Below code came from a post about C++ interview questions here. I've never known this technique :) (though it's claimed a good one :)). My questions are: In which situation do we need to use it? Do you often see it in your real production/legacy code?
Question:
Implement a method to get topSecretValue for any given Something* object. The method should be cross-platform compatible and not depend on sizeof (int, bool, string).
class Something {
Something() {
topSecretValue = 42;
}
bool somePublicBool;
int somePublicInt;
std::string somePublicString;
private:
int topSecretValue;
};
Answer:
Create another class which has all the members of Something in the same order, but has additional public method which returns the value. Your replica Something class should look like:
class SomethingReplica {
public:
int getTopSecretValue() { return topSecretValue; } // <-- new member function
bool somePublicBool;
int somePublicInt;
std::string somePublicString;
private:
int topSecretValue;
};
int main(int argc, const char * argv[]) {
Something a;
SomethingReplica* b = reinterpret_cast<SomethingReplica*>(&a);
std::cout << b->getTopSecretValue();
}
It’s important to avoid code like this in a final product, but it’s nevertheless a good technique when dealing with legacy code, as it can be used to extract intermediate calculation values from a library class. (Note: If it turns out that the alignment of the external library is mismatched to your code, you can resolve this using #pragma pack.)
