nostradamus

Last modified: den 17 juli 2006 20:09:40 --

Re: C++ style casting

Zac Howland17-Jul-06 16:12

Zac Howland

17-Jul-06 16:12

Jörgen Sigvardsson wrote:
In essence, I only to constructor like casts when dealing with integral types.

I have used these types of casts for integral types myself; however, I tend to look at them more as constructors instead of casts. For example:

long l = 1234L;
short s = (short)l;

Is really just creating a short by "passing" a long into an implicitly overriden constructor. It is easier to see if you view it as objects instead of primitive types. In that sense, I agree with you, but I still use caution when doing it for one simple reason: many beginning programmers tend to be lazy and see the above code and think nothing of extending it to do the following:

class A {};

class B : public A {};

B b;
B* pB = &b;
A* pA = (A*)pB;

Which while safe in this example, can get much more complex with a different inheritance model.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: C++ style casting

Zac Howland17-Jul-06 9:46

Zac Howland

17-Jul-06 9:46

The cast long l = 1234L; short s = (short)l; is equivalent to long l = 1234L; short s = short(l);. There is absolutely no difference between them and no advantage for using 1 over another.

That said, in C++, you should always try to cast using the proper C++-style casting operator (dynamic_cast, static_cast, const_cast, reinterpret_cast, etc.) See "Effective C++" by Scott Meyers for both why to use them and which ones to use when (and which ones to avoid using).

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Background color

goodoljosh198017-Jul-06 5:06

goodoljosh1980

17-Jul-06 5:06

Anyone have an idea how to find the RGB value of the default MFC dialog.

I am building a program and need to know what color the screen comes up as so that after I am done using the CDC->TextOut command, I can call it again with a font of the same color as the background.

Please advise.

Thank you.

Re: Background color

SimonSays17-Jul-06 6:05

SimonSays

17-Jul-06 6:05

Well, I believe that the background color is GetSysColor(COLOR_3DFACE), as long as XP Visual Styles don't come into play -- they it could be anything, including a bitmap.

You would be better served to just call dc->SetBkMode(TRANSPARENT) and just draw transparent text.

Re: Background color

goodoljosh198017-Jul-06 9:14

goodoljosh1980

17-Jul-06 9:14

I was using the SetBkMode(Transparent) call, but I wanted to be able to find the color of the background so that I could clean up the screen by simply calling the same TextOut function, but with the background color to effectively "mask" away the text I already painted. The GetSysColor function seems to be working. Thanks for the information.

Re: Background color

Hamid_RT17-Jul-06 18:14

Hamid_RT

17-Jul-06 18:14

See GetBkColor(GetDC()->m_hDC)

whitesky

Exporting function objects from DLL

napoleaninlondon17-Jul-06 4:52

napoleaninlondon

17-Jul-06 4:52

In a file i have declared a function object.I am unable to access the function with the overloaded operator in the client side.
For example:
i have in my dll file

<br />
class try_func  <br />
{<br />
public:<br />
	try_func();<br />
	virtual ~try_func();<br />
<br />
	bool operator()(int value)const;<br />
<br />
};<br />
<br />
<br />
try_func::try_func()<br />
{<br />
<br />
}<br />
<br />
try_func::~try_func()<br />
{<br />
<br />
}<br />
<br />
bool try_func::operator()(int value)const<br />
{<br />
	if(value< 20)<br />
		return true;<br />
	else<br />
		return false;<br />
}<br />
<br />
<br />
try_func export_func;<br />

And then in the client side i have tried the following variations

<br />
#include<iostream.h><br />
#include<windows.h><br />
<br />
typedef VOID (CALLBACK* LPFNDLLFUNC1)(VOID);<br />
<br />
<br />
HINSTANCE hDLL;              <br />
LPFNDLLFUNC1 MyFunc1;    <br />
<br />
int main()<br />
{<br />
	<br />
	hDLL = LoadLibrary("C:\\my_func_obj_dll\\Debug\\my_func_obj_dll.dll");<br />
<br />
	if (hDLL != NULL)<br />
	{<br />
		MyFunc1 = (LPFNDLLFUNC1)GetProcAddress(hDLL,"export_func");<br />
		if (!MyFunc1)<br />
		{<br />
			//Show error message<br />
			FreeLibrary(hDLL);       <br />
			return false;<br />
		}<br />
		else<br />
		{<br />
			MyFunc1();<br />
		}<br />
	}<br />
<br />
	return true;<br />
}<br />
<br />

Am i missing something here?

Re: Exporting function objects from DLL

Cedric Moonen17-Jul-06 4:57

Cedric Moonen

17-Jul-06 4:57

napoleaninlondon wrote:
try_func export_func;

napoleaninlondon wrote:
MyFunc1 = (LPFNDLLFUNC1)GetProcAddress(hDLL,"export_func");

export_func is an instance of the class try_func. It is not a function at all. So, you will never be able to 'retrieve' it with GetProcAddress. GetProcAddress is only used to get the address of a function !

Cédric Moonen
Software developer

Charting control

Re: Exporting function objects from DLL

napoleaninlondon17-Jul-06 5:04

napoleaninlondon

17-Jul-06 5:04

Does that mean i need to declare the function object as follows
try_func export_func;

and then also explicitly declare the operator overloaded function associated with the function object
something like

bool operator()(int value);

and use the common extern 'C' function type to wrap the function and then call that 'C' function ?

hitherto shall thou come but not further thee

Re: Exporting function objects from DLL

Cedric Moonen17-Jul-06 5:10

Cedric Moonen

17-Jul-06 5:10

No, it simply means that try_func is a class and will always be a class even if you overload the operator(). You can only use GetProcAddress to retrieve the address of a global function.

But what you can do is write a function that will create and return an instance of the class. That is perfectly legal.

Cédric Moonen
Software developer

Charting control

Re: Exporting function objects from DLL

napoleaninlondon17-Jul-06 5:18

napoleaninlondon

17-Jul-06 5:18