chenBright opened a new pull request, #2620:
URL: https://github.com/apache/brpc/pull/2620
### What problem does this PR solve?
Issue Number:
Problem Summary:
目前FlatMap是两阶段初始化：构造函数+init函数。这种初始化方式很容易用错并出core，所以使用上必须小心翼翼，体验不好。例如以下代码：
```
butil::FlatMap<int, int> m1;
if (m1.init(32) != 0) {}
butil::FlatMap<int, int> m2 = m1;
```
根据`operator=`可知，即使m1已经初始化了，但是因为m1是空的，所以将m1赋值给m2后，m2仍然是未初始化的，一旦被使用，基本上就会出core。
https://github.com/apache/brpc/blob/68aff6947c10d92bf44aa585b1913deb91cf8f92/src/butil/containers/flat_map_inl.h#L265-L274
当然上述例子是bug，但是可以反映出目前FlatMap使用上的不便性和易错性。
### What is changed and the side effects?
Changed:
FlatMap构造完成即完成初始化。使用sso、IOBuf::SmallView的策略，初始化_buckets和_thumbnail使用类内的内存块，没有init函数中Alloc失败的顾虑。
Side effects:
- Performance effects(性能影响):
- Breaking backward compatibility(向后兼容性):
---
### Check List:
- Please make sure your changes are compilable(请确保你的更改可以通过编译).
- When providing us with a new feature, it is best to add related
tests(如果你向我们增加一个新的功能, 请添加相关测试).
- Please follow [Contributor Covenant Code of
Conduct](../../master/CODE_OF_CONDUCT.md).(请遵循贡献者准则).
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