Templates and Default Arguments

　　Default parameters for templates in C++:
　　Like function default arguments, templates can also have default arguments. For example, in the following program, the second parameter U has the default value as char.

 1 #include<iostream>
 2 using namespace std;
 3
 4 template<class T, class U = char> class A
 5 {
 6 public:
 7     T x;
 8     U y;
 9 };
10
11 int main()
12 {
13     A<char> a;
14     A<int, int> b;
15     cout<<sizeof(a)<<endl;
16     cout<<sizeof(b)<<endl;
17     return 0;
18 }

　　Output: (char takes 1 byte and int takes 4 bytes)
　　2
　　8

　　Also, similar to default function arguments, if one template parameter has a default argument, then all template parameters following it must also have default arguments.

　　For example, the compiler will not allow the following program:

 1 #include<iostream>
 2 using namespace std;
 3
 4 template<class T = char, class U, class V = int> class A  // Error
 5 {
 6    // members of A
 7 };
 8
 9 int main()
10 {
11
12 } 

　　template的默认参数列表规则与函数的默认参数列表规则一样。

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　　2013-11-26　　22:21:23