fill in C++ STL (original) (raw)

Last Updated : 20 Jan, 2026

std::fill is a C++ STL algorithm defined in the header that assigns a specified value to every element in a given range [first, last). It works with arrays and STL containers that provide at least forward iterators (such as vector, list, and deque).

C++ `

#include #include #include using namespace std;

int main() { vector v(8);

// Fill elements from index 2 to index 6 with value 4
fill(v.begin() + 2, v.end() - 1, 4);

for (int x : v)
    cout << x << " ";

return 0;

}

**Explanation: This code fills the elements from index 2 to 6 of the vector with the value 4 using std::fill.

Syntax

std::fill(first, last, value);

Parameters

**first: Iterator pointing to the first element of the range.
**last: Iterator pointing to the position just after the last element of the range.
**value: The value to be assigned to each element in the range.

Note: The range is half-open — first is included, but last is excluded.

**Example 1: Using std::fill with an Array

C++ `

#include #include using namespace std;

int main() { int arr[10];

// Fill entire array with value 4
fill(arr, arr + 10, 4);

for (int i = 0; i < 10; i++)
    cout << arr[i] << " ";

return 0;

}

Output

4 4 4 4 4 4 4 4 4 4

**Explanation: This code assigns the value 4 to all elements of the array by applying std::fill on the entire array range.

**Example 2: Using std::fill with a List

C++ `

#include #include #include using namespace std;

int main() { list ml = {10, 20, 30};

// Fill all elements of the list with value 4
fill(ml.begin(), ml.end(), 4);

for (int x : ml)
    cout << x << " ";

return 0;

}

**Explanation: This code updates every element of the list to 4 by calling std::fill from begin() to end()