c++ - Some trouble with derived classes -
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i trying implement functions , classes below, output arr[i].x wrong. correctly arr[0].x = 0 , arr[1].x = 0, arr[2].x not return 0. ideas why?
class base { public: int x; }; class derived : public base { public: int y; void init(base *b); void foo(); }; void derived :: init(base *b) { for( int = 0; < 3; ++i) { b[i].x = 0; } } void derived :: foo() { derived arr[3]; init(arr); for( int = 0; < 3; ++i) { cout<<"b["<<i<<"] "<<arr[i].x; } } int main() { derived der; der.foo(); return 0; }
void derived :: foo() { derived arr[3]; init(arr); a pointer derived, derived*, passed function init(base *b). happens next instead of moving sizeof(derived)=8 in table function move sizeof(base)=4 results in initialization of x member first , second derived in array , y of first derived, not x of last derived.
pointer arithmetic done based on size of type of pointer
consider memory layout (on x64):
in derived::foo():
derived arr[3]; 0x7fffffffe310 // 1st derived, address of arr[0].x +8 = 0x7fffffffe318 // 2nd derived, address of arr[1].x +8 = 0x7fffffffe320 // 3rd derived, address of arr[2].x but in derived::init( base* b):
b[0].x = 0x7fffffffe310 // set arr[0].x 0 +4 = b[1].x = 0x7fffffffe314 // set arr[0].y 0 +4 = b[2].x = 0x7fffffffe318 // set arr[1].x 0 thus, have set arr[0].x 0, arr[1].x 0 , incidentally arr[0].y 0. not want. solution change derived::init to
void derived::init( derived *d) { for( int = 0; < 3; ++i) { d[i].x = 0; } } or better, following principle of more generic programming:
template < size_t n> void derived::init( derived (&d)[n] ) { for( int = 0; < n; ++i) { d[i].x = 0; } }
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