GCC hidden/little-known features

Question

This is my attempt to start a collection of GCC special features which usually do not encounter. this comes after @jlebedev in the another question mentioned "Effective C++" option for g++,

-Weffc++ This option warns about C++ code which breaks some of the programming guidelines given in the books "Effective C++" and "More Effective C++" by Scott Meyers. For example, a warning will be given if a class which uses dynamically allocated memory does not define a copy constructor and an assignment operator. Note that the standard library header files do not follow these guidelines, so you may wish to use this option as an occasional test for possible problems in your own code rather than compiling with it all the time.

What other cool features are there?

Answer 1

From time to time I go through the current GCC/G++ command line parameter documentation and update my compiler script to be even more paranoid about any kind of coding error. Here it is if you are interested.

Unfortunately I didn't document them so I forgot most, but -pedantic, -Wall, -Wextra, -Weffc++, -Wshadow, -Wnon-virtual-dtor, -Wold-style-cast, -Woverloaded-virtual, and a few others are always useful, warning me of potentially dangerous situations. I like this aspect of customizability, it forces me to write clean, correct code. It served me well.

However they are not without headaches, especially -Weffc++. Just a few examples:

• It requires me to provide a custom copy constructor and assignment operator if there are pointer members in my class, which are useless since I use garbage collection. So I need to declare empty private versions of them.
• My NonInstantiable class (which prevents instantiation of any subclass) had to implement a dummy private friend class so G++ didn't whine about "only private constructors and no friends"
• My Final<T> class (which prevents subclassing of T if T derived from it virtually) had to wrap T in a private wrapper class to declare it as friend, since the standard flat out forbids befriending a template parameter.
• G++ recognizes functions that never return a return value, and throw an exception instead, and whines about them not being declared with the noreturn attribute. Hiding behind always true instructions didn't work, G++ was too clever and recognized them. Took me a while to come up with declaring a variable volatile and comparing it against its value to be able to throw that exception unmolested.
• Floating point comparison warnings. Oh god. I have to work around them by writing x <= y and x >= y instead of x == y where it is acceptable.
• Shadowing virtuals. Okay, this is clearly useful to prevent stupid shadowing/overloading problems in subclasses but still annoying.
• No previous declaration for functions. Kinda lost its importance as soon as I started copypasting the function declaration right above it.

It might sound a bit masochist, but as a whole, these are very cool features that increased my understanding of C++ and general programming.

What other cool features G++ has? Well, it's free, open, it's one of the most widely used and modern compilers, consistently outperforms its competitors, can eat almost anything people throw at it, available on virtually every platform, customizable to hell, continuously improved, has a wide community - what's not to like?

Answer 2

A function that returns a value (for example an int) will return a random value if a code path is followed that ends the function without a 'return value' statement. Not paying attention to this can result in exceptions and out of range memory writes or reads.

For example if a function is used to obtain the index into an array, and the faulty code path is used (the one that doesn't end with a return 'value' statement) then a random value will be returned which might be too big as an index into the array, resulting in all sorts of headaches as you wrongly mess up the stack or heap.