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C++ Preprocessor: Always Assert Your Code Is Right

Are you looking for a way to speed up the debugging process in C++? This article explains how to use asserts to do just that, showing that not all macros are evil.

Author Info:
By: J. Nakamura
Rating: 4 stars4 stars4 stars4 stars4 stars / 49
November 07, 2005
  1. · C++ Preprocessor: Always Assert Your Code Is Right
  2. · Implementing Assert
  3. · Implementing a Simplified Assert
  4. · Refining Assert

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C++ Preprocessor: Always Assert Your Code Is Right - Implementing Assert
(Page 2 of 4 )

Though we will be implementing our customized version of assert here, all you need to do is include assert.h and you are ready to use the assert macro that comes with it:

// assert.h
#ifndef NDEBUG
void __assert(const char *, const char *, int);
#define assert(e) \
      ((e) ? (void)0 : __assert(#e,__FILE__,__LINE__))
#define assert(unused) ((void)0)

The __assert helper function prints an error message to stderr and the program is halted by calling abort(). It is possible that the implementation that comes with your compiler varies slightly, but you get the idea.

Lets use a simple example function:

void writeString(char const *string) {
 assert( 0 != string );

In the unfortunate situation that the pointer to string is NULL, execution will halt and you will be offered the possibility of opening the debugger and jumping to the location in the source where the assertion failed. This can be very handy as you can examine the call stack, memory, registers, and so forth, and are likely to catch the perpetrator red handed!

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