std::ranges:: fold_left

From cppreference.com

Defined in header `<algorithm>`
Call signature
	(1)
template < std:: input_iterator I, std:: sentinel_for < I > S, class T, /* indirectly-binary-left-foldable */ < T, I > F > constexpr auto fold_left ( I first, S last, T init, F f ) ;			(since C++23) (until C++26)
template < std:: input_iterator I, std:: sentinel_for < I > S, class T = std:: iter_value_t < I > , /* indirectly-binary-left-foldable */ < T, I > F > constexpr auto fold_left ( I first, S last, T init, F f ) ;			(since C++26)
		(2)
template < ranges:: input_range R, class T, /* indirectly-binary-left-foldable */ < T, ranges:: iterator_t < R >> F > constexpr auto fold_left ( R && r, T init, F f ) ;				(since C++23) (until C++26)
template < ranges:: input_range R, class T = ranges:: range_value_t < R > , /* indirectly-binary-left-foldable */ < T, ranges:: iterator_t < R >> F > constexpr auto fold_left ( R && r, T init, F f ) ;				(since C++26)
Helper concepts
template < class F, class T, class I > concept /* indirectly-binary-left-foldable / = / see description */ ;			(3)	( exposition only* )

Left- folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(f(f(f(init, x ₁ ), x ₂ ), ...), x _n ) , where x ₁ , x ₂ , ..., x _n are elements of the range.

Informally, ranges::fold_left behaves like std::accumulate 's overload that accepts a binary predicate.

The behavior is undefined if [ first , last ) is not a valid range.

1) The range is

first

last

. Equivalent to return ranges:: fold_left_with_iter ( std :: move ( first ) , last, std :: move ( init ) , f ) . value .

2) Same as (1) , except that uses r as the range, as if by using ranges:: begin ( r ) as first and ranges:: end ( r ) as last .

3) Equivalent to:

Helper concepts
template < class F, class T, class I, class U > concept /indirectly-binary-left-foldable-impl/ = std:: movable < T > && std:: movable < U > && std:: convertible_to < T, U > && std:: invocable < F & , U, std:: iter_reference_t < I >> && std:: assignable_from < U & , std:: invoke_result_t < F & , U, std:: iter_reference_t < I >>> ;	(3A)	( exposition only* )
template < class F, class T, class I > concept /indirectly-binary-left-foldable/ = std:: copy_constructible < F > && std:: indirectly_readable < I > && std:: invocable < F & , T, std:: iter_reference_t < I >> && std:: convertible_to < std:: invoke_result_t < F & , T, std:: iter_reference_t < I >> , std:: decay_t < std:: invoke_result_t < F & , T, std:: iter_reference_t < I >>>> && /indirectly-binary-left-foldable-impl/ < F, T, I, std:: decay_t < std:: invoke_result_t < F & , T, std:: iter_reference_t < I >>>> ;	(3B)	( exposition only* )

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the range of elements to fold
r	-	the range of elements to fold
init	-	the initial value of the fold
f	-	the binary function object

Return value

An object of type U that contains the result of left- fold of the given range over f , where U is equivalent to std:: decay_t < std:: invoke_result_t < F & , T, std:: iter_reference_t < I >>> .

If the range is empty, U ( std :: move ( init ) ) is returned.

Possible implementations

struct fold_left_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class T = std::iter_value_t<I>,
             /* indirectly-binary-left-foldable */<T, I> F>
    constexpr auto operator()(I first, S last, T init, F f) const
    {
        using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>;
        if (first == last)
            return U(std::move(init));
        U accum = std::invoke(f, std::move(init), *first);
        for (++first; first != last; ++first)
            accum = std::invoke(f, std::move(accum), *first);
        return std::move(accum);
    }
 
    template<ranges::input_range R, class T = ranges::range_value_t<R>,
             /* indirectly-binary-left-foldable */<T, ranges::iterator_t<R>> F>
    constexpr auto operator()(R&& r, T init, F f) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
    }
};
 
inline constexpr fold_left_fn fold_left;

Complexity

Exactly ranges:: distance ( first, last ) applications of the function object f .

Notes

The following table compares all constrained folding algorithms:

Fold function template	Starts from	Initial value	Return type
ranges :: fold_left	left	init	U
ranges:: fold_left_first	left	first element	std:: optional < U >
ranges:: fold_right	right	init	U
ranges:: fold_right_last	right	last element	std:: optional < U >
ranges:: fold_left_with_iter	left	init	(1) ranges:: in_value_result < I, U > (2) ranges:: in_value_result < BR, U > , where BR is ranges:: borrowed_iterator_t < R >
ranges:: fold_left_first_with_iter	left	first element	(1) ranges:: in_value_result < I, std:: optional < U >> (2) ranges:: in_value_result < BR, std:: optional < U >> where BR is ranges:: borrowed_iterator_t < R >

Feature-test macro	Value	Std	Feature
`__cpp_lib_ranges_fold`	202207L	(C++23)	`std::ranges` fold algorithms
`__cpp_lib_algorithm_default_value_type`	202403L	(C++26)	List-initialization for algorithms ( 1,2 )

Example

Run this code

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>
 
int main()
{
    namespace ranges = std::ranges;
 
    std::vector v{1, 2, 3, 4, 5, 6, 7, 8};
 
    int sum = ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
    std::cout << "sum: " << sum << '\n';
 
    int mul = ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
    std::cout << "mul: " << mul << '\n';
 
    // get the product of the std::pair::second of all pairs in the vector:
    std::vector<std::pair<char, float>> data {{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}};
    float sec = ranges::fold_left
    (
        data | ranges::views::values, 2.0f, std::multiplies<>()
    );
    std::cout << "sec: " << sec << '\n';
 
    // use a program defined function object (lambda-expression):
    std::string str = ranges::fold_left
    (
        v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
    );
    std::cout << "str: " << str << '\n';
 
    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto res = ranges::fold_left(nums, {7, 0}, std::multiplies{}); // (2)
    #else
        auto res = ranges::fold_left(nums, CD{7, 0}, std::multiplies{}); // (2)
    #endif
    std::cout << "res: " << res << '\n';
}

Output:

sum: 36
mul: 40320
sec: 42
str: A:1:2:3:4:5:6:7:8
res: (42,42)

References

C++23 standard (ISO/IEC 14882:2024):

27.6.18 Fold [alg.fold]

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::fold_left_first (C++23)	left-folds a range of elements using the first element as an initial value (algorithm function object)
ranges::fold_right (C++23)	right-folds a range of elements (algorithm function object)
ranges::fold_right_last (C++23)	right-folds a range of elements using the last element as an initial value (algorithm function object)
ranges::fold_left_with_iter (C++23)	left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object)
ranges::fold_left_first_with_iter (C++23)	left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional ) (algorithm function object)
accumulate	sums up or folds a range of elements (function template)
reduce (C++17)	similar to std::accumulate , except out of order (function template)