std::adjacent_difference

Since C++17

Header: <numeric>

Let T be the value type of decltype(first).

# Declarations

template< class InputIt, class OutputIt >
OutputIt adjacent_difference( InputIt first, InputIt last,
OutputIt d_first );

(constexpr since C++20)

template< class ExecutionPolicy,
class ForwardIt1, class ForwardIt2 >
ForwardIt2 adjacent_difference( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first );

(since C++17)

template< class InputIt, class OutputIt, class BinaryOp >
OutputIt adjacent_difference( InputIt first, InputIt last,
OutputIt d_first, BinaryOp op );

(constexpr since C++20)

template< class ExecutionPolicy,
class ForwardIt1, class ForwardIt2, class BinaryOp >
ForwardIt2 adjacent_difference( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first, BinaryOp op );

(since C++17)

# Parameters

first, last: the range of elements
d_first: the beginning of the destination range
policy: the execution policy to use
op: binary operation function object that will be applied. The signature of the function should be equivalent to the following: Ret fun(const Type1 &a, const Type2 &b); The signature does not need to have const &. The types Type1 and Type2 must be such that an object of type iterator_traits::value_type can be implicitly converted to both of them. The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret.

# Return value

Iterator to the element past the last element written, or d_first if [first,last) is empty.

# Notes

acc was introduced because of the resolution of LWG issue 539. The reason of using acc rather than directly calculating the differences is because the semantic of the latter is confusing if the following types mismatch:

acc serves as the intermediate object to cache values of the iterated elements:

# Example

#include <array>
#include <functional>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
 
void println(auto comment, const auto& sequence)
{
    std::cout << comment;
    for (const auto& n : sequence)
        std::cout << n << ' ';
    std::cout << '\n';
};
 
int main()
{
    // Default implementation - the difference between two adjacent items
    std::vector v{4, 6, 9, 13, 18, 19, 19, 15, 10};
    println("Initially, v = ", v);
    std::adjacent_difference(v.begin(), v.end(), v.begin());
    println("Modified v = ", v);
 
    // Fibonacci
    std::array<int, 10> a {1};
    std::adjacent_difference(std::begin(a), std::prev(std::end(a)),
                             std::next(std::begin(a)), std::plus<>{});
    println("Fibonacci, a = ", a);
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 242	C++98	op could not have side effects	it cannot modifythe ranges involved
LWG 539	C++98	the type requirements needed for the resultevaluations and assignments to be valid were missing	added
LWG 3058	C++17	for overloads (2,4), the result of each invocationof operator- or op was assigned to a temporaryobject, and that object is assigned to the output range	assign the resultsto the outputrange directly