1. 预定义函数对象
C++标准库内含许多预定义的函数对象，也就是内置的函数对象。
你可以充分利用他们，不必自己费心去写一些自己的函数对象。
要使用他们，你只要包含如下头文件
#include <functional>
eg:
set<int, less<int>> coll; // sort elements with <
set<int, greater<int>> coll; // sort elements with >
predefinedFuncObjectTest.cpp
deque<int> coll = { 1, 2, 3, 5, 7, 11, 13, 17, 19 }; PRINT_ELEMENTS(coll, "initialized: "); // negate all values in coll transform(coll.cbegin(), coll.cend(), // source coll.begin(), // destination negate<int>()); // operation PRINT_ELEMENTS(coll, "negated: "); // square all values in coll transform(coll.cbegin(), coll.cend(), // first source coll.cbegin(), // second source coll.begin(), // destination multiplies<int>()); // operation PRINT_ELEMENTS(coll, "squared: ");
运行结果:
---------------- predefinedFuncObject(): Run Start ----------------
initialized: 1 2 3 5 7 11 13 17 19
negated: -1 -2 -3 -5 -7 -11 -13 -17 -19
squared: 1 4 9 25 49 121 169 289 361
---------------- predefinedFuncObject(): Run End ----------------
2. 预定义函数对象绑定
你可以使用binder将预定义函数对象和其他数值进行绑定。
pdFuncObjectBind.cpp
using namespace std::placeholders; set<int, greater<int>> coll1 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 }; deque<int> coll2; // Note: due to the sorting criterion greater<>() elements have reverse order: PRINT_ELEMENTS(coll1, "initialized: "); // transform all elements into coll2 by multiplying them with 10 transform(coll1.cbegin(), coll1.cend(), // source back_inserter(coll2), // destination bind(multiplies<int>(), _1, 10)); // operation PRINT_ELEMENTS(coll2, "transformed: "); // replace value equal to 70 with 42 replace_if(coll2.begin(), coll2.end(), // range bind(equal_to<int>(), _1, 70), // replace criterion 42); // new value PRINT_ELEMENTS(coll2, "replaced: "); // remove all elements with values between 50 and 80 coll2.erase(remove_if(coll2.begin(), coll2.end(), bind(logical_and<bool>(), bind(greater_equal<int>(), _1, 50), bind(less_equal<int>(), _1, 80))), coll2.end()); PRINT_ELEMENTS(coll2, "removed: ");
运行结果:
---------------- pdFuncObjectBind(): Run Start ----------------
initialized: 9 8 7 6 5 4 3 2 1
transformed: 90 80 70 60 50 40 30 20 10
replaced: 90 80 42 60 50 40 30 20 10
removed: 90 42 40 30 20 10
---------------- pdFuncObjectBind(): Run End ----------------
