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std::generate

Defined in header `<algorithm>`
	(1)
template< class ForwardIt, class Generator > void generate( ForwardIt first, ForwardIt last, Generator g );		(until C++20)
template< class ForwardIt, class Generator > constexpr void generate( ForwardIt first, ForwardIt last, Generator g );		(since C++20)
template< class ExecutionPolicy, class ForwardIt, class Generator > void generate( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Generator g );	(2)	(since C++17)

1) Assigns each element in range [first, last) a value generated by the given function object g.

2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

`std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>` is `true`.	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`.	(since C++20)

Parameters

first, last - the range of elements to generate

policy - the execution policy to use. See execution policy for details.

generator function object that will be called.

The signature of the function should be equivalent to the following:

Ret fun();

The type Ret must be such that an object of type ForwardIt can be dereferenced and assigned a value of type Ret.

Type requirements

-ForwardIt must meet the requirements of LegacyForwardIterator.

Return value

(none).

Complexity

Exactly std::distance(first, last) invocations of g() and assignments.

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

template<class ForwardIt, class Generator>
constexpr //< since C++20
void generate(ForwardIt first, ForwardIt last, Generator g)
{
    for (; first != last; ++first)
        *first = g();
}

Example

#include <algorithm>
#include <iostream>
#include <vector>
 
void println(std::string_view fmt, auto const& v)
{
    for (std::cout << fmt; auto const& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
};
 
 
int f()
{
    static int i;
    return ++i;
}
 
int main()
{
    std::vector<int> v(5);
 
    std::generate(v.begin(), v.end(), f);
    println("v: ", v);
 
    // Initialize with default values 0,1,2,3,4 from a lambda function
    // Equivalent to std::iota(v.begin(), v.end(), 0);
    std::generate(v.begin(), v.end(), [n = 0] () mutable { return n++; });
    println("v: ", v);
}

Output:

v: 1 2 3 4 5
v: 0 1 2 3 4

fill	copy-assigns the given value to every element in a range (function template)
generate_n	assigns the results of successive function calls to N elements in a range (function template)
iota (C++11)	fills a range with successive increments of the starting value (function template)
ranges::generate (C++20)	saves the result of a function in a range (niebloid)