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C & C++ Language Tricks

, 3 Mar 2000
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More tricks of use to a programmer who shuns run-time libraries.

Introduction

This document is a collection of some of the interesting constructs that are possible in C and C++. While "nice", these features may not serve to increase code clarity but to obfuscate it. Your mileage may vary.


Playing with the C stack

Parameters on the stack appear in memory with the leftmost item at the lowest memory location. Thus you can get a pointer to your functions stack frame, and walk the parameter list, or even cast your stackframe to a struct that can be passed to other functions.

The first sample demonstrates how to retrieve the pointer to any variable parameters passed to the function.

int __cdecl printf(char const* pszFmt,...)
{
  char buf[1024];
  return wvsprintf(buf,pszFmt,&pszFmt+1);
}

This sample demonstrates how the functions passed to a function can be passed to another function.

BOOL InvalidateRect(
  HWND hwnd,
  LONG left,
  LONG top,
  LONG right,
  LONG bottom,
  BOOL fErase)
{
  return InvalidateRect(hwnd,(LPRECT)&left,fErase);
}


Constructing variable length structs

A trick to using structs that have a variable sized member is to use an overloaded operator new to allocate space for the struct.

struct string {
  int cb; // a data member
  char sz[]; // the string data

  void* operator new(int cb, int cbExtra){
    return new char[cb+cbExtra];
  }
};

Now contrast the following code to create such a struct:

  ...
  string* pStr = new(strlen(pszInString)) string;
  ...

The "C" method of working with variable sized looks like this:

  ...
  string* = (string*)new char[sizeof(string) + strlen(pszInString)];
  ...


Working with big endian data

A have always favored little endian number representation over big-endian, which why I was saddened when I found that TCP/IP, and most internet protocols (such as SSL3) use a big endian representation for numbers. Following is the type of construct I use when I need to work with a non-native data type in structs that I pull off of the network.

typedef unsigned char uint8;

struct uint16 {
  uint8 n[2];

  operator unsigned short(){
    return ((unsigned short)n[0] << 8) + n[1];
  }
};

Using a struct like this has two advantages:

  1. If a typedef unsigned short was used you could accidently use the number in situations without converting it.
  2. The operator unsigned short ensures that the struct can be used as a normal unsigned short where required.

Please send any comments or bug reports to me via email. For any updates to this article, check my site here.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Chris Becke

United States United States
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Comments and Discussions

 
Generalproject c++ Pinmemberemansamiee6-Feb-08 5:24 
Generalc++ Pinmember2rtl0v3r9-Oct-07 20:11 
QuestionEver heard of Winsock? PinmemberMike Smith23-May-02 12:05 
GeneralThat's not the C way PinsussBob Smith8-Aug-00 16:50 
GeneralCool, but not portable! PinsussEddie Velasquez5-Mar-00 14:15 
GeneralRe: Cool, but not portable! PinsussJames Curran 9-Mar-00 9:59 
GeneralRe: Cool, but not portable! PinsussSTEP4-May-00 19:23 
GeneralRe: Cool, but not portable! PinsussFab7-Jul-00 6:34 
GeneralRe: Cool, but not portable! PinmemberJason Douglas10-Jan-01 7:20 

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