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**Section:** 29.5 [complex.numbers] **Status:** CD1
**Submitter:** Gabriel Dos Reis **Opened:** 2002-11-08 **Last modified:** 2017-02-03

**Priority: **Not Prioritized

**View other** active issues in [complex.numbers].

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**Discussion:**

The absence of explicit description of `std::complex<T>` layout
makes it imposible to reuse existing software developed in traditional
languages like Fortran or C with unambigous and commonly accepted
layout assumptions. There ought to be a way for practitioners to
predict with confidence the layout of `std::complex<T>` whenever `T`
is a numerical datatype. The absence of ways to access individual
parts of a `std::complex<T>` object as lvalues unduly promotes
severe pessimizations. For example, the only way to change,
independently, the real and imaginary parts is to write something like

complex<T> z; // ... // set the real part to r z = complex<T>(r, z.imag()); // ... // set the imaginary part to i z = complex<T>(z.real(), i);

At this point, it seems appropriate to recall that a complex number
is, in effect, just a pair of numbers with no particular invariant to
maintain. Existing practice in numerical computations has it that a
complex number datatype is usually represented by Cartesian
coordinates. Therefore the over-encapsulation put in the specification
of `std::complex<>` is not justified.

**Proposed resolution:**

Add the following requirements to 29.5 [complex.numbers] as 26.3/4:

If

zis an lvalue expression of typecvstd::complex<T>then

- the expression
reinterpret_cast<cv T(&)[2]>(z)is well-formed; andreinterpret_cast<cv T(&)[2]>(z)[0]designates the real part ofz; andreinterpret_cast<cv T(&)[2]>(z)[1]designates the imaginary part ofz.Moreover, if

ais an expression of pointer typecvcomplex<T>*and the expressiona[i]is well-defined for an integer expressionithen:

reinterpret_cast<cv T*>(a)[2*i]designates the real part ofa[i]; andreinterpret_cast<cv T*>(a)[2*i+1]designates the imaginary part ofa[i].

In 29.5.2 [complex] and 29.5.3 [complex.special] add the following member functions
(changing `T` to concrete types as appropriate for the specializations).

void real(T); void imag(T);

Add to 29.5.4 [complex.members]

T real() const;

Returns:the value of the real componentvoid real(T val);Assigns

valto the real component.T imag() const;

Returns:the value of the imaginary componentvoid imag(T val);Assigns

valto the imaginary component.

*[Kona: The layout guarantee is absolutely necessary for C
compatibility. However, there was disagreement about the other part
of this proposal: retrieving elements of the complex number as
lvalues. An alternative: continue to have real() and imag() return
rvalues, but add set_real() and set_imag(). Straw poll: return
lvalues - 2, add setter functions - 5. Related issue: do we want
reinterpret_cast as the interface for converting a complex to an
array of two reals, or do we want to provide a more explicit way of
doing it? Howard will try to resolve this issue for the next
meeting.]*

*[pre-Sydney: Howard summarized the options in n1589.]*

*[
Bellevue:
]*

Second half of proposed wording replaced and moved to Ready.

*[
Pre-Sophia Antipolis, Howard adds:
]*

Added the members to 29.5.3 [complex.special] and changed from Ready to Review.

*[
Post-Sophia Antipolis:
]*

Moved from WP back to Ready so that the "and 29.5.3 [complex.special]" in the proposed resolution can be officially applied.

**Rationale:**

The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.