Is there a way to introspect an array's size?

Question

In C++ given an array like this:

unsigned char bogus1[] = {
  0x2e, 0x2e, 0x2e, 0x2e
};

Is there a way to introspect bogus1 to find out is is four characters long?

Answer 1

int bogus1_size = sizeof(bogus1) / sizeof(unsigned char);

Answer 2

Sure:

#include <iostream>

int main()
{
  unsigned char bogus1[] = {
    0x2e, 0x2e, 0x2e, 0x2e
  };

  std::cout << sizeof(bogus1) << std::endl;

  return 0;
}

emits 4. More generally, sizeof(thearray)/sizeof(thearray[0]) is the number of items in the array. However, this is a compile-time operation and can only be used where the compiler knows about that "number of items" (not, for example, in a function receiving the array as an argument). For more generality, you can use std::vector instead of a bare array.

Answer 3

One thing to be careful about with sizeof is making sure you don't accidentally do the sizeof an array that has decayed into a pointer:

void bad(int a[10])
{
    cout << (sizeof(a) / sizeof(a[0])) << endl;
}

int main()
{
    int a[10];
    cout << (sizeof(a) / sizeof(a[0])) << endl;
    bad(a);
}

On my 64 bit machine, this outputs:

10
2

The size computed inside bad is incorrect because a will have decayed to a pointer. In general, don't trust the sizeof an array that was passed as a function argument.

Answer 4

This gets around the pointer decay problem:

#include <cstddef>

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

In that it will not compile unless the array size is known.