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#include <iostream>

int main(){
  std::cout << "Hello World";
  return 0;
}

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g++ hello.cpp -o hello
./hello
Hello World

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int main(){
    int a = 1;
    return 0;
}

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cpp_tutorial# g++ warning_example.cpp -Wall -o a
warning_example.cpp: In function ‘int main()’:
warning_example.cpp:2:9: warning: unused variable ‘a’ [-Wunused-variable]
     int a = 1;
         ^

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int main(){
    int array[100];
    return array[1];
}

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cpp_tutorial# g++ warning_example.cpp  -Wuninitialized -o a -Werror
warning_example.cpp: In function ‘int main()’:
warning_example.cpp:4:19: error: ‘array[1]’ is used uninitialized in this function [-Werror=uninitialized]
     return array[1];
                   ^
cc1plus: all warnings being treated as errors

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#include <iostream>
#ifndef SIZE_ARRAY
#define SIZE_ARRAY 4
#endif 

int main(){
    int array[SIZE_ARRAY] = {0};
    std::cout << "num of elements: " << sizeof(array)/sizeof(array[0]) << "\n";
    return 0;
}

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#include "vars.h"

int main(){
  return 0;
}

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int a = 1;

int foo(){
  return 2;
}

int main(){
  return 0;
}

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g++ -E example.cpp -o _preprocess.out