Bits & Bytes

Operator Overloading in C++ – part 1

Operator overloading in C++ is a topic that is full of confusion arising from two main sources: differences with other languages and chronic misuse. The rich and distinct structure of C++ provides the language with many features that have no counterpart in other languages. Hence, operator overloading is often seen from outside of C++ as unnecessary complexity; frequent misuse of this feature only serves to reinforce this perception. In reality, the peculiar strucure of C++ gives operator overloading a particularly powerful role in developing algorithms.

With all of the confusion surrounding operator overloading, it has frequently been refered as “syntactic sugar”–meaning that it serves to make code look nice. It may true that “a + b” is nicer looking than “sum(a, b)”, but the implications of the signature on the function that is being overloaded are far more significant.

Consider this simple bubblesort function:

template <typename PData>
void Bubblesort(PData xaArray[], int iLength) {
     for (int iEnd = iLength - 1; iEnd > 0; --iEnd) {
          for (int iIndex = 0; iIndex < iEnd; ++iIndex) {
               if (xaArray[iIndex] > xaArray[iIndex + 1]) {
                    PData xTemp  = xaArray[iIndex];
                    xaArray[iIndex]  = xaArray[iIndex + 1];
                    xaArray[iIndex + 1] = xTemp;
               }
           }
     }
}

Nevermind that this is a slow bubblesort, it could easily be any sorting algorithm. The important thing to note is that the algorithm works to sort any fundamental data type (int, double, etc.). It also works to sort any other data type that has overloaded the “>” and “=” operators.

That’s the power of operator overloading. It allows our algorithms to work on both fundamental and programmer-defined types. Better still, this algorithm uses the built-in operators for fundamental types so that no additional function call is needed for comparisons. Additionally, if we inline our overloaded operators on our programmer-defined types, we can avoid an expensive function call for comparisons on those types as well. Prior to C++ and operator overloading, C programmers used functions like qsort(), which is now in the file <cstdlib>, to sort arrays of any type of item. The declaration for qsort() looks like this:

void qsort( const void* kvpArray,
            size_t qNumberOfItems,
            size_t qItemSizeInBytes,
            int (*pfnCompareFn) ( const void *, const void *));

Notice that the last argument is a function pointer to a comparison function that is passed in. Since this is a function pointer, the comparison function cannot be inlined and that makes a quicksort in C much slower than a comparable version in C++ with operator overloading. Of course, you can get around this in C by writing a version of the sort for fundamental types and one for each new data type that you create. However, this was clearly seen as unmanageable, even by C programmers, or the qsort() function would not exist.

To be continued . . .

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By: Michael Hall

One Response to “Operator Overloading in C++ – part 1”

  1. Elsie says:

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