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Showing content from https://cplusplus.com/reference/numeric/adjacent_difference/ below:

function template

<numeric>

std::adjacent_difference difference (1) 
template <class InputIterator, class OutputIterator>   OutputIterator adjacent_difference (InputIterator first, InputIterator last,                                       OutputIterator result);
custom (2) 
template <class InputIterator, class OutputIterator, class BinaryOperation>   OutputIterator adjacent_difference ( InputIterator first, InputIterator last,                                        OutputIterator result, BinaryOperation binary_op );

Compute adjacent difference of range

Assigns to every element in the range starting at result the difference between its corresponding element in the range [first,last) and the one preceding it (except for *result, which is assigned *first).

If x represents an element in [first,last) and y represents an element in result, the ys can be calculated as:

y0 = x0
y1 = x1 - x0
y2 = x2 - x1
y3 = x3 - x2
y4 = x4 - x3
... ... ...


The default operation is to calculate the difference, but some other operation can be specified as binary_op instead.

The behavior of this function template is equivalent to:


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template <class InputIterator, class OutputIterator>
   OutputIterator adjacent_difference (InputIterator first, InputIterator last,
                                       OutputIterator result)
{
  if (first!=last) {
    typename iterator_traits<InputIterator>::value_type val,prev;
    *result = prev = *first;
    while (++first!=last) {
      val = *first;
      *++result = val - prev;  // or: *++result = binary_op(val,prev)
      prev = val;
    }
    ++result;
  }
  return result;
}

Parameters
first, last
Input iterators to the initial and final positions in a sequence. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
result
Output iterator to the initial position in the destination sequence where the differences are stored. The range starts at result and shall have a size large enough to contain as many elements as the range above ([first,last)).
binary_op
Binary operation taking as arguments two elements of the type pointed by InputIterator, and returning the result of the replacement for the difference operation.
This can either be a function pointer or a function object.

Return value An iterator pointing to past the last element of the destination sequence where resulting elements have been stored.

Example 
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// adjacent_difference example
#include <iostream>     // std::cout
#include <functional>   // std::multiplies
#include <numeric>      // std::adjacent_difference

int myop (int x, int y) {return x+y;}

int main () {
  int val[] = {1,2,3,5,9,11,12};
  int result[7];

  std::adjacent_difference (val, val+7, result);
  std::cout << "using default adjacent_difference: ";
  for (int i=0; i<7; i++) std::cout << result[i] << ' ';
  std::cout << '\n';

  std::adjacent_difference (val, val+7, result, std::multiplies<int>());
  std::cout << "using functional operation multiplies: ";
  for (int i=0; i<7; i++) std::cout << result[i] << ' ';
  std::cout << '\n';

  std::adjacent_difference (val, val+7, result, myop);
  std::cout << "using custom function: ";
  for (int i=0; i<7; i++) std::cout << result[i] << ' ';
  std::cout << '\n';
  return 0;
}

Output: 
using default adjacent_difference: 1 1 1 2 4 2 1
using functional operation multiplies: 1 2 6 15 45 99 132
using custom function: 1 3 5 8 14 20 23


Complexity Linear in the distance between first and last, minus one (in number of subtractions or applications of binary_op).

Data races The elements in the range [first1,last1) are accessed (each element is accessed exactly once).
The elements in the range beginning at result are modified.

Exceptions Throws if any of binary_op, the assignments or an operation on an iterator throws.
Note that invalid arguments cause undefined behavior.

See also
accumulate
Accumulate values in range (function template)
inner_product
Compute cumulative inner product of range (function template)
partial_sum
Compute partial sums of range (function template)

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