[THRIFT-4060] Thrift printTo ostream overload mechanism breaks down when types are nested

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.9.3, 0.10.0
• Fix Version/s: 0.11.0
• Component/s: C++ - Compiler

Environment

Ubuntu 14.04 LTS

• Assignee: James E. King III
• Reporter: James E. King III
• Created: 30/Jan/17 19:20
• Updated: 14/Dec/17 13:55
• Resolved: 13/Feb/17 21:41

Description

I'm in the middle of converting a project that provides some ostream operators (for logging purposes) for a number of thrift structures. The project was using 0.8.0 and in 0.9.3 some ostream operator overloads were added with THRIFT-3336. The solution that was provided here runs into complications if a thrift structure is contained within another one. Take this simple example:

Thrift

namespace cpp example.detail

struct Lower {
   1: binary lowerBinary
}

struct Higher {
    1: set<Lower> lowers
}

C++

namespace example {

class MyLower : public detail::Lower
{
    virtual void printTo(std::ostream& os) const override;
}

class MyHigher : public detail::Higher
{
    virtual void printTo(std::ostream& os) const override
    {
        os << lowers;
        // here's the problem, lowers is a set<detail::Lower> when it serializes in,
        // not a set<MyLower>, so I cannot override it...
    }
}

}

I'm considering adding an annotation to the thrift IDL that the compiler will recognize that allows someone to say, "I am going to provide my own operator<< for this structure, don't generate one". This would replace the printTo mechanism that was added in THRIFT-3336.

Here is an example:

namespace cpp example.detail

struct MyLower {
   1: binary lowerBinary
} (cpp.customostream)

struct MyHigher {
    1: set<Lower> lowers
}

The annotation cpp.customostream (or the compiler option --gen cpp:no_ostream_operators for global effect) tells the compiler to emit only the declaration of the operator << but does not emit a definition (implementation). It would be up to the implementation building against the generated code to provide an operator << if such an operator is needed for conversion to a stream (cout, lexical_cast to string, etc..).

Issue Links

• is broken by: THRIFT-3336 Thrift generated streaming operators added in 0.9.2 cannot be overridden
• links to: GitHub Pull Request #1172