C++标准库string类效率咨询：传参与赋值是否复制数据？

Great question—moving from a custom string class to the standard library is a smart move, and it makes total sense to want to map your existing knowledge to how std::string works today. Let's break this down specifically for your use case, including Windows behavior and portability:

How std::string Handles Copies and Assignments (Current Implementations) #

First, a key point: the old "copy-on-write (COW)" mechanism your custom class uses was common in early std::string implementations, but modern C++ standards and compiler designs have largely moved away from it. Here's what you need to know:

1. Passing to Functions or Assigning #

• Pass by value: On modern implementations (like MSVC, GCC 5+, Clang), this will copy the string data—except if you're passing a temporary object (or use std::move to transfer ownership). In those cases, C++11's move semantics kicks in: instead of copying data, the string just transfers its internal pointer to the new object, leaving the original in an empty (but valid) state. This is nearly zero-cost.
• Pass by const reference (const std::string&): This is always the most efficient way to pass strings to functions if you don't need to modify them. No data is copied at all—you're just passing a reference to the existing string object.
• Assignment: Similar to pass-by-value: regular assignment (a = b) copies data, but a = std::move(b) triggers a move, transferring the pointer instead of copying.

2. No More Reference Counting (Mostly) #

Your custom class uses reference counting + copy-on-write to avoid unnecessary copies until modification. Modern std::string implementations have abandoned COW for two main reasons:

• COW has significant overhead in multi-threaded code (atomic operations on reference counts add cost).
• C++11's standard requires certain behaviors (like operator[] returning a mutable reference that can be modified immediately) that are hard to implement efficiently with COW.

Instead, modern std::string uses Small String Optimization (SSO):

• Each std::string object reserves a small chunk of internal memory (usually 15-22 bytes, depending on platform/implementation).
• If your string fits in this internal buffer, no heap allocation is done—characters are stored directly in the object itself. Copying such small strings is just a quick stack copy, no heap operations involved.
• For strings larger than the SSO buffer, the string uses a heap-allocated buffer. Copying these will duplicate the heap data, but again, move semantics let you avoid this when transferring ownership.

Windows-Specific Behavior #

On Windows, MSVC's std::string has never used COW—it's relied on SSO for years. So:

• Small strings (under ~16 bytes) are copied directly in the object's internal buffer, no heap overhead.
• Larger strings will copy heap data when passed by value or assigned normally, but moves are still zero-cost.
• Using const std::string& for function parameters is always safe and efficient here.

Portability Considerations #

Nearly all modern C++ compilers (MSVC, GCC, Clang) across Windows, Linux, and macOS now use SSO instead of COW. So your code will behave consistently across platforms as long as you follow modern C++ practices:

• Prefer const std::string& for function parameters when you don't need to modify the string.
• Use std::move when you want to transfer ownership of a string (e.g., returning a large string from a function, or assigning a string you no longer need).
• For read-only access to string data (without needing to modify), consider std::string_view (C++17+)—it's a lightweight wrapper that points to existing string data, no copies at all.

Comparing to Your Custom Class #

The SSO + move semantics approach is generally more efficient than your reference-counted COW class, especially for small strings (which are extremely common in most codebases). It also avoids the multi-threading pitfalls of COW, and aligns fully with standard C++ behavior—so you won't have to maintain your custom implementation anymore.

内容的提问来源于stack exchange，提问作者user7637348