zhangqiongyu commented on PR #2911:
URL: https://github.com/apache/brpc/pull/2911#issuecomment-2706092592
> > ```
> > 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:
>
> ```c++
> 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`).
>
> ```c++
> 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. 😊
Hi，It's a pleasure to have a friendly discussion with you.
1.Google's programming standards clearly state that global variables should
be used with caution.
https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables
2.Dependency order problems (UB). The snippet below is nothing todo with
brpc, but what if someone wants to use this global variable in the future?
File1.cpp
```
#include <iostream>
// Declaration of the global variable in another file
extern int g_var2;
int g_var1 = g_var2 + 1; // Depends on the initialization order of g_var2
void func1() {
std::cout << "func1: g_var1 = " << g_var1 << std::endl;
}
```
File2.cpp
```
#include <iostream>
// Declaration of the global variable in another file
extern int g_var1;
int g_var2 = g_var1 + 1; // Depends on the initialization order of g_var1
void func2() {
std::cout << "func2: g_var2 = " << g_var2 << std::endl;
}
int main() {
func1();
func2();
return 0;
}
```
3.About memory usage, lazy initialization is achieved by using functions to
return static local variables, which are initialized only when needed, saving
unnecessary resource overhead.
4.About performance, I have the same view with you.
In my usual projects, I prefer to use functions. In fact, I have done a lot
of performance profiling, and this method has never become a performance
bottleneck. Maybe what we are discussing is just some programming habits and
trade-offs, but I don't think this will hinder the inclusion of PRs.
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