c++ - -Wundef is not being ignored with pragma in g++ -

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given following code: 

#if macro_without_a_value int var; #endif  int main(){}

when compiled with, g++ -std=c++1z -wundef -o main main.cpp,
produces following warning: 

main.cpp:1:5: warning: "macro_without_a_value" not defined [-wundef]  #if macro_without_a_value      ^

i'd keep warning flag enabled, suppress particular instance.
apply following:

#ifdef __gnuc__ #pragma gcc diagnostic ignored "-wundef" #pragma gcc diagnostic push #endif  #if macro_without_a_value int var; #endif  #ifdef __gnuc__ #pragma gcc diagnostic pop #endif  int main(){}

this solves problem in clang++.

the command clang++ -std=c++1z -wundef -o main main.cpp builds without warnings.
command g++ -std=c++1z -wundef -o main main.cpp builds same [-wundef] warning before.

how can suppress -wundef warnings in g++? 

g++ (ubuntu 5.1.0-0ubuntu11~14.04.1) 5.1.0 clang version 3.8.0

what i've done before when third party headers inducing warnings wrap them in own private header uses #pragma gcc system_header silence warnings header. use own wrapper keep includes neat , allow additional customization point in future if needed.


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