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

Parameters

first, last - the range of elements to generate

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

g

-

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)
{
    while (first != last) {
        *first++ = g();
    }
}

Example

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
 
int f()
{ 
    static int i;
    return ++i;
}
 
int main()
{
    std::vector<int> v(5);
    auto print = [&] {
        for (std::cout << "v: "; auto iv: v)
            std::cout << iv << " ";
        std::cout << "\n";
    };
 
    std::generate(v.begin(), v.end(), f);
    print();
 
    // 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++; });
    print();
}

Output:

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

Compiler support
Freestanding and hosted
Language
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)
Technical specifications
Symbols index
External libraries

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)