重载istream的>>运算符与operator=引发无限循环问题求助

Hey there! I totally get the pain of debugging custom string classes—those memory management bugs can be tricky to track down. Let's break down the common issues you might be facing, starting with the infinite loop you fixed in operator= and moving to other potential bugs lurking in your code.

Common Pitfalls to Check #

1. Incomplete Fix for the Assignment Operator Infinite Loop #

If your original infinite loop came from accidentally writing *this = rhs inside the operator= itself (a classic mistake!), fixing that is a start—but there are still two critical things to verify:

• Self-assignment check: Did you add a guard like if (this == &rhs) return *this;? Without this, assigning an object to itself could lead to freeing memory before you try to copy it, causing a crash or undefined behavior.
• Proper memory handling: Are you first deleting the existing char* buffer before allocating new memory? Skipping this step leads to memory leaks.

2. Shallow Copy vs Deep Copy Mistakes #

This is the #1 bug in custom string classes:

• If your copy constructor or assignment operator only copies the char* pointer (shallow copy) instead of allocating new memory and copying the actual string data, two MyString objects will point to the same memory block. When one is destroyed, it frees that memory, leaving the other with a dangling pointer—leading to crashes when you try to use it.
• Double-check that in both your copy constructor and operator=, you're doing something like:

  m_size = rhs.m_size;
  m_data = new char[m_size + 1]; // +1 for the null terminator!
  strcpy(m_data, rhs.m_data);
  

3. Null Terminator Oversights #

It's easy to forget that C-style strings need a \0 at the end. Make sure:

• When you allocate memory for your string, you always add 1 to the length returned by strlen() (or your calculated size) to make room for the null terminator.
• When you copy or modify the string data, you explicitly set the last character to \0 if needed (like when implementing substr() or operator+=).

4. Destructor Issues #

• Does your destructor properly delete the char* buffer with delete[] (not delete—using the wrong one leads to undefined behavior)?
• While not strictly necessary, setting m_data = nullptr; in the destructor can help catch dangling pointer bugs during debugging.

5. Const Correctness Violations #

Are your const member functions marked correctly? For example:

• The c_str() method should be const char* c_str() const;—without the trailing const, you can't call it on a const MyString object.
• Your copy constructor and assignment operator should take a const MyString& parameter (since you shouldn't modify the source object during copying).

6. Copy-and-Swap Idiom (A Safer Alternative) #

If you're still having issues with operator=, consider using the copy-and-swap idiom—it eliminates most memory management bugs by leveraging the copy constructor and a swap function:

// In MyString.h
void swap(MyString& other);
MyString& operator=(MyString rhs); // Pass by value to trigger copy constructor

// In MyString.cpp
void MyString::swap(MyString& other) {
    std::swap(m_data, other.m_data);
    std::swap(m_size, other.m_size);
}

MyString& MyString::operator=(MyString rhs) {
    swap(rhs);
    return *this;
}

This approach automatically handles self-assignment and avoids memory leaks, since the temporary rhs object will be destroyed and free its memory when the function exits.

Next Steps #

To pinpoint exactly what's going wrong, sharing snippets of your code would help a ton—like:

• The member variable declarations in MyString.h
• Your copy constructor, assignment operator, and destructor implementations
• Any other overloaded operators (like operator+, operator[]) that might be causing issues

内容的提问来源于stack exchange，提问作者Compeador