std::for_each
| Defined in header <algorithm>
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template< class InputIt, class UnaryFunc > UnaryFunc for_each( InputIt first, InputIt last, UnaryFunc f ); |
(1) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt, class UnaryFunc > void for_each( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryFunc f ); |
(2) | (since C++17) |
Applies the given unary function object f to the result of dereferencing every iterator in the range [first, last). If f returns a result, the result is ignored.
f is applied in order starting from first.
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If |
(since C++11) |
f might not be applied in order. The algorithm is executed according to policy.|
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(until C++20) |
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(since C++20) |
If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range through the dereferenced iterator.
Unlike the rest of the parallel algorithms, for_each is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.
Parameters
| first, last | - | the pair of iterators defining the range of elements to which the function object will be applied |
| policy | - | the execution policy to use |
| 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:
The signature does not need to have |
| Type requirements | ||
-InputIt must meet the requirements of LegacyInputIterator.
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-ForwardIt must meet the requirements of LegacyForwardIterator.
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Return value
fComplexity
Exactly std::distance(first, last) 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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined. - If the algorithm fails to allocate memory, std::bad_alloc is thrown.
Possible implementation
See also the implementations in libstdc++, libc++ and MSVC stdlib.
template<class InputIt, class UnaryFunc>
constexpr UnaryFunc for_each(InputIt first, InputIt last, UnaryFunc f)
{
for (; first != last; ++first)
f(*first);
return f; // implicit move since C++11
}
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Notes
For overload (1), f can be a stateful function object. The return value can be considered as the final state of the batch operation.
For overload (2), multiple copies of f may be created to perform parallel invocation. No value is returned because parallelization often does not permit efficient state accumulation.
Example
The following example uses a lambda-expression to increment all of the elements of a vector and then uses an overloaded operator() in a function object (i.k.a., "functor") to compute their sum. Note that to compute the sum, it is recommended to use the dedicated algorithm std::accumulate.
#include <algorithm>
#include <iostream>
#include <vector>
int main()
{
std::vector<int> v{3, -4, 2, -8, 15, 267};
auto print = [](const int& n) { std::cout << n << ' '; };
std::cout << "before:\t";
std::for_each(v.cbegin(), v.cend(), print);
std::cout << '\n';
// increment elements in-place
std::for_each(v.begin(), v.end(), [](int &n) { n++; });
std::cout << "after:\t";
std::for_each(v.cbegin(), v.cend(), print);
std::cout << '\n';
struct Sum
{
void operator()(int n) { sum += n; }
int sum {0};
};
// invoke Sum::operator() for each element
Sum s = std::for_each(v.cbegin(), v.cend(), Sum());
std::cout << "sum:\t" << s.sum << '\n';
}
Output:
before: 3 -4 2 -8 15 267
after: 4 -3 3 -7 16 268
sum: 281
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 475 | C++98 | it was unclear whether f can modify the elementsof the sequence being iterated over ( for_each isclassified as “non-modifying sequence operations”) |
made clear (allowed if the iterator type is mutable) |
| LWG 2747 | C++11 | overload (1) returned std::move(f)
|
returns f (which implicitly moves)
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See also
| applies a function to a range of elements, storing results in a destination range (function template) | |
(C++17) |
applies a function object to the first N elements of a sequence (function template) |
(C++20) |
applies a unary function object to elements from a range (algorithm function object) |
(C++20) |
applies a function object to the first N elements of a sequence (algorithm function object) |
range-for loop(C++11)
|
executes loop over range |