### 2459. `std::polar` should require a non-negative rho

**Section:** 28.4.7 [complex.value.ops] **Status:** C++17
**Submitter:** Marshall Clow **Opened:** 2014-10-22 **Last modified:** 2017-07-30 20:15:43 UTC

**Priority: **0

**View all other** issues in [complex.value.ops].

**View all issues with** C++17 status.

**Discussion:**

Different implementations give different answers for the following code:

#include <iostream>
#include <complex>
int main ()
{
std::cout << std::polar(-1.0, -1.0) << '\n';
return 0;
}

One implementation prints:

(nan, nan)

Another:

(-0.243068, 0.243068)

Which is correct? Or neither?

In this list,
Howard Hinnant wrote:

I've read this over several times. I've consulted C++11, C11, and IEC 10967-3. [snip]

I'm finding:

The magnitude of a complex number `== abs(c) == hypot(c.real(), c.imag())` and is always non-negative
(by all three of the above standards).

Therefore no complex number exists for which `abs(c) < 0`.

Therefore when the first argument to `std::polar` (which is called `rho`) is negative, no complex number
can be formed which meets the post-conidtion that `abs(c) == rho`.

One could argue that this is already covered in 28.4 [complex.numbers]/3, but I think it's worth making explicit.

*[2015-02, Cologne]*

Discussion on whether theta should also be constrained.

TK: infinite theta doesn't make sense, whereas infinite rho does (theta is on a compact domain, rho is on a non-compact domain).

AM: We already have a narrow contract, so I don't mind adding further requirements. Any objections to requiring that theta be finite?

Some more discussion, but general consensus. Agreement that if someone finds the restrictions problematic, they should write
a proper paper to address how `std::polar` should behave. For now, we allow infinite rho (but not NaN and not negative),
and require finite theta.

No objections to tentatively ready.

**Proposed resolution:**

This wording is relative to N4296.

Change 28.4.7 [complex.value.ops] around p9 as indicated

template<class T> complex<T> polar(const T& rho, const T& theta = 0);

-?- *Requires*: `rho` shall be non-negative and non-NaN. `theta` shall be finite.

-9- *Returns*: The complex value corresponding to a complex number whose magnitude is `rho` and whose
phase angle is `theta`.