C++ Notes: Arrays - Problems and Solutions

Inherent Problems

Arrays can be difficult to use reliably for several reasons.

Unknown maximum size of an array

Because arrays are implemented as a pointer to the first element of the allocated storage, the length, or maximum allocated size, is not generally available. It's necessary to pass this maximum size to functions which need to add elements to an array. [Note: Vectors solve this problem.]

Lack of current number of elements

The number of elements currently in an array is another quantity that the programmer must keep track of. [Note: Vectors solve this problem.]

Fixed size

The programmer has to decide how much memory to allocate for an array, but sometimes there may be large variations in the requirements. For example, I was just writing a program to process daily mutual fund values. The data is downloaded from a service that provides this data, but I don't know how much data there will be for any fund. It can vary from a few entries for new funds to many years of data. How big should I declare the arrays? If I make all arrays large enuf for the largest possible number of data points (50 years?), then memory usage will be very wasteful and the program might not even fit into memory on some machines. If I make the arrays smaller, there might not be suffcient space for all data. [Note: Vectors solve this problem.]

Lack of subscript checking

C++ doesn't check subscription to see that the subscript is in a legal range. This lack of checking means that errors are often undetected for a long time. This is also the source of the most common hacker exploitation of programs, known as the buffer overflow exploit. [Note: Vectors can solve this problem. Also, Java and C# provide bounds checking.]

Buffer Overflow

Imagine that in the previous example, which involved reading numbers into an array, the input consisted of more than 1000 (the size of the array) numbers. What happens? The numbers are read into memory locations after the end of the array. What is the consequence of this? Hmmmm, that's what hacker knowledge is all about. [Note: Vectors solve this problem.]


Dynamic allocation - Solution to fixed size restriction

Arrays can be dynamically allocated, which is a nice solution for some problems. But you have to be comfortable with pointers before using dynamic allocation (Dynamic Allocation of Arrays). [Note: This is how vectors are implemented.]

Vectors - Solution to buffer overflow, fixed size, unknown maximum and current size, ...

Object-oriented programming libraries, such as the STL (Standard Template Library) which comes with modern versions of C++, provide data structures which are nice replacements for arrays. The simplest alternative is vector, which is essentially a kind of expandable array.