My Favorite g++ Flags for a New Project

3/2/2022 2-minute read

C++ is hard; make it a little bit easier. The summaries are taken from https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html.

The effectiveness of some of these options depends on various optmization passes. You may also be able to selectively disable warnings for certain chunks of code (https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html). This can be especially useful for header files with contents you can’t control very well.

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wuninitialized"
  foo(b);                       /* no diagnostic for this one */
#pragma GCC diagnostic pop
  • -Wall: turns on many warnings, but not “all.”
  • -Wextra: turns on even more warnings, but still not all.
  • -Wpedantic: Issue all the warnings demanded by strict ISO C++.
  • -Wcast-align: warn whenever a pointer is cast such that the required alignment is increased (char* -> int*).
  • -Wcast-qual: warn when qualifier (const) is cast away, or introduces a qualifier in an unsafe way.
  • -Wdisabled-optimization: warn if a requested optimization pass is disabled (e.g. code is too large, has some other feature that makes g++ give up).
  • -Wduplicated-branches: warn if if-else branches have identical bodies.
  • -Wduplicated-cond: warn about duplicated conditions in an if-else-if chain.
  • -Wformat=2: same as -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. make sure printf-style function arguments match their format strings.
  • -Wlogical-op: warn about suspicious use of logical operators, i.e. contexts where bitwise is more likely.
  • -Wmissing-include-dirs: warn if a user-supplied include dir does not exist.
  • -Wnull-dereference: warn if paths that dereference a null pointer are detected.
  • -Woverloaded-virtual: warn when a function declaration hides virtual functions from a base class
  • -Wpointer-arith: warn about sizeof for function types or void.
  • -Wshadow: warn about variable shadowing and global function shadowing.
  • -Wswitch-enum: warn when a switch on an enum type is missing one of the enums.
  • -Wvla: warn about using variable-length arrays.

Also, in debug mode:

Honorable Mentions

  • -fno-omit-frame-pointer: g++ only turns on -fomit-frame-pointer on machines where it does not interfere with debugging, so no need for this one.
  • -Wuseless-cast: Warns when casting to the same type. Unfortunately, doesn’t work well with MPI, since MPI_COMM_WORLD may be an int and casting it to MPI_Comm is a “useless cast”
  • -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]: I haven’t spent much time with this one, it may be useful.