Why should I always enable compiler warnings?

Question

I often hear that when compiling C and C++ programs I should \"always enable compiler warnings\". Why is this necessary? How do I do that?

Sometimes I also hear tha

Answer 1

The other answers are excellent and I don't want to repeat what they have said.

One other aspect to "why enable warnings" that hasn't properly been touched on is that they help enormously with code maintenance. When you write a program of significant size, it becomes impossible to keep the whole thing in your head at once. You typically have a function or three that you're actively writing and thinking about, and perhaps a file or three on your screen that you can refer to, but the bulk of the program exists in the background somewhere and you have to trust that it keeps working.

Having warnings on, and having them as energetic and in your face as possible, helps to alert you if something you change makes trouble for something that you can't see.

Take for example, the clang warning -Wswitch-enum. That triggers a warning if you use a switch on an enum and miss out one of the possible enum values. It's something you might think would be an unlikely mistake to make: you probably at least looked at the list of enum values when you wrote the switch statement. You might even have an IDE that generated the switch options for you, leaving no room for human error.

This warning really comes into its own when, six months later you add another possible entry to the enum. Again, if you're thinking about the code in question you'll probably be fine. But if this enum is used for multiple different purposes and it's for one of those that you need the extra option, it's very easy to forget to update a switch in a file you haven't touched for 6 months.

You can think of warnings in the same way as you'd think of automated test cases: they help you make sure that the code is sensible and doing what you need when you first write it, but they help even more to make sure that it keeps doing what you need while you prod at it. The difference is that test cases work very narrowly to the requirements of your code and you have to write them, while warnings work broadly to sensible standards for almost all code, and they're very generously supplied by the boffins who make the compilers.