(Original callback) how to use function object? (C/C ++) (STL)

Many STL algorithm are at the end of _ if, so that we can bring the function in. If function object is used together, the function can be more coherent !!

In the following example, we want to use count_if () algorithm to know that there are more than n vector?

1 /**/ /*  
2 (C) oomusou 2007 Http://oomusou.cnblogs.com
3
4 Filename: functionobjectsimple. cpp
5 Compiler: Visual C + + 8.0/iso c ++
6 Description: Demo how to use function object
7 Release: 01/18/2007 1.0
8 */
9 # Include < Iostream >
10 # Include < Vector >
11 # Include < Algorithm >
12
13 Using   Namespace STD;
14
15 Bool Biggerthan3 ( Int );
16
17 // Function object
18 Struct Biggerthan {
19 Int N;
20 Biggerthan ( Int N): n (n) {}   // Constructor
21 Bool   Operator ()( Int Val) {ReturnVal>N ;}
22 } ;
23
24 Int Main () {
25 Int IA [] =   {1,2,3,4,5} ;
26 Vector < Int > Ivec (IA, IA +   Sizeof (Ia) /   Sizeof ( Int ));
27 Int I = Count_if (ivec. Begin (), ivec. End (), biggerthan3 );
28 Cout < I < Endl;
29 Int J = Count_if (ivec. Begin (), ivec. End (), biggerthan ( 3 ));
30 Cout < J < Endl;
31 }
32
33 Bool Biggerthan3 ( Int Val) {
34ReturnVal> 3;
35}

Results

2
2

If there is no function object, we can only bring a function name in, because the signature is fixed, so we can only bring a fixed n function as 33 rows, but STL algorithm allows us to bring function objects in. If we can use function objects, they will be very intrusive ~ In line 22, the function is wrapped into a function object. Of course, the class is also used. However, if struct is used, the public: word can be omitted. First, the constructor is used to take over the numeric value, and then () operator performs overload. In this case, 29 rows can carry forward data, regardless of the number of N bands. Of course, operator has more flexibility.

See also
(Original token) How to Use the token circle (using for_each )? (中c) (C ++) (STL) (oo c ++) (template C ++)
(Original example) how to add a line feed to a program notebook? (C/C ++) (STL)