std::generate() algorithm

since C++20

// (1)
template< class ForwardIt, class Generator >
constexpr void generate( ForwardIt first, ForwardIt last, Generator g );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Generator >
void generate( ExecutionPolicy&& policy,
               ForwardIt first, ForwardIt last, Generator g );

since C++17

// (1)
template< class ForwardIt, class Generator >
void generate( ForwardIt first, ForwardIt last, Generator g );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Generator >
void generate( ExecutionPolicy&& policy,
               ForwardIt first, ForwardIt last, Generator g );

until C++17

// (1)
template< class ForwardIt, class Generator >
void generate( ForwardIt first, ForwardIt last, Generator g );

• (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.

Overload Resolution

These overloads participate in overload resolution only if 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

LegacyForwardIterator

Return value

(none)

Complexity

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

Exceptions

The overloads with a template parameter named ExecutionPolicy report 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 none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

generate (1)

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

Examples

Main.cpp

#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