std::accumulate(3)

NAME
std::accumulate - std::accumulate

Synopsis
   Defined in header <numeric>
   template< class InputIt, class T > (1) (constexpr since C++20)
   T accumulate( InputIt first, InputIt last, T init );
   template< class InputIt, class T, class BinaryOp >
   T accumulate( InputIt first, InputIt last, T init, (2) (constexpr since C++20)
   BinaryOp op );
   Computes the sum of the given value init and the elements in the range
   [first, last).
   1) Initializes the accumulator acc (of type T) with the initial value init and then
   modifies it with
   acc = acc + *i
   (until C++20)
   acc = std::move(acc) + *i
   (since C++20) for every iterator i in the range [first, last) in order.
   2) Initializes the accumulator acc (of type T) with the initial value init and then
   modifies it with
   acc = op(acc, *i)
   (until C++20)
   acc = op(std::move(acc), *i)
   (since C++20) for every iterator i in the range [first, last) in order.
   If any of the following conditions is satisfied, the behavior is undefined:
     * T is not CopyConstructible.
     * T is not CopyAssignable.
     * op modifies any element of [first, last).
     * op invalidates any iterator or subrange in [first, last].

Parameters
   first, last - the range of elements to sum
   init - initial value of the sum
                 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);
   op -
                 The signature does not need to have const &.
                 The type Type1 must be such that an object of type T can be
                 implicitly converted to Type1. The type Type2 must be such that an
                 object of type InputIt can be dereferenced and then implicitly
                 converted to Type2. The type Ret must be such that an object of type
                 T can be assigned a value of type Ret.

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.

Return value
   acc after all modifications.

Possible implementation
                        accumulate (1)
   template<class InputIt, class T>
   constexpr // since C++20
   T accumulate(InputIt first, InputIt last, T init)
   {
       for (; first != last; ++first)
           init = std::move(init) + *first; // std::move since C++20
       return init;
   }
                        accumulate (2)
   template<class InputIt, class T, class BinaryOperation>
   constexpr // since C++20
   T accumulate(InputIt first, InputIt last, T init, BinaryOperation op)
   {
       for (; first != last; ++first)
           init = op(std::move(init), *first); // std::move since C++20
       return init;
   }

Notes
   std::accumulate performs a left fold. In order to perform a right fold, one must
   reverse the order of the arguments to the binary operator, and use reverse
   iterators.
   If left to type inference, op operates on values of the same type as init which can
   result in unwanted casting of the iterator elements. For example,
   std::accumulate(v.begin(), v.end(), 0) likely does not give the result one wishes
   for when v is of type std::vector<double>.

Example
// Run this code
 #include <functional>
 #include <iostream>
 #include <numeric>
 #include <string>
 #include <vector>
 int main()
 {
     std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
     int sum = std::accumulate(v.begin(), v.end(), 0);
     int product = std::accumulate(v.begin(), v.end(), 1, std::multiplies<int>());
     auto dash_fold = [](std::string a, int b)
     {
         return std::move(a) + '-' + std::to_string(b);
     };
     std::string s = std::accumulate(std::next(v.begin()), v.end(),
                        std::to_string(v[0]), // start with first element
                        dash_fold);
     // Right fold using reverse iterators
     std::string rs = std::accumulate(std::next(v.rbegin()), v.rend(),
                        std::to_string(v.back()), // start with last element
                        dash_fold);
     std::cout << "sum: " << sum << '\n'
               << "product: " << product << '\n'
               << "dash-separated string: " << s << '\n'
               << "dash-separated string (right-folded): " << rs << '\n';
 }

Output:
 sum: 55
 product: 3628800
 dash-separated string: 1-2-3-4-5-6-7-8-9-10
 dash-separated string (right-folded): 10-9-8-7-6-5-4-3-2-1
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 242 C++98 op could not have side effects it cannot modify the ranges
                        involved
   LWG 2055 C++20 acc was not moved while being it is moved
   (P0616R0) accumulated

See also
   adjacent_difference computes the differences between adjacent elements in a range
                       (function template)
   inner_product computes the inner product of two ranges of elements
                       (function template)
   partial_sum computes the partial sum of a range of elements
                       (function template)
   reduce similar to std::accumulate, except out of order
   (C++17) (function template)
   ranges::fold_left left-folds a range of elements
   (C++23) (niebloid)