Another C++11 ‘countof’

Note: There’s an update here.

Read “Better array ‘countof’ implementation with C++ 11” for context. Specifically, it presents Listing 5 as an implementation of countof() using C++11 constexpr:

• template<typename T, std::size_t N>
  constexpr std::size_t countof(T const (&)[N]) noexcept
  {
    return N;
  }

But this falls short. Just a little.

There are arguments that could be passed to a naive sizeof(a)/sizeof(a[0]) macro that will cause the above to fail to compile.

Consider:

struct S
{
  int a[4];
};

void f(S* s)
{
  constexpr size_t s_a_count = countof(s->a); 
  int b[s_a_count]; 
  // do things...
}

This does not compile. s is not constant, and countof() is a constexpr function whose result is needed at compile time, and so expects a constexpr-friendly argument. Even though it is not used.

Errors from this kind of thing can look like this from clang-3.7.0:

error: constexpr variable 's_a_count' must be initialized by a 
       constant expression
note:  read of non-constexpr variable 's' is not allowed in a 
       constant expression

or this from Visual Studio 2015 Update 1:

error: C2131: expression did not evaluate to a constant

(Aside: At the time of writing, the error C2131 seems to be undocumented for VS2015. But Visual Studio 6.0 had an error with the same number)

Here’s a C++11 version of countof() that will give the correct result for countof(s->a) above:

#include <type_traits>

template<typename Tin>
constexpr std::size_t countof()
{
  using T = typename std::remove_reference<Tin>::type;
  static_assert(std::is_array<T>::value, 
                "countof() requires an array argument");
  static_assert(std::extent<T>::value > 0,  // [0]
                "zero- or unknown-size array");
  return std::extent<T>::value;
}

#define countof(a) countof<decltype(a)>()

Some of the details:

Adding a countof() macro allows use of decltype() in the caller’s context, which provides the type of the member array of a non-const object at compile time.

std::remove_reference is needed to get the array type from the result of decltype(). Without it, std::is_array and std::extent produce false and zero, respectively.

The first static assert ensures that countof() is being called on an actual array. The upside over failed template instantiation or specialization is that you can write your own human-readable, slightly more context aware error message (better than mine).

The second static assert validates that the array size is known, and is greater than zero. Without it, countof<int[]>() will return zero (which will be wrong) without error. And zero-sized arrays will also result in zero — in practice they rarely actually contain zero elements. This isn’t a function for finding the size of those arrays.

And then std::extent<T>::value produces the actual count of the elements of the array.

---

Addendum:

If replacing an existing sizeof-based macro with a constexpr countof() alternate, Visual Studio 2015 Update 1 will trigger warnings in certain cases where there previously were no warnings.

warning C4267: conversion from 'size_t' to 'int', possible loss of data

It is unfortunate to have to add explicit casts when the safety of such operations is able to be determined by the compiler. I have optimistically submitted this as an issue at connect.microsoft.com.

[0] Typo fix thanks to this commentor