C++ Function Pointers Simplified!

18 10 2009

Background information
Pointer is a variable which holds the address of another variable. Where, function pointer is again a variable which holds the address of a function.

If you think pointers are evil, then function pointers must be Satan for you. 🙂 Well, is there any easy way to create function pointers from function prototype? Indeed, there is. Its the “BAT” technique. Never heard about it before? No problem. Its invented by me just now. After watching BatMan series from cartoon network.

FunctionPointers


The “BAT” technique is this –

  1. Put Bracket or parenthesis around the function name.
  2. Add Asterisk or star in-front of function name.
  3. Now Typedef it to create a new datatype. Means change the function name to new datatype name and add typedef infront of it.
  4. Now you can use the new function pointer datatype like ordinary variables.

For instance, Assume we want to make a function pointer for function – DWORD MyFunction( int a, int b).

1) Bracket
DWORD (MyFunction)( int a, int b);

2) Asterisk
DWORD (*MyFunction)( int a, int b);

3) Typedef
typedef DWORD (*MyFunctionPtr)( int a, int b);

Ah! you have created a function pointer – MyFunctionPtr for function type – ‘DWORD MyFunction( int a, int b)’
Now you can use it like any other variable in your code. For instance, just see the following code snippet with real world usage of function pointers.

// Callback function for progress notification.
bool NotifyProgress( int Percentage )
{
 // Display progress and return true to continue.
 return true;
}

// typedef function pointer.
typedef bool (*NotifyProgressPtr)( int Percentage );

// DVD Burning function with pointer to NotifyProgress
// to update progress.
void BurnDVD( NotifyProgressPtr FnPtr )
{
 for( int Progress = 0; Progress <= 100; ++Progress )
 {
 // Call the function.
 (*FnPtr)(Progress);
 }
}

// Main function.
int _tmain(int argc, _TCHAR* argv[])
{
 // Ummm... Burn one DVD.
 BurnDVD( NotifyProgress );
 return 0;
}


Function pointers are not that much evil. Isn’t it? 😉


Targeted Audience – Beginners.





How to Set Dialog as TopMost Window?

11 10 2009


I always wondered about popularity of Winamp. It has rich custom drawn UI, which made it stand out of the crowd. Did you noticed its “Always on top” feature and wondered about how its implemented? Its time to reveal the secret – How winamp implemented that feature – Staying at the top?

SetTopMostWindow


You can use – SetWindowPos() with HWND_TOPMOST flag. Have a look at the code snippet.

void CRabbitDlg::OnSetTopmost()
{
    // Set window position to topmost window.
    ::SetWindowPos( GetSafeHwnd(),
                    HWND_TOPMOST,
                    0, 0, 0, 0,
                    SWP_NOMOVE | SWP_NOREDRAW | SWP_NOSIZE );
}


Single line of code. But wowing feature. isn’t it?


Targeted Audience – Beginners.





How to Watch this Pointer – The Wizards Way!

30 07 2009


How to watch the this pointer? Just add ‘this’ to watch window. Everyone does like that. Isn’t it? But how Visual C++ wizards watch ‘this’ pointer? 😉

thiswizardway


The secret is, visual C++ compiler passes this pointer via ECX register. So add (ClassName*)(@ECX) to watch window will give you this pointer. Have a look at the screenshot.

thiswizardway2


Interesting, the internals of Visual C++. Isn’t it?


Targeted Audiance – Intermediate.





How to get the CPU Name String?

21 06 2009


While taking the System properties, you have noticed the processor name string. For instance, in my laptop it is – “Intel(R) Core(TM)2 Duo CPU     T5250  @ 1.50GHz“. Ever though about how to get this processor name string?

cpuid
Image Courtesy – Wallpaper Mania.


You can use the function – __cpuid(), which generates the instruction – cpuid. Have a look at the code snippet. Code taken and modified from MSDN.

#include <iostream>
#include <intrin.h>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    // Get extended ids.
    int CPUInfo[4] = {-1};
    __cpuid(CPUInfo, 0x80000000);
    unsigned int nExIds = CPUInfo[0];

    // Get the information associated with each extended ID.
    char CPUBrandString[0x40] = { 0 };
    for( unsigned int i=0x80000000; i<=nExIds; ++i)
    {
        __cpuid(CPUInfo, i);

        // Interpret CPU brand string and cache information.
        if  (i == 0x80000002)
        {
            memcpy( CPUBrandString,
            CPUInfo,
            sizeof(CPUInfo));
        }
        else if( i == 0x80000003 )
        {
            memcpy( CPUBrandString + 16,
            CPUInfo,
            sizeof(CPUInfo));
        }
        else if( i == 0x80000004 )
        {
            memcpy(CPUBrandString + 32, CPUInfo, sizeof(CPUInfo));
        }
}

    cout << "Cpu String: " << CPUBrandString;
}


You can get a lot of information about cpu by using __cpuid. Have a look at the MSDN Documentation.


Targeted Audiance – Intermeidate.





How to Change the Display Orientation?

10 05 2009


Are you using Windows XP? Press Ctrl+Atl+DownArrow, and then Ctrl+Atl+UpArrow. The screen changes its orientation upside down. isn’t it? But how to turn the screen, upside down programmatically?

ChangeDisplayOrientation
Image Courtesy – marieforleo.com


Get the current DEVMODE by calling –EnumDisplaySettings(). Then change orientation by setting DEVMODE.dmDisplayOrientation and calling ChangeDisplaySettings(). Have a look at the code snippet. Code taken from MSDN.

// Get current Device Mode.
DEVMODE DeviceMode = { 0 };
EnumDisplaySettings( NULL,
 ENUM_CURRENT_SETTINGS,
 &DeviceMode );

// Change display mode upside down.
DeviceMode.dmDisplayOrientation = DMDO_180;
ChangeDisplaySettings( &DeviceMode, 0 );

// Sleep for 10 seconds.
Sleep( 10000 );

// Change display mode back.
DeviceMode.dmDisplayOrientation = DMDO_DEFAULT;
ChangeDisplaySettings( &DeviceMode, 0 );


Be careful to restore the display orientation back. Or else 😉


Targeted Audiance – Intermeidate.





How to Delete Duplicate entries from STL containers?

16 04 2009


If you want to remove duplicate items, you can go for stl::set. But what to do if you want to delete duplicate data from other containers?

removeduplicate
Picture Courtesy – Squidoo


You can use std::unique() algorithm to remove adjacent duplicate items. So at first, sort your data, then call std::unique(). Now all the duplicate data will be rearranged to end of container. Now delete the unwanted range of duplicate data. Have a look at code snippet below.

#include <vector>
#include <string>
#include <algorithm>

using namespace std;

int main(int argc, char* argv[])
{
    // Election list.
    vector<string> ElectionList;
    ElectionList.push_back( "Sam" );
    ElectionList.push_back( "John" );
    ElectionList.push_back( "Ron" );
    ElectionList.push_back( "Sam" );
    ElectionList.push_back( "John" );

    // Sort the list to make same items be together.
    sort( ElectionList.begin(), ElectionList.end());

    // Rearrange unique items to front.
    vector<string>::iterator Itr = unique(
        ElectionList.begin(),
        ElectionList.end());

    // Delete the duplicate range.
    ElectionList.erase( Itr, ElectionList.end());
}


Take care that std::unique() just removes the adjacent duplicate entries. It wont remove the entire duplicate entries present in the container. That’s why we need to sort the container at first, which will arrange all duplicate entries to adjacent  locations. 😉


Targeted Audience – Beginners.





How to Change the Icon of MFC application?

5 04 2009


When you create an MFC application, did you notice the icon of executable? Yes! its that same old icon. But I’ve seen other application with different icon. Well, how to set the icon of executable to give a new face for it? 😉

setappicon
Image Courtesy – Flickr


The secret is, windows will choose the first icon present in executable as exe icon. By default for an MFC application, IDR_MAINFRAME will be the icon resource name and it have the lowest resource value – 128. Follow the steps to add an icon and make set it the first one in executable.

1. Import a new icon by using resource editor.

setappicon1

2. Let the icon be IDR_ICON1.
3. Now open resource.h and you can see, IDR_MAINFRAME which is the mfc icon, have lowest resource id.

setappicon2
4. Now edit the resource.h to make IDI_ICON1 as lowest resource id.

setappicon3
5. Now clean and build your application and check the application icon. Wow! its changed!!!


The point is, the icon should be the first icon in executable. You can set icon value even to zero. It will work!


Targeted Audiance – Intermediate.