Initialization of data members

　　In C++, class variables are initialized in the same order as they appear in the class declaration.

　　Consider the below code.

 1 #include<iostream>
 2
 3 using namespace std;
 4
 5 class Test
 6 {
 7 private:
 8     int y;
 9     int x;
10 public:
11     Test() : x(10), y(x + 10)
12     {
13     }
14     void print();
15 };
16
17 void Test::print()
18 {
19     cout<<"x = "<<x<<" y = "<<y;
20 }
21
22 int main()
23 {
24     Test t;
25     t.print();
26     getchar();
27     return 0;
28 }

　　The program prints correct value of x, but some garbage value for y, because y is initialized before x as it appears before in the class declaration.

　　So one of the following two versions can be used to avoid the problem in above code.

 1 // First: Change the order of declaration.
 2 class Test
 3 {
 4 private:
 5     int x;
 6     int y;
 7 public:
 8     Test() : x(10), y(x + 10)
 9     {
10     }
11     void print();
12 };
13
14 // Second: Change the order of initialization.
15 class Test
16 {
17 private:
18     int y;
19     int x;
20 public:
21     Test() : x(y-10), y(20)
22     {
23     }
24     void print();
25 };

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　　2013-11-26　　10:23:29