CSTL使用函数对象操作容器元素的foreach和transform的区别

　　for_each() 和transform()都可以使用一个函数指针或函数对象对一个容器区间的元素施加操作。Algorithm for_each applies a general function to every element in a certain range.
　　Algorithm transform applies a general function to every element in a certain range and replaces each element with the result of function.
　　for_each()使用的参数都是input iterator，只是对容器施加读操作，并不更改容器。
　　transform()使用了更多的参数，可以将结果存储到另一容器（或容器自身）中。
　　transform()还可以操作两个容器之间的运算，并将结果存储到另一容器（或容器自身）中。1 std::for_eachtemplate  Function for_each (InputIterator first, InputIterator last, Function fn);
　　Apply function to range
　　Applies function fn to each of the elements in the range [first,last).
　　The behavior of this template function is equivalent to: template Function for_each(InputIterator first, InputIterator last, Function fn) {     while(first!=last) {         fn (*first);         ++first;     }     return fn;      // or, since C++11: return move(fn); }
　　demo: #include      // std::cout #include     // std::for_each #include        // std::vector  void myfunction (int i) {  // function:     std::cout << ＂ ＂ << i; }  struct myclass {           // function object type:     void operator() (int i) {std::cout << ＂ ＂ << i;} } myobject;  int main () {     std::vector myvector;     myvector.push_back(10);     myvector.push_back(20);     myvector.push_back(30);          std::cout << ＂myvector contains:＂;     for_each (myvector.begin(), myvector.end(), myfunction);     std::cout << ＂ ＂;      // or:     std::cout << ＂myvector contains:＂;     for_each (myvector.begin(), myvector.end(), myobject);     std::cout << ＂ ＂;          return 0; } /* output: Output: myvector contains: 10 20 30 myvector contains: 10 20 30 */2 std::transformunary operation(1) template    OutputIterator transform (InputIterator first1, InputIterator last1,                             OutputIterator result, UnaryOperation op); binary operation(2) template    OutputIterator transform (InputIterator1 first1, InputIterator1 last1,                             InputIterator2 first2, OutputIterator result,                             BinaryOperation binary_op);
　　Transform range
　　Applies an operation sequentially to the elements of one (1) or two (2) ranges and stores the result in the range that begins at result.
　　(1) unary operation  Applies op to each of the elements in the range [first1,last1) and stores the value returned by each operation in the range that begins at result.
　　(2) binary operation  Calls binary_op using each of the elements in the range [first1,last1) as first argument, and the respective argument in the range that begins at first2 as second argument. The value returned by each call is stored in the range that begins at result.
　　The behavior of this function template is equivalent to: template  OutputIterator transform (InputIterator first1, InputIterator last1, OutputIterator result, UnaryOperator op) {     while(first1 != last1) {         *result = op(*first1);  // or: *result=binary_op(*first1,*first2++);         ++result; ++first1;     }     return result; }
　　demo:#include      // std::cout #include     // std::transform #include        // std::vector #include    // std::plus  int op_increase (int i) { return ++i; }  int main () {     std::vector foo;     std::vector 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());     // foo: 21 41 61 81 101          std::cout << ＂foo contains:＂;     for(std::vector::iterator it=foo.begin(); it!=foo.end(); ++it)         std::cout << ＂ ＂ << *it;     std::cout << ＂ ＂;          return 0; } /*Output: foo contains: 21 41 61 81 101 */
　　ref:
　　http://www.cplusplus.com/reference/algorithm/for_each/
　　http://www.cplusplus.com/reference/algorithm/transform/