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std::generate

Defined in header <algorithm>

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

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

(1)

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

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 ForwardIterator.

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 three 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>
void generate(ForwardIt first, ForwardIt last, Generator g)
{
    while (first != last) {
        *first++ = g();
    }
}

Example

The following code fills a vector with random numbers:

#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
 
int main()
{
    std::vector<int> v(5);
    std::generate(v.begin(), v.end(), std::rand); // Using the C function rand()
 
    std::cout << "v: ";
    for (auto iv: v) {
        std::cout << iv << " ";
    }
    std::cout << "\n";
 
    // Initialize with default values 0,1,2,3,4 from a lambda function
    // Equivalent to std::iota(v.begin(), v.end(), 0);
    int n = {0};
    std::generate(v.begin(), v.end(), [&n]{ return n++; });
 
    std::cout << "v: ";
    for (auto iv: v) {
        std::cout << iv << " ";
    }
    std::cout << "\n";
}

Possible output:

v: 52894 15984720 41513563 41346135 51451456
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)