you may want to specify the C++ version with
g++ -std=c++11 tmp.cpp -o tmp
I don't have gcc 4.8.1 at hand , but in older versions of GCC,
you can use
g++ -std=c++0x tmp.cpp -o tmp
At least gcc 4.9.2 I believe also support part of C++14 by specifying
g++ -std=c++1y tmp.cpp -o tmp
Update:
gcc 5.3.0 (I am using the cygwin version) supports both -std=c++14
and -std=c++17
now.
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