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Longest C++ Variable Declaration

(478 words) Sat, Oct 26, 2019

Here’s a challenge: what’s the longest variable declaration you can come up with?

That’s not useful in any way. Quite the opposite - the result is something you’d never want to see in your codebase. But it’s an interesting mental challenge, or at least so I think.

Ground rules:

  1. Each keyword used gets you 1 point, repetitions allowed;

  2. Must be global variable;

  3. Must be valid C++17;

  4. Must not declare new structs / classes / unions / functions / methods;

  5. Variable must be initialized with empty braces, i.e {};

  6. No comments of any type are allowed, nor usage of preprocessor;

  7. Each ‘trick’ may only be used once.

Take a moment to think about it! Maybe you’ll come up with ideas I haven’t.

My Solution

Here’s roughly how my solution evolved:

// Most basic declaration - 1 point.
int a{};

// Some types are longer than others - 4 points
unsigned long long int a{};

// Adding constness - 6 points.
constexpr const unsigned long long int a{};

// With pointers you can add an additional const - 7 points.
constexpr const unsigned long long int* const a{};

// static - 8 points
static constexpr const unsigned long long int* const a{};

// Can you use thread_local with static? Apparently! - 9 points.
static thread_local constexpr const unsigned long long int* const a{};

// How 'bout static thread_local inline? Seems to work... - 10 points.
static thread_local inline constexpr const unsigned long long int* const a{};

// Here's a feature rarely used - volatile! - 11 points.
// (volatile constexpr - such a weird statement...)
static thread_local inline constexpr volatile const unsigned long long int* const a{};

Cheating, aka Stack Overflow

At this point, curious if I’m the only very bored C++ developer who came up with this challenge I decided to Google it.

I found this Stack Overflow thread with some interesting ideas. Someone came up with an easy way to get infinite points:

int const* const* const* const* const* const* const* const* /* ... */ a{};

This is the reason I added that last rule. It’s not well formulated I guess, but you get the spirit.

Additional idea is to use decltype with typeid:

// Adding decltype with typeid buys us additional 4 points, with a total of 15 points.
static thread_local inline constexpr volatile const decltype(typeid(const volatile unsigned long long int).name())* const a{};

And the last idea I read was to geniusly use alignas with sizeof, although I personally prefer to use alignof instead:

// Adding alignas with alignof / sizeof gets us to 25 points!
alignas(alignof(decltype(typeid(const volatile unsigned long long int)))) static thread_local inline constexpr volatile const decltype(typeid(const volatile unsigned long long int).name())* const a{};

This is as far as I got. I’m certain there are many more tricks I hadn’t thought of, or unaware of. Do you have any? Let me know in a comment!

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