c++ - What are the dangers of uninitialised variables?

Question

In a program I am writing I currently have several uninitialised variables in my .h files, all of which are initialised at run-time. However, in Visual Studio it warns me every time I do this to "Always initialise a member variable" despite how seemingly pointless it feels to do so. I am well aware that attempting to use a variable when uninitialised will lead to undefined behaviour, but as far as I know, this can be avoided by not doing so. Am I overlooking something?

Thanks.

Answer 1

These variables could contain any value if you don't initialize them and reading them in an uninitialized stated is undefined behavior. (except if they are zero initalized)

And if you forgot to initialize one of them, and reading from it by accident results in the value you expect it should have on your current system configuration (due to undefined behavior), then your program might behave unpredictable/unexpected after a system update, on a different system or when you do changes in your code.

And these kinds of errors are hard to debug. So even if you set them at runtime it is suggested to initialize them to known values so that you have a controlled environment with predictable behavior.

There are a few exceptions, e.g. if you set the variable right after you declared it and you can't set it directly, like if you set its value using a streaming operator.