What's the point of including -std=c++0x in a G++ compile command?

Question

I have recently started learning C++ and, since I'm on Linux, I'm compiling using G++.

Now, the tutorial I'm following says

If you happen to have a Linux or Mac environment with development features, you should be able to compile any of the examples directly from a terminal just by including C++11 flags in the command for the compiler:

and tells me to compile using this command: g++ -std=c++0x MY_CODE.cpp -o MY_APP.

Now, what I'm wondering, what is the point of the std=c++0x flag? Is it required, or can I just run g++ MY_CODE.cpp -o MY_APP?

Answer 1

By default, GCC compiles C++-code for gnu++98, which is a fancy way of saying the C++98 standard plus lots of gnu extenstions.

You use -std=??? to say to the compiler what standard it should follow.
Don't omit -pedantic though, or it will squint on standards-conformance.

The options you could choose:

standard          with gnu extensions

c++98             gnu++98
c++03             gnu++03
c++11 (c++0x)     gnu++11 (gnu++0x)
c++14 (c++1y)     gnu++14 (gnu++1y)

Coming up:

c++1z             gnu++1z (Planned for release sometime in 2017, might even make it.)

GCC manual: https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Standards.html#Standards

Also, ask for full warnings, so add -Wall -Wextra.

There are preprocessor-defines for making the library include additional checks:

• _GLIBCXX_CONCEPT_CHECKS to add additional compile-time-checks for some templates prerequisites. Beware that those checks don't actually always do what they should, and are thus deprecated.
• _GLIBCXX_DEBUG. Enable the libraries debug-mode. This has considerable runtime-overhead.
• _GLIBCXX_DEBUG_PEDANTIC Same as above, but checks against the standards requirements instead of only against the implementations.

Answer 2

You want to use the C++11 standard (and you are right to want that), but C++11 made a huge progress w.r.t. its older C++98 standard.

But old versions of GCC (i.e. GCC 4.8 or earlier) where not finalized before the standard itself (so they accepted the -std=c++0x flag). I strongly recommend (if you want C++11) to use the latest version of GCC, that is GCC 4.9. A bug fixing GCC 4.9.2 release appeared at end of october 2014. So use it please, and pass it the std=c++11 flag to tell the compiler you want C++11 conformance.

I actually suggest to pass std=c++11 -Wall -Wextra -g to get C++11, all warnings, and debug info. Once you have debugged your program (with gdb, and you'll better also use a recent version of gdb!) you might ask the compiler to optimize with -O2 (and perhaps -mtune=native if you want to optimize for your own computer)

Answer 3

Source for your reference:

main.cpp

#include <iostream>
using namespace std;

int main()
{
    cout << "Test main CPP" << endl;
    return 0;
}

build.sh

rm demoASI*
echo "**cleaned !!**"

##### C++ 11 Compliance #####
# type ONE
g++ -o demoASI_1 -std=c++0x main.cpp
echo "**rebuild-main-done (C++ 11 Compilation) !**"

# type TWO
g++ -o demoASI_2 -std=c++11 main.cpp
echo "**rebuild-main-done (C++ 11 Compilation) !**"

##### C++ 11+ Compliance #####
# type THREE
g++ -o demoASI_3 -std=c++1y main.cpp
echo "**rebuild-main-done (C++ 11+ (i.e. 1y, but not C++14) Compilation) !**"

###### C++ 14 Compliance  ######
# type FOUR
g++ -o demoASI_4 -std=c++14 main.cpp
if [ $? -eq 0 ]
then
    echo "**rebuild-main-done (C++ 14 Compilation) !** :: SUCCESS"
else
    echo "**rebuild-main-done (C++ 14 Compilation) !** :: FAILED"
fi

Now, execute the script as; ./build.sh (assuming build.sh has execution permission)

You can first check the version of your g++ compiler, as;

g++ --version

The version of g++, after 4.3, has support for the c++11.

Please see, for c++14 support info in compiler.