Reference-type in Functions

Pass by reference

• Allows modifications of the actual parameters:

  // modifies the actual parameter
  // no return value
  void addOne(int &var) {
    ++var;
  }
  
  int main() {
    int a = 10;
    addOne(a);  // the actual parameter is a, must be an lvalue
    cout << a << endl;  // 11
    return 0;
  }
  

• Avoid duplication of the value (reduce overhead when the value is an object):

  string processText(const string &inputText) {
    string processedText;
    // process the text
    return processedText;
  }
  
  int main() {
    string inputText = "This is a very long text";
    string result = processText(inputText);
    cout << result << endl;
  }
  

• It is a good practice to always use pass-by-reference for objects (class-typed variables)

• Extensively used in many scenarios

  • copy constructor
  • copy assignment operator overloading

Returning a Reference

Returning a reference allows a function call to be used as a lvalue. It allows modifications to the result of a function call.

class MyArr {
 private:
  int *arr;
  int size;
 public:
  MyArr();
  MyArr(int size, int value);
  ~MyArr();
  int &at(int i);
};

// returns a reference
int & MyArr::at(int i) {
  return arr[i];  // the reference to arr[i] is passed out
}

int main() {
  MyArr myArr1(10, 10);
  myArr1.at(0) = 10;  // you can assign a value to a function call now
                      // because myArr1.at() is an alias of myArr1.arr[i]
  myArr1.at(1)++;
}