How to copy data from base class to derived class fastly?-CodePudding

I know move constructor can avoid copy, with a high performance.

but can i use it in copy data from base class to derived class?

for example:

// base.h
class Base {
 public:
  int a;
  char buff[24];
  double bp_[10];
};

// derived.h
class Derived : public Base {
 public:
  int b;
  char buff2[16];
};

// main.cpp
int main() {
  Base b;
  b.a = 1;
  snprintf(b.buff, sizeof(b.buff), "helloworld");
  b.bp_[0] = 100;
  Derived d;
  d.b = 9;
  snprintf(d.buff2, sizeof(d.buff2), "mytest");
  // TODO: how can i move b's data into d with high performance?
}

please see the code, I have a Base object with data, but i want to create a Derived class object, so i need copy data from Base object, is there any good methods like move constructor?

CodePudding user response：

There is no general way to do this, as the layout of the data in the base and derived classes may be different. You'll need to write a specific function to do the copying for each class.

One approach would be to write a template function that takes a base class and a derived class, and uses a static_cast to copy the data:

template <typename Base, typename Derived>
void copy_data(Base& b, Derived& d)
{
    d = static_cast<Derived>(b);
}

This will work if the layout of the data in the base and derived classes is the same. If it's not, you'll need to write a specific function to do the copying.