How do I find the length of an array?

Question

Is there a way to find how many values an array has? Detecting whether or not I've reached the end of an array would also work.

Answer 1

If you mean a C-style array, then you can do something like:

int a[7]; std::cout << "Length of array = " << (sizeof(a)/sizeof(*a)) << std::endl; 

This doesn't work on pointers (i.e. it won't work for either of the following):

int *p = new int[7]; std::cout << "Length of array = " << (sizeof(p)/sizeof(*p)) << std::endl; 

or:

void func(int *p) {     std::cout << "Length of array = " << (sizeof(p)/sizeof(*p)) << std::endl; }  int a[7]; func(a); 

In C++, if you want this kind of behavior, then you should be using a container class; probably std::vector.

Answer 2

As others have said, you can use the sizeof(arr)/sizeof(*arr), but this will give you the wrong answer for pointer types that aren't arrays.

template<class T, size_t N> constexpr size_t size(T (&)[N]) { return N; } 

This has the nice property of failing to compile for non-array types (Visual Studio has _countof which does this). The constexpr makes this a compile time expression so it doesn't have any drawbacks over the macro (at least none I know of).

You can also consider using std::array from C++11, which exposes its length with no overhead over a native C array.

C++17 has std::size() in the <iterator> header which does the same and works for STL containers too (thanks to @Jon C).