SamYuan1990 commented on PR #2911:
URL: https://github.com/apache/brpc/pull/2911#issuecomment-2705705399
> ```
> const std::string& AdaptiveMaxConcurrency::UNLIMITED() {
> static const std::string s = "unlimited";
> return s;
> }
>
> const std::string& AdaptiveMaxConcurrency::CONSTANT() {
> static const std::string s = "constant";
> return s;
> }
> ```
>
> 1.This way of initialization is much more common，uncontrolled use of
global variables may cause dependency order problems(UB). 2.These short strings
can be optimized to be stored on the stack using the SSO(Small String
Optimization) mechanism of std::string. 3.The original lazy initialization
mechanism is retained to reduce unnecessary memory usage. 4.It is thread-safe.
Hi @zhangqiongyu ,
I hope you're doing well! I wanted to check if this is a maintainer change
request, as it doesn't seem to be in a CR. I also tried searching via [this
link](https://github.com/search?q=repo%3Aapache%2Fbrpc+zhangqiongyu&type=commits)
but didn't find anything relevant.
I'm relatively new to C++, and I'm here to learn from everyone. If I make
any mistakes, please feel free to correct me—I really appreciate it!
I believe we both agree on using `const std::string` instead of `static
std::string* s = new` to ensure thread safety. However, I'm curious about
whether we should use lazy initialization or not. When would be the best time
to initialize this variable?
Initially, I considered using the following approach:
```cpp
const std::string& AdaptiveMaxConcurrency::UNLIMITED() {
static const std::string s = "unlimited";
return s;
}
const std::string& AdaptiveMaxConcurrency::CONSTANT() {
static const std::string s = "constant";
return s;
}
```
But when I realized that the function simply returns a constant string, I
thought about making a broader change.
From what I understand, the differences are:
- Defining a `const` in a class affects the scope of the class and its
instances (and possibly all child classes and their instances). It’s unique
across all instances.
Regarding memory usage:
As far as I know, and as mentioned in the PR description, compiler
optimization is a bit beyond my current knowledge. Since it's a short string,
it might benefit from SSO (Short String Optimization). However, I'm not
entirely sure how the compiler will handle it.
I’m aware of the concept of dependency order problems (UB), but I lack
practical experience in C++. Could you provide an example in this context?
Regarding performance, when invoking this value from other parts of the
code, previously it involved a function call. After changing it to a `const`,
it becomes a direct access to a constant. From my understanding, if the
compiler optimizes a function call that returns a static value, it should be
equivalent to accessing a constant. If the function isn't loaded onto the
stack, it might involve a page change/reload. If SSO is in play, the string
would be on the stack in either case (function call or `const`).
```cpp
const std::string& AdaptiveMaxConcurrency::UNLIMITED() {
static const std::string s = "unlimited";
return s;
}
```
I’d love to hear your thoughts on this! Thank you for your time and
guidance. 😊
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