How to Use Reference Values in C++
Reference values are a form of pointer that has additional restrictions, this makes it useful as the compiler can make certain optimizations. Once a reference pointer has been defined it can’t be moved, so assigning to the reference modifies the original value (except in the case of a const reference which can’t be modified). You can use reference values to return more than 1 value from a function, to modify values passed to the function, to pass values efficiently, or to return the address of an array element.
- More than 1 value. Most assembly languages return the dividend and the remainder after a division instruction, and there are times when you need both values, so using a function like
void divmod(int number, int divide, int ÷nd, int &mod) this would allow you to return both values, in an more intuitive manner than returning a struct with multiple values.
- Modify values passed to a function. A function reduce that would round a value towards zero (if it is negative it would add 1, if it was positive it would subtract 1). You could use the code void reduce(int &value) and call it with reduce(n).
- Passing values efficiently. If you have a method that takes a string, you can pass it by value or by reference, the problem that you get when passing by value is that it has to make a copy, this could be inefficient since you are allocating memory for the characters, and then freeing the memory as the function exits. A reference means that no copy is made, you should use a const reference if the function is not supposed to modify the value. For example int value(const string& number, int base) could be used to convert a string to a number (similar to itoa for a char*).
- Returning the address of an array element. This allows you to write a class that overloads the , so that you can write code like words[“hello”] += 1. You can also use it to search through a binary tree, or another data structure, you might want to return a const reference if you don’t want the code to be able to modify the returned value.
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