tinloaf opened a new pull request, #2755:
URL: https://github.com/apache/thrift/pull/2755
Overview
=======
Both the default constructor and operator== implementations reference
certain member functions of the class' members. As an example, the default
constructor references (i.e., "uses") the default constructors of its members.
If a class contains a std::vector<Foo>, and Foo has only been *forward*-
declared (which happens often in Thrift-generated code), this creates undefined
behavior: The std::vector specification states that as long as Foo is an
incomplete type, it is fine to reference std::vector<Foo>, but not any members
(such as its default constructor).
Thus, we must defer our default constructor's implementation (which
references the default constructor of std::vector<Foo>) to a point where Foo is
a complete type. That is the case in the .cpp file.
The same holds for operator==.
Example
======
Take this very simple Thrift file:
```
struct StructA {
11:required list<StructB> myBs
}
struct StructB
{
1:required string someString
}
```
If I compile this using `thrift --gen cpp:no_skeleton -o out ./file.thrift`
I get a file that contains the following (full file
[here](https://gist.github.com/tinloaf/1f7ae57df383d7b87221598e4f7a700d) ):
```
class StructA;
class StructB;
class StructA : … {
public:
…
StructA() noexcept {
}
…
std::vector<StructB> myBs;
…
};
…
class StructB : … {
…
};
```
In this case, the default constructor for `StructA` references the default
constructor of `std::vector<StrungB>` while `StructB` is still an incomplete
type. *This is undefined behavior.* It did apparently compile with all big
compilers until recently, but with C++20, Clang 15 stops accepting this kind of
construct, as you can see [here at
CompilerExplorer](https://godbolt.org/z/xcc4av6cb).
Checklist
======
- [ ] Did you create an [Apache
Jira](https://issues.apache.org/jira/projects/THRIFT/issues/) ticket? (not
required for trivial changes)
**No**: I do not have a Jira account. I was told the right way to get a Jira
account is to post to the mailing list, which [I
did](https://lists.apache.org/thread/wmk0nb8lgrtyxrznp1ogd7b0b9whossn). I did
not receive a response, so I just went ahead and created this PR.
- [ ] If a ticket exists: Does your pull request title follow the pattern
"THRIFT-NNNN: describe my issue"?
*Note:* As far as I can tell, no ticket exists for this.
- [x] Did you squash your changes to a single commit? (not required, but
preferred)
- [x] Did you do your best to avoid breaking changes? If one was needed,
did you label the Jira ticket with "Breaking-Change"?
- [ ] If your change does not involve any code, include `[skip ci]` anywhere
in the commit message to free up build resources.
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