transform的功能类似于for_each,但是transform,传参数一次拷贝,返回时一次拷贝,不能保证按序执行,而for_each传参使用引用,不需要拷贝,可以保证有序执行。
std::transform
Defined in header
<algorithm> |
||
template< class InputIt, class OutputIt, class UnaryOperation > OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first, |
(1) | |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryOperation > ForwardIt2 transform( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, |
(2) | (since C++17) |
template< class InputIt1, class InputIt2, class OutputIt, class BinaryOperation > OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2, |
(3) | |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class BinaryOperation > ForwardIt3 transform( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, |
(4) | (since C++17) |
std::transform
applies the given function to a range and stores the result in another range, beginning at d_first
.
unary_op
is applied to the range defined by [first1, last1)
.binary_op
is applied to pairs of elements from two ranges: one defined by [first1, last1)
and the other beginning at first2
.policy
. This overload only participates in overload resolution ifstd::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true
|
(until C++11) |
|
(since C++11) |
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_op | - | unary operation function object that will be applied. The signature of the function should be equivalent to the following: Ret fun(const Type &a); The signature does not need to have const &. |
binary_op | - | binary operation function object that will be applied. 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 &. |
Type requirements | ||
-
InputIt, InputIt1, InputIt2 must meet the requirements of InputIterator . |
||
-
OutputIt must meet the requirements of OutputIterator . |
||
-
ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of ForwardIterator . |
Return value
Output iterator to the element past the last element transformed.
Complexity
unary_op
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 three 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
First version |
---|
template<class InputIt, class OutputIt, class UnaryOperation>
OutputIt transform(InputIt first1, InputIt last1, OutputIt d_first,
UnaryOperation unary_op)
{
while (first1 != last1) {
*d_first++ = unary_op(*first1++);
}
return d_first;
}
|
Second version |
template<class InputIt1, class InputIt2,
class OutputIt, class BinaryOperation>
OutputIt transform(InputIt1 first1, InputIt1 last1, InputIt2 first2,
OutputIt d_first, BinaryOperation binary_op)
{
while (first1 != last1) {
*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
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 #include <string> 2 #include <cctype> 3 #include <algorithm> 4 #include <iostream> 5 6 int main() 7 { 8 std::string s("hello"); 9 std::transform(s.begin(), s.end(), s.begin(), 10 [](unsigned char c) { return std::toupper(c); }); 11 std::cout << s; 12 }