c++ - Which compilation flags should I use to avoid run time errors

Question

Just learned here that -Wsequence-point comiplation flag will pop a warning when the code can invoke UB. I tried it on a statement like

int x = 1;
int y = x+ ++x;

and it worked very nicely. Until now I have compiled with gcc or g++ only using -ansi -pedantic -Wall . Do you have any other helpful flags to make the code more safe and robust?

Answer 1

As alk summed up, use these flags:

-pedantic -Wall -Wextra -Wconversion

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First, I think you don't want to use the -ansi flag, as suggested in Should I use "-ansi" or explicit "-std=..." as compiler flags?

Secondly, -Wextra seems to be quite useful too, as suggested in -Wextra how useful is it really?

Thirdly, -Wconversion seems also useful, as suggested in Can I make GCC warn on passing too-wide types to functions?

Fourthly, -pedantic is also helpul, as suggested in What is the purpose of using -pedantic in GCC/G++ compiler?.

Lastly, enabling -Wall should be fine in this case, so I am pretty doubtful about what you said.

Example with gcc:

Georgioss-MacBook-Pro:~ gsamaras$ cat main.c 
int main(void)
{
    int x = 1;
    int y = x+ ++x;
    return 0;
}
Georgioss-MacBook-Pro:~ gsamaras$ gcc -Wall main.c 
main.c:4:16: warning: unsequenced modification and access to 'x' [-Wunsequenced]
    int y = x+ ++x;
            ~  ^
main.c:4:9: warning: unused variable 'y' [-Wunused-variable]
    int y = x+ ++x;
        ^
2 warnings generated.
Georgioss-MacBook-Pro:~ gsamaras$ gcc -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 8.1.0 (clang-802.0.38)
Target: x86_64-apple-darwin16.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Example with g++, same version:

Georgioss-MacBook-Pro:~ gsamaras$ cp main.c main.cpp
Georgioss-MacBook-Pro:~ gsamaras$ g++ -Wall main.cpp 
main.cpp:4:16: warning: unsequenced modification and access to 'x'
      [-Wunsequenced]
    int y = x+ ++x;
            ~  ^
main.cpp:4:9: warning: unused variable 'y' [-Wunused-variable]
    int y = x+ ++x;
        ^
2 warnings generated.

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Relevant answer of mine, that Wall saves the day once more with a similar problem.