What is the importance of functions in c plus plus?

Answer 1

Without functions, a non-trivial program would contain repeated code segments and would be extremely difficult to maintain. Even if you don't write any functions of your own, you still have to call the built-in functions to actually do anything worthwhile.

User-defined functions allow you to structure your source code more efficiently. If you find yourself writing the same code segment over and over, then it makes sense to turn it into a function. Apart from anything else, it reduces the chance of making a mistake or a typo. And with only one copy of the code it becomes that much easier to adjust the function to improve its efficiency or add additional functionality. Moreover, you can break large and complex problems down into smaller functions, each of which can be broken down further if required. This is known as re-factoring. The end result is your code becomes much easier to read and is therefore much easier to maintain.

Every function you write should do a small amount of work, and should do it extremely well and as efficiently as possible. A one-line comment is usually more than sufficient to describe the purpose of a function. But if your function is large and complex, and heavily commented, re-factoring should be considered -- especially if you find it difficult to follow yourself.

While there is a performance penalty in making a function call, your compiler should be intelligent enough to work out which functions can be inline expanded (just as if you'd duplicated the code yourself). In general, only trivial and heavily repeated functions will be inline expanded, and only if the compiler believes there to be an advantage in doing so. While you can tell it which functions to inline expand, the compiler is usually free to ignore the request if it sees no advantage in doing so. Additional optimisers will check the resulting machine code more closely and make finer adjustments as they see fit. The end result should be reasonably efficient code, the only real compromise being overall code size vs. overall code speed (large code is not necessarily fast code).

Answer 2

Every program requires an entry point. The global main function serves this purpose and is the only function that is required. Additional functions make code easier to read and maintain, so the main function is reduced to handling the program's input parameters and invoking the appropriate function or functions.