std::for_each

< cpp‎ | algorithm
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Concepts and utilities: std::Sortable, std::projected, ...
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
for_each
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
(C++11)
Minimum/maximum operations
(C++11)
(C++11)
(C++17)
(C++20)
Permutations
Numeric operations
C library
 
Defined in header <algorithm>
(1)
template< class InputIt, class UnaryFunction >
UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );
 (until C++20)
template< class InputIt, class UnaryFunction >
constexpr UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryFunction2 >
void for_each( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryFunction2 f );
 (2) (since C++17)
1) Applies the given function object f to the result of dereferencing every iterator in the range [first, last), in order.
2) Applies the given function object f to the result of dereferencing every iterator in the range [first, last) (not necessarily in order). The algorithm is executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

For both overloads, if the iterator type is mutable, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored.

Unlike the rest of the algorithms, for_each is not allowed to make copies of the elements in the sequence even if they are trivially copyable.

Parameters

first, last - the range to apply the function to
policy - the execution policy to use. See execution policy for details.
f - function object, to be applied to the result of dereferencing every iterator in the range [first, last)

The signature of the function should be equivalent to the following:

 void fun(const Type &a);

The signature does not need to have const &.
The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.

​

Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryFunction must meet the requirements of MoveConstructible. Does not have to be CopyConstructible
-
UnaryFunction2 must meet the requirements of CopyConstructible.

Return value

1) f (until C++11) std::move(f) (since C++11)
2) (none)

Complexity

Exactly last - first applications of f

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

template<class InputIt, class UnaryFunction>
constexpr UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
{
    for (; first != last; ++first) {
        f(*first);
    }
    return f; // implicit move since C++11
}

Example

The following example uses a lambda function to increment all of the elements of a vector and then uses an overloaded operator() in a functor to compute their sum. Note that to compute the sum, it is recommended to use the dedicated algorithm std::accumulate.

Run this code
#include <vector>
#include <algorithm>
#include <iostream>
 
struct Sum
{
    Sum(): sum{0} { }
    void operator()(int n) { sum += n; }
    int sum;
};
 
int main()
{
    std::vector<int> nums{3, 4, 2, 8, 15, 267};
 
    auto print = [](const int& n) { std::cout << " " << n; };
 
    std::cout << "before:";
    std::for_each(nums.begin(), nums.end(), print);
    std::cout << '\n';
 
    std::for_each(nums.begin(), nums.end(), [](int &n){ n++; });
 
    // calls Sum::operator() for each number
    Sum s = std::for_each(nums.begin(), nums.end(), Sum());
 
    std::cout << "after: ";
    std::for_each(nums.begin(), nums.end(), print);
    std::cout << '\n';
    std::cout << "sum: " << s.sum << '\n';
}

Output: 

before: 3 4 2 8 15 267
after:  4 5 3 9 16 268
sum: 305

See also

 applies a function to a range of elements
(function template)
range-for loop executes loop over range (since C++11)
(C++17)
 applies a function object to the first n elements of a sequence
(function template)