Discussion

In the low-level efficiency tradition of C and C++ alike, the compiler is often not required to initialize variables unless you do it explicitly (e.g., local variables, forgotten members omitted from constructor initializer lists). Do it explicitly.

There are few reasons to ever leave a variable uninitialized. None is serious enough to justify the hazard of undefined behavior.

If you've used a procedural language (e.g., Pascal, C, Fortran, or Cobol) you might be used to defining variables in separation from the code that uses them, and then assigning them values later when they're about to be used. This approach is obsolete and not recommended (see Item 18).

A common misconception about uninitialized variables is that they will crash the program, so that those few uninitialized variables lying around here and there will be quickly revealed by simple testing. On the contrary, programs with uninitialized variables can run flawlessly for years if the bits in the memory happen to match the program's needs. Later, a call from a different context, a recompilation, or some change in another part of the program will cause failures ranging from inexplicable behavior to intermittent crashes.