*This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of Resolved status.*

**Section:** 20.16.3 [ratio.ratio] **Status:** Resolved
**Submitter:** Vicente Juan Botet Escribá **Opened:** 2009-12-07 **Last modified:** 2018-05-20

**Priority: **Not Prioritized

**View all other** issues in [ratio.ratio].

**View all issues with** Resolved status.

**Discussion:**

CopyConstruction and Assignment between `ratio`s having the same
normalized form. Current
N3000
do not allows to copy-construct or assign `ratio` instances of
`ratio` classes having the same normalized form.

Two `ratio` classes `ratio<N1,D1>` and
`ratio<N2,D2>` have the same normalized form if

ratio<N1, D1>::num == ratio<N2, D2>::num && ratio<N1, D1>::den == ratio<N2, D2>::den

This simple example

ratio<1,3> r1; ratio<3,9> r2; r1 = r2; // (1)

fails to compile in (1). Other example

ratio<1,3> r1; ratio_subtract<ratio<2,3>, ratio<1,3>>::type r2; r1 = r2;

The nested type of `ratio_subtract<ratio<2,3>,
ratio<1,3>>` could be `ratio<3,9>` so the compilation
could fail. It could also be `ratio<1,3>` and the compilation
succeeds.

In 20.16.4 [ratio.arithmetic] 3 and similar clauses

3 The nested typedef

typeshall be a synonym forratio<T1, T2>whereT1has the valueR1::num * R2::den - R2::num * R1::denandT2has the valueR1::den * R2::den.

the meaning of synonym let think that the result shall be a normalized
`ratio` equivalent to `ratio<T1, T2>`, but there is not an
explicit definition of what synonym means in this context.

Additionally we should add a typedef for accessing the normalized
`ratio`, and change 20.16.4 [ratio.arithmetic] to return only this
*normalized* result.

*[
2010 Pittsburgh:
]*

There is no consensus to add the converting copy constructor or converting copy assignment operator. However there was consensus to add the typedef.

Proposed wording modified. Original proposed wording preserved here. Moved to Review.

Make

ratiodefault constructible, copy-constructible and assignable from anyratiowhich has the same reduced form.Add to 20.16.3 [ratio.ratio] synopsis

template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; typedef ratio<num, den> type; ratio() = default; template <intmax_t N2, intmax_t D2> ratio(const ratio<N2, D2>&); template <intmax_t N2, intmax_t D2> ratio& operator=(const ratio<N2, D2>&); };Add to 20.16.3 [ratio.ratio]:

Two ratio classes

ratio<N1,D1>andratio<N2,D2>have the same reduced form ifratio<N1,D1>::typeis the same type asratio<N2,D2>::typeAdd a new section: [ratio.cons]

Construction and assignment [ratio.cons]template <intmax_t N2, intmax_t D2> ratio(const ratio<N2, D2>& r);

Effects:Constructs aratioobject.

Remarks:This constructor shall not participate in overload resolution unlessrhas the same reduced form as*this.template <intmax_t N2, intmax_t D2> ratio& operator=(const ratio<N2, D2>& r);

Effects:None.

Returns:*this.

Remarks:This operator shall not participate in overload resolution unlessrhas the same reduced form as*this.Change 20.16.4 [ratio.arithmetic]

Implementations may use other algorithms to compute these values. If overflow occurs, a diagnostic shall be issued.

template <class R1, class R2> struct ratio_add { typedefsee belowtype; };The nested typedef

typeshall be a synonym forratio<T1, T2>::typewhereT1has the valueR1::num * R2::den + R2::num * R1::denandT2has the valueR1::den * R2::den.template <class R1, class R2> struct ratio_subtract { typedefsee belowtype; };The nested typedef

typeshall be a synonym forratio<T1, T2>::typewhereT1has the valueR1::num * R2::den - R2::num * R1::denandT2has the valueR1::den * R2::den.template <class R1, class R2> struct ratio_multiply { typedefsee belowtype; };The nested typedef

typeshall be a synonym forratio<T1, T2>::typewhereT1has the valueR1::num * R2::numandT2has the valueR1::den * R2::den.template <class R1, class R2> struct ratio_divide { typedefsee belowtype; };The nested typedef

typeshall be a synonym forratio<T1, T2>::typewhereT1has the valueR1::num * R2::denandT2has the valueR1::den * R2::num.

*[
2010-03-27 Howard adds:
]*

Daniel brought to my attention the recent addition of the typedef

typeto the FCD N3092:typedef ratio type;This issue was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidently applied to the FCD, and incorrectly. The FCD version of the typedef refers to

ratio<N, D>, but the typedef is intended to refer toratio<num, den>which in general is not the same type.I've updated the wording to diff against N3092.

*[Batavia: NAD EditorialResolved - see rationale below]*

**Rationale:**

Already fixed in working draft

**Proposed resolution:**

Add to 20.16.3 [ratio.ratio] synopsis

template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; typedef ratio<num, den> type; };