class B { public: B(): vec(128) { for (int i = 0; i < 128; ++i) { vec[i] = 0; } } // PRE: ... // POST: ... void add(const char c) { ++vec[c]; } // PRE: ... // POST: ... int get(const char c) const { return vec[c]; } add the pre and post

Here's the updated code with the added preconditions and postconditions:

```cpp
class B {
public:
    /**
     * @brief Default constructor. Initializes a vector of size 128 with all elements set to 0.
     */
    B(): vec(128) {
        for (int i = 0; i < 128; ++i) {
            vec[i] = 0;
        }
    }
  
    /**
     * @brief Adds a character to the internal vector.
     * 
     * @pre The character c is a valid ASCII character (0 <= c <= 127).
     * @post The internal vector has been updated to reflect the addition of the character c.
     * 
     * @param c The character to be added.
     */
    void add(const char c) {
        ++vec[c];
    }
  
    /**
     * @brief Retrieves the count of a character in the internal vector.
     * 
     * @pre The character c is a valid ASCII character (0 <= c <= 127).
     * @post The count of the character c in the internal vector has been returned.
     * 
     * @param c The character whose count is to be retrieved.
     * @return The count of the character c in the internal vector.
     */
    int get(const char c) const {
        return vec[c];
    }

private:
    int vec[128];  // Internal vector to store character counts
};
```

Note that I've used Doxygen-style comments to document the class and its methods. This is a common convention in C++ for documenting code. The preconditions and postconditions are specified in the comments above each method, as per the requirements.