Computing length of array

Question

I have a C++ array declared as mentioned below:

CString carray[] =
{
        "A",
        "B",
        "C",
        "D",
        "E"
}

I want to determine the length of carray at runtime. I am doing:

int iLength = sizeof(carray)/sizeof(CString);

Is this correct?

Answer 1

You can use the following function template. If you're using Boost, you can call boost::size.

template <typename T, std::size_t N>
std::size_t size(T (&)[N])
{
    return N;
}

int iLength = size(carray);

As others have already stated, however, you should prefer std::vector to C-style arrays.

Answer 2

Yes. In case the declared element type ever changes, you could also write

int iLength = sizeof(carray)/sizeof(carray[0]);

Answer 3

That is correct, as it is using metaprogramming as this:

template <typename T, std::size_t N>
inline std::size_t array_size( T (&)[N] ) {
   return N;
};

You must know that this works when the compiler is seeing the array definition, but not after it has been passed to a function (where it decays into a pointer):

void f( int array[] )
{
   //std::cout << array_size( array ) << std::endl; // fails, at this point array is a pointer
   std::cout << sizeof(array)/sizeof(array[0]) << std::endl; // fails: sizeof(int*)/sizeof(int)
}
int main()
{
   int array[] = { 1, 2, 3, 4, 5 };
   f( array );
   std::cout << array_size( array ) << std::endl; // 5
   std::cout << sizeof(array)/sizeof(array[0]) << std::endl; // 5 
}