C++中使用new操作string对象的内存分配错误分析及相关疑问解答

Let's walk through your code step by step to break down exactly why it's failing, plus clarify what's happening with memory allocation here.

为什么代码会报错？ #

Your code has two critical issues that will cause compilation errors (and catastrophic undefined behavior if you somehow forced it to run):

1. Type Mismatch & Invalid Assignment
   std::string is a C++ class built to manage string data automatically. When you write string str = new char[30];, you're trying to assign a raw char* pointer (returned by new[]) directly to a std::string object. C++ doesn't allow this implicit conversion—std::string can be constructed from a const char* (like a string literal), but not from a dynamic char* pointer. The compiler will throw an error here because no valid assignment operator or constructor exists for this combination.

2. Incorrect Use of delete[]
   Even if you hacked around the type mismatch, delete[] str; is completely invalid. delete[] is only meant to free memory allocated by new[] via a raw pointer. str is a std::string object, not a pointer to dynamically allocated chars. Using delete[] on a non-pointer type triggers undefined behavior—this would crash your program at runtime, if it even compiled.

内存分配的实际情况 #

To answer your question: No, your code does NOT allocate 90 bytes for str, and it won't even compile to reach the point of runtime memory allocation. Here's the breakdown:

• The new char[30] expression does allocate 30 bytes of heap memory, but since you're trying to assign this pointer to a std::string (which isn't allowed), you lose track of this pointer immediately. This creates a memory leak—those 30 bytes are never freed.
• The new char[60] line would similarly allocate 60 bytes, but again, the assignment to std::string is invalid. Even if it worked, you'd leak both the 30 and 60 byte blocks, because std::string doesn't take ownership of raw pointers assigned to it. It manages its own internal memory separately.

正确的做法 #

If you want to work with std::string and manage its internal memory, let the class handle it for you—this is the whole point of using std::string instead of raw char* arrays:

• To pre-allocate space for future characters (without initializing them), use reserve():

  std::string str;
  str.reserve(30); // Pre-allocates space for 30 characters internally
  

• To create a string with a specific number of initialized characters:

  std::string str(30, '\0'); // Creates a string of 30 null characters
  

std::string will automatically handle memory allocation, reallocation, and deallocation when the object goes out of scope—you never need to use new or delete with it directly.

内容的提问来源于stack exchange，提问作者masou2312