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c++temporary-objects

Accessing an array within a struct causes warnings with clang


struct test{
   char c_arr[1];
};

test array[1] = {{1}};

test get(int index){
 return array[index];
}

int main(){
  char* a =  get(0).c_arr;
  return 0;
}

Compiling this with g++ has no warnings but with clang++ prints the following:

warning: temporary whose address is used as value of local variable 'a' will be destroyed at the end of the full-expression

Is this incorrect? does get(0).c_arr not return a pointer to a global array?

or does get(0) return a temporary variable and the compiler thinks c_arr is just an instance of it, and not global, by mistake?

Edit

Why passing this temp variable to a function works without warnings?

void call(char* in){}

int main(){
  call(get(0).c_arr);
  return 0;
}

Solution

  • get returns by-value, then get(0) does return a temporary which gets destroyed after the full expression, left a being a dangling pointer.

    Note that the returned temporary test is copied from array[index], including the array data member c_arr. a is supposed to point to the 1st element of the data member array c_arr of the temporary test, after the full expression (i.e. the ; in char* a = get(0).c_arr;) the whole temporary test (and its data member c_arr) is destroyed, then a becomes dangling.

    If get returns by-reference then it'll be fine.

    test& get(int index){
     return array[index];
    }
    

    EDIT

    The code you added is fine. The temporary is destroyed after the full expression, i.e. after ; in call(get(0).c_arr);. The pointer passed to call remains valid inside call.