std::ranges:: generate_random

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All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutation operations
Fold operations
Operations on uninitialized storage
Return types
Defined in header <random>
Call signature
template < class R, class G >

requires ranges:: output_range < R, std:: invoke_result_t < G & >> &&
std:: uniform_random_bit_generator < std:: remove_cvref_t < G >>
constexpr ranges:: borrowed_iterator_t < R >

generate_random ( R && r, G && g ) ;
(1) (since C++26)
template < class G, std:: output_iterator < std:: invoke_result_t < G & >> O,

std:: sentinel_for < O > S >
requires std:: uniform_random_bit_generator < std:: remove_cvref_t < G >>
constexpr O

generate_random ( O first, S last, G && g ) ;
(2) (since C++26)
template < class R, class G, class D >

requires ranges:: output_range < R, std:: invoke_result_t < D & , G & >> &&
std:: invocable < D & , G & > &&
std:: uniform_random_bit_generator < std:: remove_cvref_t < G >>
constexpr ranges:: borrowed_iterator_t < R >

generate_random ( R && r, G && g, D && d ) ;
(3) (since C++26)
template < class G, class D, std:: output_iterator < std:: invoke_result_t < D & , G & >> O,

std:: sentinel_for < O > S >
requires std:: invocable < D & , G & > &&
std:: uniform_random_bit_generator < std:: remove_cvref_t < G >>
constexpr O

generate_random ( O first, S last, G && g, D && d ) ;
(4) (since C++26)

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:

Parameters

first, last - iterator-sentinel pair that denotes the range to which random numbers are written
r - range to which random numbers are written
g - uniform random bit generator
d - random number distribution object

Notes

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.

Feature-test macro Value Std Feature
__cpp_lib_ranges_generate_random 202403L (C++26) std::ranges::generate_random

Example

Run this code 
#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

See also

(C++20)
saves the result of a function in a range
(algorithm function object)
(C++20)
specifies that a type qualifies as a uniform random bit generator
(concept)
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