自定义类型MyClass中如果含有STL顺序容器成员sequential_container<type>，可能希望定义MyClass::operator[]操作符；如果STL顺序容器是多维的sequential_constainer<sequential_constainer<...<type>...>，可能希望定义MyClass的[][]...操作符。
[]
重载[]操作符：
1. 双目操作符，第一操作数是重载此操作符的对象，第二操作数是无符号下标。
2. C++不允许把下标操作符函数作为外部函数来定义,它只能是非静态的成员函数。
3. 为了能对数组赋值,一般将返回值声明为引用类型。
ReTurnType& MyClass::operator[](unsigned int idx);
例一：
1 #include <iostream>
2 #include <iomanip>
3 #include <fstream>
4 #include <string>
5 #include <vector>
6
7 using namespace std;
8
9 class Data1d{
10
11 vector<double> data1d;
12 int length1d;
13
14 public:
15 Data1d(int lg=0, double elem=0):length1d(lg){
16 for (int i=0; i<lg; i++)
17 data1d.push_back(elem);
18 //length1d=data1d.size();
19 }
20
21 void pushback(double elem){data1d.push_back(elem); ++length1d;}
22 void popback(){data1d.pop_back(); --length1d;}
23 int length(){return length1d;}
24
25 double& operator[](const int idx){
26 if (idx < 0 || idx >= length1d){
27 cerr << "ERROR: Index Out of Range, Last Element Returned!" << endl;
28 return data1d[length1d - 1];
29 }
30 return data1d[idx];
31 }
32 friend ostream& operator<<(ostream&, Data1d&);
33 };
34
35 ostream& operator<<(ostream& o, Data1d& mydata1d){
36 cout << "Data(" << mydata1d.length1d << "):\t";
37 for (int i=0; i<mydata1d.length1d; i++)
38 o << mydata1d[i] << ' ';
39 return o;
40 }
41
42 int main(){
43 Data1d mydata1d(5,0);
44 cout << mydata1d << endl;
45
46 for (int i=1; i<=5; i++)
47 mydata1d.pushback(i);
48 cout << mydata1d << endl;
49
50 for (int i=0; i<=mydata1d.length(); i++)
51 mydata1d[i]=i;
52 cout << mydata1d << endl;
53
54 mydata1d.popback();
55 cout << mydata1d << endl;
56 }
输出：
Data(5): 0 0 0 0 0 Data(10): 0 0 0 0 0 1 2 3 4 5 ERROR: Index Out of Range, Last Element Returned! Data(10): 0 1 2 3 4 5 6 7 8 10 Data(9): 0 1 2 3 4 5 6 7 8
[][]
[][]操作符。[][]无法作为一个操作符整体来定义或重载，因为C++不允许定义新的操作符。[][]的实现实际上是后面一个[]作用于前面一个[]的返回值上，以下例子中[][]是这样实现的：MyClass重载operator[]，其返回类型为&vector<type>, 继续使用非重载的vector<type>::operator[], 得到type&.
例二：
1 #include <iostream>
2 #include <iomanip>
3 #include <fstream>
4 #include <string>
5 #include <vector>
6
7 using namespace std;
8
9 class Data2d{
10
11 vector<vector<double>> data2d;
12 int lengthx, lengthy;
13
14 public:
15 Data2d(int lgx = 0, int lgy=0, double elem = 0):lengthx(lgx), lengthy(lgy){
16 vector<double> data1d;
17 for (int i = 0; i < lgx; i++){
18 data1d.clear();
19 for (int j = 0; j < lgy; j++)
20 data1d.push_back(elem);
21 data2d.push_back(data1d);
22 }
23 }
24
25 vector<int> size(){ return vector<int>{lengthx, lengthy}; }
26
27 //reload operator[] and we can use "operator"[][]
28 vector<double>& operator[](const int idx){ return data2d[idx]; }
29
30 friend ostream& operator<<(ostream&, Data2d&);
31 };
32
33 ostream& operator<<(ostream& o, Data2d& mydata2d){
34 cout << "Data(" << mydata2d.lengthx << ", " << mydata2d.lengthy << ")" << endl;
35
36 for (int i = 0; i < mydata2d.lengthx; i++){
37 for (int j = 0; j < mydata2d.lengthy; j++)
38 o << mydata2d.data2d[i][j] << '\t';
39 o << endl;
40 }
41 return o;
42 }
43
44 int main(){
45 Data2d mydata2d(3, 5, 0);
46 cout << mydata2d << endl;
47
48 vector<int> sz = mydata2d.size();
49 for (int i = 0; i < sz[0]; i++)
50 for (int j = 0; j < sz[1]; j++)
51 mydata2d[i][j] = i+j;
52 cout << mydata2d << endl;
53 }
输出：
Data(3, 5) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Data(3, 5) 0 1 2 3 4 1 2 3 4 5 2 3 4 5 6
来源：https://www.cnblogs.com/zhangyz/p/6077293.html