I want to suppress specific warnings from g++. I'm aware of the -Wno-XXX flag, but I'm looking for something more specific. I want some of the warnings in -Weffc++, but not all of them. Something like what you can do with lint - disable specific messages.
Is there a built in way in gcc to do this? Do I have to write a wrapper script?
Unfortunately, this feature isn't provided by g++. In VC++, you could use #pragma warning to disable some specific warnings. In gcc, the closest you can have is diagnostic pragmas, which let you enable/disable certain types of diagnostics for certain files or projects.
Edit: GCC supports pushing/popping warnings since 4.6.4 (see changelog)
For some warnings, there is a command line switch to disable them. In order to know which switch to use, pass -fdiagnostics-show-option to gcc.
You could just use grep -v on the output.
Depending on the warning you wish to disable, you can sometimes correct in code. E.g.:
int main()
{
int i;
}
Generates: foo.cc:4: warning: unused variable 'i'
Whereas this does not:
#define MARKUSED(X) ((void)(&(X)))
int main()
{
int i;
MARKUSED(i);
}
pipe standard error to a filter that removes things you don't want to see. For example, this is my make file:
main.o: main.cpp
g++ -c -Wall main.cpp 2>&1 | grep -v Wunused-variable
Related
I wanted to compile C++11 source code within Mac Terminal but failed. I tried g++ -std=c++11, g++ -std=c++0x, g++ -std=gnu++11 and g++ -std=gnu++0x but nothing worked. Terminal always read unrecognized command line option. However, g++ -std=gnu and things like that worked fine (of course C++11 source code could not pass).
Which option should I use to turn on C++11 support?
By the way, the command line tool I'm using is installed within Xcode, and I'm pretty sure that they are up-to-date.
As others have pointed out you should use clang++ rather than g++. Also, you should use the libc++ library instead of the default libstdc++; The included version of libstdc++ is quite old and therefore does not include C++11 library features.
clang++ -std=c++11 -stdlib=libc++ -Weverything main.cpp
If you haven't installed the command line tools for Xcode you can run the compiler and other tools without doing that by using the xcrun tool.
xcrun clang++ -std=c++11 -stdlib=libc++ -Weverything main.cpp
Also if there's a particular warning you want to disable you can pass additional flags to the compiler to do so. At the end of the warning messages it shows you the most specific flag that would enable the warning. To disable that warning you prepend no- to the warning name.
For example you probably don't want the c++98 compatibility warnings. At the end of those warnings it shows the flag -Wc++98-compat and to disable them you pass -Wno-c++98-compat.
XCode uses clang and clang++ when compiling, not g++ (assuming you haven't customized things). Instead, try:
$ cat t.cpp
#include <iostream>
int main()
{
int* p = nullptr;
std::cout << p << std::endl;
}
$ clang++ -std=c++11 -stdlib=libc++ t.cpp
$ ./a.out
0x0
Thanks to bames53's answer for pointing out that I had left out -stdlib=libc++.
If you want to use some GNU extensions (and also use C++11), you can use -std=gnu++11 instead of -std=c++11, which will turn on C++11 mode and also keep GNU extensions enabled.
I want to use c++11/14 features like range-based loops, but get a warning while doing g++ program.cpp. If done with compiler flag g++ -std=c++11 program.cpp the warning goes away. Is there a way to use c++11/14 by default on the g++ command (i.e without passing compiler flag every time).
Please explain to someone with limited knowledge of compilers and only need the c++11/14 features for competitive programming problems (even if it's a bad idea in general, maybe due to backward compatibility?)
Short Answer: Update your g++
According to g++ documentation
The default, if no C++ language dialect options are given, is -std=gnu++17.
You are probably using an older version of g++. You can check it using by running g++ --version in your terminal. If you are using Linux, you can also extract your default c++ standard from g++ manual with the command man g++ | col -b | grep -B 1 -e '-std.* default' in your terminal.
If you do not want to update your g++, you can also set a command alias by putting something like alias g+++='g++ -std=c++14' in your .bashrc
If you're using a gcc version > 4.9.3 use this command: g++ -std=c++14 program.cpp
If you're using an older version than that use g++ -std=c++1y program.cpp
Note: Consider adding the -Wall flag before program.cpp in your command to get warnings, they help you way more than you'd think!
Tip: If you're a starting developer and don't want too steep of a learning curve, try using an IDE before going full command-line.
EDIT: If you want a command to be "the default" you can add something like alias mycc='g++ -std=c++14 -Wall' in your .bashrc or .bash_profile file (see this link), then you'll be able to use mycc program.cpp
I know that if you execute GCC as such:
gcc -O3 -O2 foo.c
GCC will use the last optimization flag passed (in this case O2). However, is this true for all flags? For example, if I execute GCC like so:
gcc -mno-sse -msse bar.c
Will it support SSE since that was the last flag passed, or would this result in undefined behavior? My initial experimentation seems to indicate that it will support SSE, but I'm not sure if this is true for all cases.
Normally later options on the line override ones passed previously, as you mention in your first example. I haven't personally come across any different behaviour for -m or -f flags, but I don't know of a specific reference in the documentation.
Note that some options don't behave this way:
$ gcc example.c -DABC -DABC=12
<command-line>: warning: "ABC" redefined
<command-line>: warning: this is the location of the previous definition
So there would need to be a -UABC in between there to shut that warning up.
As an aside, clang is particularly good at solving this problem - it will produce a warning if it ignores a command line option, which can help you out.
I'm trying to change the settings of g++ from the Code::Blocks IDE. I went to the Settings tab, clicked Compiler... and checked various options for the compiler to use, like
Enable all warnings (-Wall)
Have g++ follow the C++11 ISO C++ language standard (-std=c++11)
......
These are just two of many others; when I compile on the command line, here is what comes up:
g++ -o example example.cpp
# warning: initializer lists only available with -std=c++11 ...
Notice how there's no warning either - I have an unused variable in my program. It only works if I give the options manually:
g++ -Wall -std=c++11 -o example example.cpp
Do you think I might have done something wrong when setting up the compiler? Why aren't the options taking affect?
Invoking the compiler from the ide is completely independent from doing it in a command line shell. There's no reason for the setting and usage of one to have any effect on the other.
In g++ 4.6 (or later), what extra optimisations does -Ofast enable other than -ffast-math?
The man page says this option "also enables optimizations that are not valid for all standard compliant programs". Where can I find more information about whether this might affect my program or not?
Here's a command for checking what options are enabled with -Ofast:
$ g++ -c -Q -Ofast --help=optimizers | grep enabled
Since I only have g++ 4.4 that doesn't support -Ofast, I can't show you the output.
The -Ofast options might silently enable the gcc C++ extensions. You should check your sources to see if you make any use of them. In addition, the compiler might turn off some obscure and rarely encountered syntax checking for digraphs and trigraphs (this only improves compiler performance, not the speed of the compiled code).