Disable every single gcc extensions (C++) - c++

The title is quite clear, is there a way to disable every single non-standard feature of gcc (extension) when compiling some C++ code. I've previously always used -pedantic-errors alongside -Wall and -Wextra, the first according to the gcc man page does the following
Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some
equivalent to -Werror=pedantic, since there are errors enabled by this option and not
particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings
made into an error by -pedantic-errors.
-Wall or -Wpedantic.
As a result it disables practically all extensions that aren't standard conforming, however. I was rather shooked that the following code compiles with gcc
void foo(std::vector<auto>) { ... }
with the -std=c++20 flag, if we bump it down to -std=c++17 we receive an error message prompting the use of -std=c++20 as it's a "c++20 feature". However neither clang or msvc is willing to compile this. After some further research I was notified of the following
This is a GCC extension and accepted by design.
Which is rather upsetting as even the safe-guards of -pedantic-errors, -Wall and -Wextra didn't pick this up, which leads me to question what other extensions might be silently passed through by gcc. Thus the question comes; is there any way to disable exactly every gcc extension?
Edit: From the comment section of How to disable GNU C extensions?
You cannot

Related

Why does the -std=c++98 flag sometimes not work?

The situation is as follows:
I want to create a simple c++ program, but it has to use only C++98 functions.
I am using Ubuntu 20.04.4 LTS.
I am using c++ as compiler
I am using the following flags for compiling:
-Wall -Werror -Wextra -std=c++98 -pedantic-errors
Now if I am using the function stoi() from <string>, compiling will fail, as expected, because stoi() is C++11, as you can see here.
But here comes the weird behavior that I am not able to understand:
using round or roundf from <cmath> won't trigger the -std=c++98 flag, even though, from what I can see here, all round functions are C++11.
Is there any good explanation why this is happening?
EDIT:
as jjramsey mentioned, there is the chance that the standard-C-function of round was used, so the -std=c++98 flag will have no effect on that.
Can anyone confirm this theory?
Nothing stopped a compiler from implementing anything pre-standard. For example, Visual Studio 2010 was not a standard C++11 compiler, yet implemented (to some extent) lambdas.
The only thing to expect from a C++98 flag is that all (hopefully) features of C++98 are implemented. Anything beyond that is just a pre-standard C++11/14/17, etc "bonus" that the compiler has implemented.
Of course, these bonuses were not official, and most were based on a draft of the future C++ standard. Once the C++ standard for 11/14/17, etc. came to be, a lot of those features introduced in those older compilers were either non-standard, didn't work properly, or missing.

emscripten C++11 with boost support issue

I have an issue when building my C++ project that uses boost in Emscripten as shown on the screenshot, it says that '_Atomic' is a C11 extension however even if i add -std=c++11 or even -std=c11 i am still getting the error, the _Atomic use is defined from boost.
any idea on how to work around on this? reading around it does say that C++11 is already supported in Emscripten.
my setup.
compiler: emscripten/em++ Clang 12.0.8
Why
The message in your screenshot shows:
you are compiling with clang
error: '_Atomic' is a C11 extension [-Werror,-Wc11-extensions]
So clang warns on c11 extensions because it has the diagnostic flag set, and is also configured to fail on warning which leads to your failed compilation.
-Wc11-extensions may have been specified directly in your compilation commands
or indirectly by -Wpedantic (see https://clang.llvm.org/docs/DiagnosticsReference.html#id604)
How to fix
You could specifically deactivate this warning with the -Wno-c11-extensions compiler flag, see this good explanation here on how to do it: https://nelkinda.com/blog/suppress-warnings-in-gcc-and-clang/

Difference between -g and -gfull in Clang

The title says it all. I looked everywhere on Clang's docs, but the option -gfull hasn't got any description.
The documentation also lacks information about all of the various debug levels (like -g2), and it would be cool if someone could explain that as well.
Apparently, options -g0, -g1, -g2, -g3 and -gfull in Clang are equivalent to GCC's. I could not find their documentation in Clang, but I found some info about their use in GCC (and they should behave the same in Clang, but I'm not 100% sure):
-gfull is used on Darwin/macOS. Using -gfull is equivalent to -g -fno-eliminate-unused-debug-symbols. Sources: link1 link2.
-g0 to -g3 are debug levels. They specify how much debugging information to include (note that -g0 means no debugging info at all). See here for a more detailed explanation (you find them under the paragraph -glevel).
EDIT:
All these command line options are listed in the Clang manual as GCC-compatible options (source). Therefore, the behavior described in the GCC manual should be the same in Clang.
P.S.: I've already said that in the original answer, but now I added a link to the Clang manual as source.

MSVC++ warning flags

With gcc and clang, I routinely use -Wall -Wextra warning flags.
What command line switches for Visual C++ 2010 (and newer, if there are differences) would produce about same result?
This MSDN document page provides the technical details, but I am after a more qualitative answer from Visual C++ developers, based on practical experience: A "rule of thumb" flags you can safely throw on any new project when you start, that would still catch approximately the same things that -Wall -Wextra of the other compilers catch.
I agree with #Paulius's answer here which you linked to in the comments. /W4 will show you all warnings that are not disabled by default, which is probably the closest you can get to "approximately the same things" without searching through each of the warnings individually and enabling the equivalent. /Wall in MSVC will do what it says and turn on all warnings (even the questionable ones), which for any non-trivial project will give false positives. This is different from gcc -Wall, which will only turn on warnings that are not "questionable".

Reducing the execution time of the code using CLANG/LLVM compiler

Well... When i was searching for a good compiler I came across clang/LLVM. This compiler gives me same result as other compilers like icc, pgi. But the problem is there are very few tutorials on this compiler... Kindly let me know where can I find the tutorials on the clang compiler.
Note by:
I have compiled my c code using the following flags clang -O3 -mfpmath=sse file.c
Clang (the command line compiler) takes gcc-compatible options, but accepts and ignores a lot of flags that GCC takes (like -mfpmath=sse). We aim to generate good code out of the box. There are some flags that allow clang to violate the language standards that can be useful in some scenarios, like -ffast-math though.
If you're looking for good performance, I highly recommend experimenting with link-time-optimization, which allows clang to optimize across source files in your application. Depending on what platform you're on, this is enabled by passing -O4 to the compiler. If you're on linux, you need to use the "gold" linker (see http://llvm.org/docs/GoldPlugin.html). If you're on the mac, it should "just work" with any recent version of Xcode.
The clang is not a compiler, it is just frontend of LLVM compiler. So, when you calls clang, it parses c/c++ file but the optimization and code generation is handled in LLVM itself.
Here you can found a documentation of LLVM optimization and analysis options: http://llvm.org/docs/Passes.html
The full documentation is here http://llvm.org/docs/
Also useful options are listed here http://linux.die.net/man/1/llvmc (I suggest clang will accept most of them too)