c++ - Virtual function with default argument, weird output -

the output of code 15 , don't know why. think uses x=5 in foo function don't know why. can me ?

#include <iostream> #include <string>  using namespace std;   struct {     virtual int foo(int x = 5)     {         return x*2;     } };  struct b : public {     int foo(int x = 10)     {         return x*3;     } };    int main(int argc, char** argv) {   a* = new b;   cout << a->foo();   return 0; } 

i think uses x=5 in foo function don't know why.

yes, default argument base class a's declaration (i.e. 5) used here, because you're calling foo() on object static type a*. default arguments decided based on static type, other dynamic type.

the standard has clear explanation this, $8.3.6/10 default arguments [dcl.fct.default]:

(emphasis mine)

a virtual function call ([class.virtual]) uses default arguments in declaration of virtual function determined static type of pointer or reference denoting object. overriding function in derived class not acquire default arguments function overrides. [ example:

struct {   virtual void f(int = 7); }; struct b : public {   void f(int a); }; void m() {   b* pb = new b;   a* pa = pb;   pa->f();          // ok, calls pa->b::f(7)   pb->f();          // error: wrong number of arguments b::f() } 

— end example ]