std::transform在指定的范围内应用于给定的操作，并将结果存储在指定的另一个范围内。要使用std::transform函数需要包含<algorithm>头文件。
以下是std::transform的两个声明，一个是对应于一元操作，一个是对应于二元操作：
template <class InputIterator, class OutputIterator, class UnaryOperation>
OutputIterator transform (InputIterator first1, InputIterator last1,
OutputIterator result, UnaryOperation op);
template <class InputIterator1, class InputIterator2,
class OutputIterator, class BinaryOperation>
OutputIterator transform (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, OutputIterator result,
BinaryOperation binary_op);
对于一元操作，将op应用于[first1, last1)范围内的每个元素，并将每个操作返回的值存储在以result开头的范围内。给定的op将被连续调用last1-first1次。op可以是函数指针或函数对象或lambda表达式。
如op的一个实现 即将[first1, last1)范围内的每个元素加5，然后依次存储到result中。
int op_increase(int i) {return (i + 5)};
调用std::transform的方式如下：
std::transform(first1, last1, result, op_increase);
对于二元操作，使用[first1, last1)范围内的每个元素作为第一个参数调用binary_op,并以first2开头的范围内的每个元素作为第二个参数调用binary_op,每次调用返回的值都存储在以result开头的范围内。给定的binary_op将被连续调用last1-first1次。binary_op可以是函数指针或函数对象或lambda表达式。
如binary_op的一个实现即将first1和first2开头的范围内的每个元素相加，然后依次存储到result中。
int op_add(int, a, int b) {return (a + b)};
std::transform支持in place，即result和first1指向的位置可以是相同的。std::transform的主要作用应该就是省去了我们自己写for循环实现。
以下是摘自对std::transform的英文解释：
/*
// reference: http://en.cppreference.com/w/cpp/algorithm/transform
template< class InputIt, class OutputIt, class UnaryOperation >
OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first, UnaryOperation unary_op )
{
while (first1 != last1) {
*d_first++ = binary_op(*first1++, *first2++);
}
return d_first;
}
template< class InputIt1, class InputIt2, class OutputIt, class BinaryOperation >
OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, BinaryOperation binary_op );
std::transform applies the given function to a range and stores the result in another range, beginning at d_first.
(1): The unary operation unary_op is applied to the range defined by [first1, last1).
(2): The binary operation binary_op is applied to pairs of elements from two ranges:
one defined by [first1, last1) and the other beginning at first2.
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
unary_op: unary operation function object that will be applied.
binary_op: binary operation function object that will be applied.
Return value: Output iterator to the element past the last element transformed.
std::for_each: ignores the return value of the function and guarantees order of execution.
std::transform: assigns the return value to the iterator, and does not guarantee the order of execution.
*/
以下是std::transform用法举例：
#include "transform.hpp"
#include <algorithm> // std::transform
#include <string>
#include <cctype> // std::toupper
#include <iostream>
#include <vector>
#include <functional> // std::plus c++14
int test_transform1()
{
std::string s("Hello");
std::transform(s.begin(), s.end(), s.begin(),
[](unsigned char c) { return std::toupper(c); });
std::cout << s << std::endl; // HELLO
std::transform(s.begin(), s.end(), s.begin(), ::tolower);
std::cout << s << std::endl; // hello
////////////////////////////////
std::vector<int> arr{ 1, 3, 5 };
std::vector<int> arr2{ 1, 3, 5 };
std::vector<int> arr3{ 1, 3, 5 };
std::transform(arr.begin(), arr.end(), arr.begin(),
[](int d) -> int {return d * 5; }); // for_each
for (auto value : arr) {
std::cout << value << " "; // 5 15 25
}
std::cout<<std::endl;
std::for_each(arr2.begin(), arr2.end(), [](int& a) {a *= 5; });
for (auto value : arr2) {
std::cout << value << " "; // 5 15 25
}
std::cout << std::endl;
for (auto& value : arr3) {
value *= 5;
}
for (auto value : arr3) {
std::cout << value << " "; // 5 15 25
}
std::cout << std::endl;
std::vector<std::string> names = { "hi", "test", "foo" };
std::vector<std::size_t> name_sizes;
///////////////////////////
std::transform(names.begin(), names.end(), std::back_inserter(name_sizes),
[](std::string name) { return name.size(); });
for (auto value : name_sizes) {
std::cout << value << " "; // 2 4 3
}
std::cout << std::endl;
std::for_each(name_sizes.begin(), name_sizes.end(), [](std::size_t name_size) {
std::cout << name_size << " "; // 2 4 3
});
std::cout << std::endl;
return 0;
}
/////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/algorithm/transform/
static int op_increase(int i) { return ++i; }
int test_transform2()
{
std::vector<int> foo;
std::vector<int> bar;
// set some values:
for (int i = 1; i<6; i++)
foo.push_back(i * 10); // foo: 10 20 30 40 50
bar.resize(foo.size()); // allocate space
std::transform(foo.begin(), foo.end(), bar.begin(), op_increase);
// bar: 11 21 31 41 51
// std::plus adds together its two arguments:
std::transform(foo.begin(), foo.end(), bar.begin(), foo.begin(), std::plus<int>());
// foo: 21 41 61 81 101
std::cout << "foo contains:";
for (std::vector<int>::iterator it = foo.begin(); it != foo.end(); ++it)
std::cout << ' ' << *it; // 21 41 61 81 101
std::cout << '
';
return 0;
}