当派生类无法轻松地将参数传递给基类时，如何调用基类构造函数？

Question

Let say I have a base and a derived class. In the standard derived class constructor I can call the Base class constructor with the required parameter (CASE1 below). However, if the derived class constructor accepts a json string, I need to first extract the field containing the value for the param_base and then call the base class constructor with this parameter. How do I do that in C++?

class Base {
    int param_base;
public:
    Base(int x) : param_base(x) {}
};

class Derived : public Base{
    int param_derived;
public:
    Derived(int d, int b): Base(b), param_derived(d) {}; // CASE1
    Derived(std::string json_str) {  // CASE2
        int base_param = prase(json_str, "b_field");
        // How do I construct the base class with the parsed "base_param" ?
    }
};

Answer 1

You can write a function or lambda to do the work, then call it.

int do_stuff(const std::string& json_str) {
  return 42; // really parse json and extract int value
}

then

Derived(std::string json_str) : Base(do_stuff(json_str) {}

lambda version:

Derived(std::string json_str) : Base([&json_str](){ return 42;}()) {}