Add atomic and double

Question

I want to add two values:

auto size = new std::atomic<double>(0);
double packet_size = 64e3;
*size += packet_size;

But I got an error.

no match for ‘operator+=’ (operand types are ‘std::atomic<double>’ and ‘double’)

How should I correctly add these two numbers?

`std::atomic` is not to appear until C++20. You got a compiler supporting it? — DeiDei, Dec 19 '17 at 11:39
@DeiDei; not even complaining... http://coliru.stacked-crooked.com/a/24b749dce3f8d705 — user1810087, Dec 19 '17 at 11:42
You may want to look at this answer https://stackoverflow.com/questions/23116279/how-to-perform-basic-operations-with-stdatomic-when-the-type-is-not-integral — Emerald Weapon, Dec 19 '17 at 11:45
@user1810087 That uses `operator+`, so it implicitly converts the `atomic` to `double`, adds them together, and assigns back to the atomic variable. That's kind of not atomic. — DeiDei, Dec 19 '17 at 11:46

Michaël Roy · Answer 1 · 2019-11-20T06:57:18.417

Even tough you can create atomic<float> and atomic<double>, atomic operators are not defined for floating point atomics. This is because there is no x86 (nor ARM) assembly instruction for atomically add floating point values.

A workaround is to use the compare_exchange operations to increment/change your atomic variable.

#include <atomic>

int main()
{
    std::atomic<int> i{};
    i += 3;

    //  Peter Cordes pointed out in a comment below that using 
    //  compare_exchange_weak() may be better suited for most 
    //  uses.
    //  Then again, the needed strength of the exchange depends 
    //  on your application, and the hardware it's intended to run on.  

    std::atomic<double> f{};
    for (double g = f; !f.compare_exchange_strong(g, g + 1.0);)
      ;
    return 0;
}

If you're using a loop already, use `compare_exchange_weak` so it compiles more nicely on LL/SC ISAs like ARM. Otherwise this compiles to a nested loop on ARM. Like most RISC, ARM doesn't have instructions to do any atomic RMW except a paired load/store-conditional, with the ALU operation implemented however you want. — Peter Cordes, Nov 19 '19 at 21:42

Add atomic and double

1 Answers1