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std::transform(3) C++ Standard Libary std::transform(3) NAME std::transform - std::transform Synopsis Defined in header <algorithm> template< class InputIt, class OutputIt, class UnaryOp > OutputIt transform( InputIt first1, InputIt last1, (1) (const- expr since C++20) OutputIt d_first, UnaryOp unary_op ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryOp > ForwardIt2 transform( ExecutionPolicy&& policy, (2) (since C++17) ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 d_first, UnaryOp unary_op ); template< class InputIt1, class InputIt2, class OutputIt, class BinaryOp > OutputIt transform( InputIt1 first1, InputIt1 last1, (3) (const- expr since C++20) InputIt2 first2, OutputIt d_first, BinaryOp binary_op ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class BinaryOp > ForwardIt3 transform( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 (4) (since C++17) last1, ForwardIt2 first2, ForwardIt3 d_first, BinaryOp binary_op ); 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. 1) The unary operation unary_op is applied to the elements of [first1, last1). If unary_op invalidates an iterator or modifies an element in any of the following ranges, the behavior is undefined: * [first1, last1]. * The range of std::distance(first1, last1) + 1 elements starting from d_first. 3) The binary operation binary_op is applied to pairs of elements from two ranges: [first1, last1) and another range of std::distance(first1, last1) elements starting from first2. If binary_op invalidates an iterator or modifies an element in any of the following ranges, the behavior is undefined: * [first1, last1]. * The range of std::distance(first1, last1) + 1 elements starting from first2. * The range of std::distance(first1, last1) + 1 elements starting from d_first. 2,4) Same as (1,3), but executed according to policy. These overloads participate in overload resolution only if 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) Parameters first1, last1 - the first range of elements to transform first2 - the beginning of the second range of elements to transform d_first - the beginning of the destination range, may be equal to first1 or first2 policy - the execution policy to use. See execution policy for details. unary operation function object that will be ap- plied. The signature of the function should be equivalent to the following: Ret fun(const Type &a); unary_op - 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. binary operation function object that will be ap- plied. The signature of the function should be equivalent to the following: Ret fun(const Type1 &a, const Type2 &b); binary_op - The signature does not need to have const &. The types Type1 and Type2 must be such that ob- jects 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 derefer- enced and assigned a value of type Ret. Type requirements - InputIt, InputIt1, InputIt2 must meet the requirements of LegacyIn- putIterator. - OutputIt must meet the requirements of LegacyOutputIterator. - ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator. Return value Output iterator to the element that follows the last element trans- formed. Complexity Given \(\scriptsize N\)N as std::distance(first1, last1): 1,2) Exactly \(\scriptsize N\)N applications of unary_op. 3,4) Exactly \(\scriptsize N\)N applications of binary_op. Exceptions The overloads with a template parameter named ExecutionPolicy report 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::termi- nate 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 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; } Notes 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. Example // Run this code #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_up- percase); 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.cbe- gin(), 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(ordi- nals)); binary_transform_example(std::move(ordinals)); } Output: hello = "HELLO" world = "WORLD" ordinals: 72 69 76 76 79 ordinals: 144 138 152 152 158 Defect reports The following behavior-changing defect reports were applied retroac- tively 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 they cannot modify the effects ranges involved See also for_each applies a function to a range of elements (function template) ranges::transform applies a function to a range of elements (C++20) (niebloid) http://cppreference.com 2024.06.10 std::transform(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Possible implementation | Notes | Example | Output: | See also
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