std::fill

Since C++26

Header: <algorithm>

Assigns the given value to all elements in the range [first,last).

# Declarations

template< class ForwardIt, class T >
void fill( ForwardIt first, ForwardIt last, const T& value );

(constexpr since C++20) (until C++26)

template< class ForwardIt, class T = typename std::iterator_traits
<ForwardIt>::value_type >
constexpr void fill( ForwardIt first, ForwardIt last,
const T& value );

(since C++26)

template< class ExecutionPolicy, class ForwardIt, class T >
void fill( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, const T& value );

(since C++17) (until C++26)

template< class ExecutionPolicy,
class ForwardIt, class T = typename std::iterator_traits
<ForwardIt>::value_type >
void fill( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, const T& value );

(since C++26)

# Parameters

first, last: the range of elements to modify
value: the value to be assigned
policy: the execution policy to use

# Notes

Feature-test macro Value Std Feature __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (1,2)

# Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <vector>
 
void println(const auto& seq)
{
    for (const auto& e : seq)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8};
    println(v);
 
    // set all of the elements to 8
    std::fill(v.begin(), v.end(), 8);
    println(v);
 
    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
    println(nums);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::fill(nums.begin(), nums.end(), {4, 2});
    #else
        std::fill(nums.begin(), nums.end(), std::complex<double>{4, 2});
    #endif
    println(nums);
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	T was required to be CopyAssignable, butT is not always writable to ForwardIt	required to be writable instead