numeric_limits of atomic types

Question

Suppose someAtomic is an std::atomic with an integral underlying type, such as atomic_uint16_t. I don't want to assume WHICH integral type, however, in particular code, so I want something to accomplish the following, which right now doesn't compile:

if (newVal > numeric_limits<decltype(someAtomic)>::max()) throw "newVal too large";
else someAtomic.store(newVal, memory_order_release);

It looks like at least in VC++2015 there are no numeric_limits specializations for atomic types even if their underlying types do have such specializations. What's the best way to deal with this?

Remy Lebeau · Accepted Answer · 2017-07-26 20:24:37Z

template<class T>
struct my_numeric_limits : std::numeric_limits<T>{};

template<class T>
struct my_numeric_limits<std::atomic<T>> : my_numeric_limits<T>{};

Then you can use my_numeric_limits<SomeAtomic>::max().

This is less likely to violate (vague) parts of the standard than adding a specialization to std::numeric_limits that does not depend on a user-provided type. In C++11, there were requirements that you specialize over "user defined types", and I am uncertain if this has been resolved if std::atomic<int> is user-defined or not. I saw a fix proposal, but am uncertain if it went anywhere.

Regardless, this follows the principle of least surprise, and is just as efficient. Messing around with things in the std namespace should only be done when the alternatives are impractical.

Get something wrong, and your code suddenly becomes ill-formed, no diagnostic required. People checking your code are rightly scared. People modifying your code have to not screw up. my_numeric_limits is robust, safe, and resists error.

Ivaylo Valchev · Accepted Answer · 2017-07-26 19:40:52Z

3

The C++ Standard allows (and encourages) you to add specializations to std::numeric_limits and you can do just that.

#include <limits>
#include <atomic>
#include <iostream>

template<typename T>
class std::numeric_limits<std::atomic<T>> : public std::numeric_limits<T> {};

int main()
{
    std::cout << std::numeric_limits<std::atomic<int>>::max();
}

answered Jul 26, 2017 at 19:40

Ivaylo Valchev

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2 Comments

KABoissonneault Over a year ago

I'm not sure if you're allowed to specialize standard templates with types from the standard library

user743382 Over a year ago

I would be very surprised if this is allowed. Can you back up your claim?

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