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Attempts to generate random numbers with the generate_random member function of the random number generator or the distribution, which is expected to be more efficient. Falls back to element-wise generation if no generate_random member function is available.
Let fallback operation be calling ranges::generate (std::forward <R>(r), std::ref (g)) or ranges::generate (std::forward <R>(r), [&d, &g] { return std::invoke (d, g); }) for (1) or (3) respectively.
1) Calls
g.generate_random(std::forward <R>(r)) if this expression is well-formed.
Otherwise, let
I be
std::invoke_result_t <G&>. If
R models
sized_range, fills
r with
ranges::size (r) values of
I by performing an unspecified number of invocations of the form
g() or
g.generate_random(s), if such an expression is well-formed for a value
N and an object
s of type
std::span <I, N>.
Otherwise, performs the fallback operation.
3) Calls
d.generate_random(std::forward <R>(r), g) if this expression is well-formed.
Otherwise, let
I be
std::invoke_result_t <D&, G&>. If
R models
sized_range, fills
r with
ranges::size (r) values of type
I by performing an unspecified number of invocations of the form
std::invoke (d, g) or
d.generate_random(s, g), if such an expression is well-formed for a value
N and an object
s of type
std::span <I, N>.
Otherwise, performs the fallback operation.
2,4) Equivalent to
(1,3) respectively, where
r is obtained from
ranges::subrange <O, S>(std::move(first), last).
If the effects of (1) or (3) are not equivalent to those of the corresponding fallback operation, the behavior is undefined.
The value of N can differ between invocations. Implementations may select smaller values for shorter ranges.
The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
[edit] Parameters
first, last
-
the iterator-sentinel pair defining the
range of elements to which random numbers are written
r
-
the
range to which random numbers are written
g
-
uniform random bit generator
d
-
random number distribution object
At the time of the standardization of std::ranges::generate_random, there is no random number generator or distribution in the standard library that provides a generate_random member function.
std::ranges::generate_random can be more efficient when used with a user-defined random number generator that wraps an underlying vectorized API.
[edit] Example
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <random>
int main()
{
std::default_random_engine eng;
std::default_random_engine::result_type rs[16]{};
std::ranges::generate_random(rs, eng);
std::cout << std::left ;
for (int i{}; auto n : rs)
std::cout << std::setw (11) << n << (++i % 4 ? ' ' : '\n');
}Possible output:
16807 282475249 1622650073 984943658
1144108930 470211272 101027544 1457850878
1458777923 2007237709 823564440 1115438165
1784484492 74243042 114807987 1137522503
[edit] See also
saves the result of a function in a range
(algorithm function object)[edit]
specifies that a type qualifies as a uniform random bit generator
(concept) [edit]