Title
UniformRandomBitGenerator should validate min and max
Status
new
Section
[rand.req.urng]
Submitter
Casey Carter

Created on 2018-08-09.00:00:00, last changed 2018-08-20.12:41:52.

#### Messages

Date: 2018-08-20.12:41:52

Proposed resolution:

This wording is relative to N4762.

1. Modify [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<auto> struct require-constant; // exposition-only

template<class G>
concept UniformRandomBitGenerator =
Invocable<G&> && UnsignedIntegral<invoke_result_t<G&>> &&
requires {
G::min(); requires Same<decltype(G::min()), invoke_result_t<G&>>;
G::max(); requires Same<decltype(G::max()), invoke_result_t<G&>>;
typename require-constant<G::min()>;
typename require-constant<G::max()>;
requires G::min() < G::max();
};
```

2 Let g be an object of type G. G models UniformRandomBitGenerator only if

(2.1) — both G​::​min() and G​::​max() are constant expressions ([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 UniformRandomBitGenerator, invoke_­result_­t<G&> is an unsigned integer type ([basic.fundamental]), and G provides a nested typedef-name result_­type that denotes the same type as invoke_­result_­t<G&>.

Date: 2018-08-20.00:00:00

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

Date: 2018-08-09.00:00:00

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

2 Let g be an object of type G. G models UniformRandomBitGenerator only if

(2.1) — both G::min() and G::max() are constant expressions ([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.

History
Date User Action Args
2018-08-20 12:41:52adminsetmessages: + msg10079