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std::generate - cppreference.com

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Defined in header <algorithm>

template< class ForwardIt, class Generator > void generate( ForwardIt first, ForwardIt last, Generator g );

 (1) (constexpr 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 participates in overload resolution only if all following conditions are satisfied:

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 pair of iterators defining the range of elements to generate
policy - the execution policy to use
g - generator function object that will be called.

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

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.

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, const auto& v)
{
    for (std::cout << fmt; const auto& 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

See also

copy-assigns the given value to every element in a range
(function template) [edit]
assigns the results of successive function calls to N elements in a range
(function template) [edit]
fills a range with successive increments of the starting value
(function template) [edit]
saves the result of a function in a range
(algorithm function object)[edit]