关于左值的错误

 1 #include<iostream>
 2 #include<string> 
 3 using namespace std;
 4 
 5 class TextBlock{
 6 public:
 7     TextBlock(string x):text(x){
 8     }
 9     //operator[] for const 对象
10     const char& operator[](std::size_t position) const
11     { return text[position]; }
12 
13     //operator[] for non-const 对象
14     char operator[](std::size_t position)
15     { return text[position]; }
16     
17 private:
18     std::string text;
19 };
20 
21 int main(){
22     TextBlock tb("hello");
23     tb[0] = 'x';      //报错： lvalue required as left operand of assignment
24         
25     std::cout << tb[0] << endl;
26     
27     
28     const TextBlock ctb("World!!");
29     std::cout << ctb[0]; 
30     
31 }