Type-specific static declaration in a template function

Question

I've just written the following snippet:

// Fix for MinGW 4.9.2 bug - std::log2 is missing there
template <typename T>
T log2 (T value)
{
    static const T l2 = std::log(T(2));
    return std::log(value) / l2;
}

Obviously, l2 should be unique for each T type because it is of type T. But does it actually work that way as per the C++ standard?

did you try it? Why it should not work? each instantiation gets its own static variable — 463035818_is_not_an_ai, Jul 15 '16 at 17:26
@tobi303: It struck me that in the nature of static declarations, it may be ONLY defined once (say, for whatever type instantiates it first). I can only try it with one compiler in the next couple days, but I'm actually writing cross-platform code here. — Violet Giraffe, Jul 15 '16 at 17:28
no, it is defined ONLY once FOR EACH instantiation of the template. I had a link to a nice video with an example on that, maybe I will find it... — 463035818_is_not_an_ai, Jul 15 '16 at 17:29
here is the [link](https://www.youtube.com/watch?v=WXeu4fj3zOs) (around 6:00) and [this](http://stackoverflow.com/questions/38059936/template-function-static-variable/38059959#38059959) is almost a dupe — 463035818_is_not_an_ai, Jul 15 '16 at 17:30
[This](http://stackoverflow.com/a/14663369/241631) answers your question, even though that question is about something unrelated. — Praetorian, Jul 15 '16 at 17:34
related: http://stackoverflow.com/questions/37345946/why-are-static-members-of-template-classes-not-unique. It is for classes but it has the same behavior with functions. — NathanOliver, Jul 15 '16 at 17:43

score 2 · Accepted Answer · answered Jul 15 '16 at 17:36

Note that once instantiated

log2<double>

and

log2<float>

are two completely different functions. They both get their own static variable. After template instantiation, this is the same situation as if you had two functions:

double log2(double) {
    static double x;
    /*...*/
}

and

float log2(float) {
    static float x;
    /*...*/
}

This is also nicely explained here in an example around 6:00.

Type-specific static declaration in a template function

1 Answers1