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**Section:** 28.5.3.3 [rand.req.urng] **Status:** C++20
**Submitter:** Casey Carter **Opened:** 2018-08-09 **Last modified:** 2021-02-25

**Priority: **3

**View all other** issues in [rand.req.urng].

**View all issues with** C++20 status.

**Discussion:**

28.5.3.3 [rand.req.urng]
paragraph 2 specifies axioms for
the `UniformRandomBitGenerator` concept:

2 Let

gbe an object of typeG.GmodelsUniformRandomBitGeneratoronly if(2.1) — both

G::min()andG::max()are constant expressions (7.7 [expr.const]),(2.2) —

G::min() < G::max(),(2.3) —

G::min() <= g(),(2.4) —

g() <= G::max(), and(2.5) —

g()has amortized constant complexity.

Bullets 2.1 and 2.2 are both compile-time requirements that ought to be validated by the concept.

*[2018-08-20 Priority set to 3 after reflector discussion]*

This wording is relative to N4791.

Modify 28.5.3.3 [rand.req.urng] as follows:

1 A

uniform random bit generatorgof typeGis a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [Note:The degree to whichg's results approximate the ideal is often determined statistically.—end note]template<auto> structrequire-constant; //exposition-onlytemplate<class G> concept UniformRandomBitGenerator = Invocable<G&> && UnsignedIntegral<invoke_result_t<G&>> && requires { { G::min() } -> Same<invoke_result_t<G&>>; { G::max() } -> Same<invoke_result_t<G&>>; typenamerequire-constant<G::min()>; typenamerequire-constant<G::max()>; requires G::min() < G::max(); };2 Let

gbe an object of typeG.GmodelsUniformRandomBitGeneratoronly if

~~(2.1) — both~~G::min()andG::max()are constant expressions (7.7 [expr.const]),

~~(2.2) —~~G::min() < G::max(),(2.3) —

G::min() <= g(),(2.4) —

g() <= G::max(), and(2.5) —

g()has amortized constant complexity.3 A class

Gmeets theuniform random bit generatorrequirements ifGmodelsUniformRandomBitGenerator,invoke_result_t<G&>is an unsigned integer type (6.8.2 [basic.fundamental]), andGprovides a nestedtypedef-nameresult_typethat denotes the same type asinvoke_result_t<G&>.

*[2020-02-13; Prague]*

LWG provided some improved wording.

*[2020-02 Status to Immediate on Thursday night in Prague.]*

**Proposed resolution:**

This wording is relative to N4849.

Modify 28.5.3.3 [rand.req.urng] as follows:

1 A

*uniform random bit generator*`g`of type`G`is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [*Note:*The degree to which`g`'s results approximate the ideal is often determined statistically.—*end note*]template<class G> concept uniform_random_bit_generator = invocable<G&> && unsigned_integral<invoke_result_t<G&>> && requires { { G::min() } -> same_as<invoke_result_t<G&>>; { G::max() } -> same_as<invoke_result_t<G&>>; requires bool_constant<(G::min() < G::max())>::value; };

-2- Let

`g`be an object of type`G`.`G`models`uniform_random_bit_generator`only if~~(2.1) — both~~`G::min()`and`G::max()`are constant expressions (7.7 [expr.const]),~~(2.2) —~~`G::min() < G::max()`,(2.3) —

`G::min() <= g()`,(2.4) —

`g() <= G::max()`, and(2.5) —

`g()`has amortized constant complexity.3 A class

`G`meets the*uniform random bit generator*requirements if`G`models`uniform_random_bit_generator`,`invoke_result_t<G&>`is an unsigned integer type (6.8.2 [basic.fundamental]), and`G`provides a nested*typedef-name*`result_type`that denotes the same type as`invoke_result_t<G&>`.