lazy

1_lazy.py

# example of how to make a method invocation
# "lazy" by not doing the work until first access
class A:
    def __init__(self):
        self.mValue = None

    def getValue(self):
        if not self.mValue:
            print("doing something expensive")
            self.mValue = "value"
        return self.mValue

a = A()
print(a.getValue())
print(a.getValue())

2_lazy.cpp

// attempt to port to c++17
#include <iostream> 
#include <experimental/optional>
using namespace std;
using namespace std::experimental;

class A {
  public:
    const string &getValue() const {
      if (!mValue) {
        cout << "doing something expensive" << endl;
        mValue.emplace("value"); 
      }
      return *mValue;
    }

  private:
    mutable optional<const string> mValue;
};

int main() {
  // A is logically const but not physically const.
  const A a;
  cout << a.getValue() << endl;
  cout << a.getValue() << endl;
}