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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
(Page 1 of 4 )

If a man begins with certainties, he shall end in doubts;

But if he will be content to begin with doubts,

 He shall end in certainties.

[Francis Bacon 1561-1626]

Assertive Programming

If there is one thing I have learned over the past few years, it is not to underestimate the power of assert(). It comes with your compiler and can be found either in cassert or assert.h.

The reason I love assert is because it looks after me and helps me find bugs I was sure weren’t there. I mean, we all write bugfree software, right? Yeah right. Well there have been many Kodak moments when an assert fired on something I was dead sure could never happen; I should be a rich man as the look on my face was priceless.

If you are not familiar with asserts, you should start using them right now. Use  assert to check that your code is working the way you expect it to or be prepared to pay the price. No not for my face, but for long nights behind the debugger, ticking away possible problem sources until you have your own Kodak moment.

When do you use assert, you ask? Simple… whenever you can use it to verify the truth of a situation: ‘this pointer can never be null’, ‘this number is never smaller than zero’, ‘there is always at least one customer stored in this list’, ‘this code won’t be used the next millennium, two digits are fine’.

Use proper error handling when you can check for things that can go wrong. Use asserts on things you are sure can’t go wrong.

Trust me… the sillier the assert… the more valuable it is. I tested this once myself, grinning, thinking ‘this is ridiculous, want to bet it will never fire?’, only to have it triggered a couple of months later! And because the assert was so preposterous and silly I immediately knew which conditions were breaking my code.


  1. To state or express positively, affirm: asserted his innocence
  2. To defend or maintain (one’s rights, for example)

The important thing to remember is that asserts are only compiled into your code when the NDEBUG macro is not defined. So in the final optimized version of your application, they are not there to bloat or to slow things down! The only investment you have to make is typing them out while you are working on those classes and functions. It will pay you back greatly when it helps you shorten the time it takes to track down bugs.

Face it… we all write code on assumptions we make about the situation the code will be running in. Assert you are in that situation, because your code base will grow beyond the picture you have of it in your mind.

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