# std::generate

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< cpp‎ | 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)

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.

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 `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:

Run this code

    #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

### See also

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)

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