Mehrdad Mehrdad - 26 days ago 5
C++ Question

How to turn on (literally) ALL of GCC's warnings?

I would like to enable -- literally -- ALL of the warnings that GCC has. (You'd think it would be easy...)


  • You'd think
    -Wall
    might do the trick, but nope! Still need
    -Wextra
    .

  • You'd think
    -Wextra
    might do the trick, but nope! Not all of the warnings listed here (for example,
    -Wshadow
    ) are enabled by this. And I still have no idea if this list is comprehensive.



How do I tell GCC to enable (no if's, and's, or but's!) all the warnings it has?

Answer

You can't.

The manual for GCC 4.4.0 is only comprehensive for that version, but it does list all the possible warnings for 4.4.0. They're not all on the page you link to though, for instance some language-specific options are on the pages for C++ options or Obj-C options. To find them all you're better off looking at the Options Summary

Turning on everything would include -Wdouble-promotion which is only relevant on CPUs with a 32-bit single-precision floating-point unit which implements float in hardware, but emulates double in software. Doing calculations as double would use the software emulation and be slower. That's relevant for some embedded CPUs, but completely irrelevant for modern desktop CPUs with hardware support for 64-bit floating-point.

Another warning that's not usually useful is -Wtraditional, which warns about perfectly well formed code that has a different meaning (or doesn't work) in traditional C, e.g. "string " "concatenation", or ISO C function definitions! Do you really care about compatibility with 30 year old compilers? Do you really want a warning for writing int inc(int i) { return i+1; } ?

I think -Weffc++ is too noisy to be useful, it's based on the outdated first edition of Effective C++ and warns about constructs which are perfectly valid C++ (and for which the guidelines changed in later editions of the book.) I don't want to be warned that I haven't initialized a std::string member in my constructor; it has a default constructor that does exactly what I want, why should I write m_str() to call it? The -Weffc++ warnings that would be helpful are too difficult for the compiler to detect accurately (giving false negatives), and the ones that aren't useful, such as initializing all members explicitly, just produce too much noise, giving false positives.

Luc Danton provided a great example of useless warnings from -Waggregate-return that almost certainly never makes sense for C++ code.

i.e. you don't really want all warnings, you just think you do.

Go through the manual, read about them, decide which you might want to enable, try them. Reading your compiler's manual is a Good ThingTM anyway, taking a short cut and enabling warnings you don't understand is not a very good idea, especially if it's to avoid having to RTFM.

Anyone who just turns on everything is probably either doing so because they're clueless because or a pointy-haired boss said "no warnings."

Some warnings are important, and some aren't. You have to be discriminating or you mess up your program. Consider, for instance, -Wdouble-promotion. If you're working on an embedded system you might want this; if you're working on a desktop system you probably don't. And do you want -Wtraditional? I doubt it.

Edit: See also -Wall-all to enable all warnings which is closed as WONTFIX.

Edit 2: in response to DevSolar's complaint about makefiles needing to use different warnings depending on compiler version, if -Wall -Wextra isn't suitable then it's not difficult to use compiler-specific and version-specific CFLAGS:

compiler_name := $(notdir $(CC))
ifeq ($(compiler_name),gcc)
compiler_version := $(basename $(shell $(CC) -dumpversion))
endif
ifeq ($(compile_name),clang)
compiler_version := $(shell $(CC) --version | awk 'NR==1{print $$3}')
endif
# ...
wflags.gcc.base := -Wall -Wextra
wflags.gcc.4.7 := -Wzero-as-null-pointer-constant
wflags.gcc.4.8 := $(wflags.gcc.4.7)
wflags.clang.base := -Wall -Wextra
wflags.clang.3.2 := -Weverything
CFLAGS += $(wflags.$(compiler_name).base) $(wflags.$(compiler_name).$(compiler_version))