std::ranges:: generate

From cppreference.com

Defined in header `<algorithm>`
Call signature
template < std:: input_or_output_iterator O, std:: sentinel_for < O > S, std:: copy_constructible F > requires std:: invocable < F & > && std:: indirectly_writable < O, std:: invoke_result_t < F & >> constexpr O generate ( O first, S last, F gen ) ;	(1)	(since C++20)
template < class R, std:: copy_constructible F > requires std:: invocable < F & > && ranges:: output_range < R, std:: invoke_result_t < F & >> constexpr ranges:: borrowed_iterator_t < R > generate ( R && r, F gen ) ;	(2)	(since C++20)

1) Assigns the result of successive invocations of the function object gen to each element in the range

first

last

2) Same as (1) , but uses r as the range, as if using ranges:: begin ( r ) as first and ranges:: end ( r ) as last .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the range of elements to modify
r	-	the range of elements to modify
gen	-	the generator function object

Return value

An output iterator that compares equal to last .

Complexity

Exactly ranges:: distance ( first, last ) invocations of gen ( ) and assignments.

Possible implementation

struct generate_fn
{
    template<std::input_or_output_iterator O, std::sentinel_for<O> S,
             std::copy_constructible F>
    requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
    constexpr O operator()(O first, S last, F gen) const
    {
        for (; first != last; *first = std::invoke(gen), ++first)
        {}
        return first;
    }
 
    template<class R, std::copy_constructible F>
    requires std::invocable<F&> && ranges::output_range<R, std::invoke_result_t<F&>>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, F gen) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(gen));
    }
};
 
inline constexpr generate_fn generate {};

Example

Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <random>
#include <string_view>
 
auto dice()
{
    static std::uniform_int_distribution<int> distr{1, 6};
    static std::random_device device;
    static std::mt19937 engine {device()};
    return distr(engine);
}
 
void iota(auto& r, int init)
{
    std::ranges::generate(r, [init] mutable { return init++; });
}
 
void print(std::string_view comment, const auto& v)
{
    for (std::cout << comment; int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::array<int, 8> v;
 
    std::ranges::generate(v.begin(), v.end(), dice);
    print("dice: ", v);
    std::ranges::generate(v, dice);
    print("dice: ", v);
 
    iota(v, 1);
    print("iota: ", v);
}

Possible output:

dice: 4 3 1 6 6 4 5 5
dice: 4 2 5 3 6 2 6 2
iota: 1 2 3 4 5 6 7 8

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::generate_n (C++20)	saves the result of N applications of a function (algorithm function object)
ranges::fill (C++20)	assigns a range of elements a certain value (algorithm function object)
ranges::fill_n (C++20)	assigns a value to a number of elements (algorithm function object)
ranges::transform (C++20)	applies a function to a range of elements (algorithm function object)
ranges::generate_random (C++26)	fills a range with random numbers from a uniform random bit generator (algorithm function object)
generate	assigns the results of successive function calls to every element in a range (function template)