Undefined behavior

By Md Shoriful Islam

 Undefined behavior

Using the value from an uninitialized variable is our first example of undefined behavior. Undefined behavior (often abbreviated UB) is the result of executing code whose behavior is not well defined by the C++ language. In this case, the C++ language doesn’t have any rules determining what happens if you use the value of a variable that has not been given a known value. Consequently, if you actually do this, undefined behavior will result.

Code implementing undefined behavior may exhibit any of the following symptoms:

  • Your program produces different results every time it is run.
  • Your program consistently produces the same incorrect result.
  • Your program behaves inconsistently (sometimes produces the correct result, sometimes not).
  • Your program seems like its working but produces incorrect results later in the program.
  • Your program crashes, either immediately or later.
  • Your program works on some compilers but not others.
  • Your program works until you change some other seemingly unrelated code.

Or, your code may actually produce the correct behavior anyway. The nature of undefined behavior is that you never quite know what you’re going to get, whether you’ll get it every time, and whether that behavior will change when you make other changes.

C++ contains many cases that can result in undefined behavior if you’re not careful. We’ll point these out in future lessons whenever we encounter them. Take note of where these cases are and make sure you avoid them.

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