std::transform
(on partitioned ranges)
<algorithm>
OutputIt transform( InputIt first1, InputIt last1,
class ForwardIt1, class ForwardIt2, class UnaryOp >
ForwardIt2 transform( ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1,
class OutputIt, class BinaryOp >
OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2,
class ForwardIt1, class ForwardIt2,
class ForwardIt3, class BinaryOp >
ForwardIt3 transform( ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2,
std::transform
applies the given function to the elements of the given input range(s), and stores the result in an output range starting from d_first.
[
first1,
last1)
.[
first1,
last1]
.
[
first1,
last1)
and another range of std::distance (first1, last1) elements starting from first2.[
first1,
last1]
.
std::is_execution_policy_v <std::decay_t <ExecutionPolicy>> is true.
(until C++20)std::is_execution_policy_v <std::remove_cvref_t <ExecutionPolicy>> is true.
(since C++20)The signature of the function should be equivalent to the following:
Ret 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. The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret.
The signature of the function should be equivalent to the following:
Ret fun(const Type1 &a, const Type2 &b);
The signature does not need to have const &.
The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret.
InputIt, InputIt1, InputIt2
must meet the requirements of LegacyInputIterator.
OutputIt
must meet the requirements of LegacyOutputIterator.
ForwardIt1, ForwardIt2, ForwardIt3
must meet the requirements of LegacyForwardIterator.
Output iterator to the element that follows the last element transformed.
Given \(\scriptsize N\)N as std::distance (first1, last1):
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate is called. For any other ExecutionPolicy
, the behavior is implementation-defined.
transform (1) |
---|
template<class InputIt, class OutputIt, class UnaryOp> constexpr //< since C++20 OutputIt transform(InputIt first1, InputIt last1, OutputIt d_first, UnaryOp unary_op) { for (; first1 != last1; ++d_first, ++first1) *d_first = unary_op(*first1); return d_first; } |
transform (3) |
template<class InputIt1, class InputIt2, class OutputIt, class BinaryOp> constexpr //< since C++20 OutputIt transform(InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, BinaryOp binary_op) { for (; first1 != last1; ++d_first, ++first1, ++first2) *d_first = binary_op(*first1, *first2); return d_first; } |
std::transform
does not guarantee in-order application of unary_op or binary_op. To apply a function to a sequence in-order or to apply a function that modifies the elements of a sequence, use std::for_each .
#include <algorithm> #include <cctype> #include <iomanip> #include <iostream> #include <string> #include <utility> #include <vector> void print_ordinals(const std::vector <unsigned>& ordinals) { std::cout << "ordinals: "; for (unsigned ord : ordinals) std::cout << std::setw (3) << ord << ' '; std::cout << '\n'; } char to_uppercase(unsigned char c) { return std::toupper (c); } void to_uppercase_inplace(char& c) { c = to_uppercase(c); } void unary_transform_example(std::string & hello, std::string world) { // Transform string to uppercase in-place std::transform(hello.cbegin(), hello.cend(), hello.begin(), to_uppercase); std::cout << "hello = " << std::quoted (hello) << '\n'; // for_each version (see Notes above) std::for_each (world.begin(), world.end(), to_uppercase_inplace); std::cout << "world = " << std::quoted (world) << '\n'; } void binary_transform_example(std::vector <unsigned> ordinals) { // Transform numbers to doubled values print_ordinals(ordinals); std::transform(ordinals.cbegin(), ordinals.cend(), ordinals.cbegin(), ordinals.begin(), std::plus <>{}); print_ordinals(ordinals); } int main() { std::string hello("hello"); unary_transform_example(hello, "world"); std::vector <unsigned> ordinals; std::copy (hello.cbegin(), hello.cend(), std::back_inserter (ordinals)); binary_transform_example(std::move(ordinals)); }
Output:
hello = "HELLO" world = "WORLD" ordinals: 72 69 76 76 79 ordinals: 144 138 152 152 158
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 | unary_op and binary_op could not have side effects | they cannot modify the ranges involved |