How to avoid operator's or method's code duplication for const and non-const objects? [duplicate]

This question already has answers here: Closed 11 years开发者_JAVA技巧 ago.

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How do I remove code duplication between similar const and non-const member functions?

My task is to implement c++ vector analogue. I've coded operator[] for 2 cases.

T myvector::operator[](size_t index) const {//case 1, for indexing const vector
    return this->a[index];   
} 
T & myvector::operator[](size_t index) {//case 2, for indexing non-const vector and assigning values to its elements
    return this->a[index];
}

As you can see, the code is completely equal. It's not a problem for this example (only one codeline), but what should I do if I need to implement some operator or method for both const and non-const case and return const or reference value, respectively? Just copy-paste all the code everytime I make changes in it?

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One of the few good uses of const_cast here. Write your non const function as normal, then write your const function like so:

const T & myvector::operator[](size_t index) const {
    myvector<T> * non_const = const_cast<myvector<T> *>(this);
    return (*non_const)[index];
}