Which C++ standard is the default when compiling with g++?

Question

I have a piece of code that looks like the following. Let\'s say it\'s in a file named example.cpp

#include 
#include 

        

Answer 1

I'm guessing a default version of the C++ compiler gets called, but I don't know which?

This is only guessable by reading the documentation of your particular compiler version.

If using a recent GCC, I recommend first to understand what version are you using by running

g++ -v

or

g++ --version

and then refer to the version of the particular release of GCC. For example for GCC 7, read GCC 7 changes etc

Alternatively, run

g++ -dumpspecs

and decipher the default so called spec file.

BTW, you could ensure (e.g. in some of your common header file) that C++ is at least C++17 by coding

 #if __cplusplus < 201412L
 #error expecting C++17 standard
 #endif

and I actually recommend doing it that way.

PS. Actually, think of C++98 & C++17 being two different languages (e.g. like Ocaml4 and C++11 are). Require your user to have a compiler supporting some defined language standard (e.g. C++11), not some particular version of GCC. Read also about package managers.

Answer 2

If your version of g++ is later than 4.7 I think you can find the default version of C++ standard supported like so:

g++ -dM -E -x c++  /dev/null | grep -F __cplusplus

An example from my machine:

mburr@mint17 ~ $ g++ --version | head -1
g++ (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4
mburr@mint17 ~ $ g++ -dM -E -x c++  /dev/null | grep -F __cplusplus
#define __cplusplus 199711L

Some references:

• Details on the g++ options used
• Why this only works for g++ 4.7 or later

Answer 3

g++ man page actually tells what is the default standard for C++ code.

Use following script to show the relevant part:

man g++ | col -b | grep -B 1 -e '-std.* default'

For example, in RHEL 6 g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-23), the output:

         gnu++98
           GNU dialect of -std=c++98.  This is the default for C++ code.

And in Fedora 28 g++ (GCC) 8.1.1 20180502 (Red Hat 8.1.1-1), the output:

       gnu++1y
           GNU dialect of -std=c++14.  This is the default for C++ code.  The name gnu++1y is deprecated.