when do we need to pass the size of array as a parameter

Question

I am a little bit confused about pass an array in C/C++. I saw some cases in which the signature is like this

void f(int arr[])

some is lik

Answer 1

Anytime you need to know the size of the array, it needs to be provided. There is nothing special about the two forms of passing the array itself; the first parameter is the same either way. The second method simply provides the information needed to know the size of the array while the first does not.

Sometimes the array itself holds the information about its size, though. In your first example, for instance, perhaps arr[0] is set to the size of the array, and the actual data begins at arr[1]. Or consider the case of c-strings... you provide just a char[], and the array is assumed to end at the first element equal to \0. In your example, a negative value may act as a similar sentinel. Or perhaps the function simply doesn't care about the array's size, and will simply assume it is large enough.

Such methods are inherently unsafe, though... it is easy to forget to set arr[0] or to accidently overwrite the null terminator. Then, f suddenly has no way of knowing how much space it has available to it. Always prefer to provide the size explicitly, either via a size parameter like you show, or with a second pointer to the end of the array. The latter is the method generally taken by the standard library functions in C++. You still have the issue of providing an incorrect size, though, which is why in C++ it isn't recommended you ever use such an array in the first place... use an actual container that will keep track of that information for you.