template/type deduction and 'oneof' in message

[Alex Shaver]

Suppose I have a message:

message Foo{
  oneof b_message {
    Bar = 16;
    Baz = 17;
  }
}

and I have some C++ handler that is a templated function:

template<class T>
void b_handler(const T&);
template void b_handler(const Bar& bar){...}
template void b_handler(const Baz& baz){...}  

What I'd like to be able to do is:

void foo_handler(const Foo& foo){
  ...
  b_handler(foo.b_message());
}  

Essentially, if protoc generated some kind of function like

template<class T>
const T& Foo::b_message() const;
const Bar& Foo::b_message() const;
const Baz& Foo::b_message() const;

that would make for automatic handling of whatever kind of b_message Foo had carried, rather than doing a switch(foo.b_message_case()) type decision tree.

I'm sure there are a lot of edge cases that I'm not thinking of, or cross-language considerations, but is anything like this possible?

[Feng Xiao]

On Thu, May 19, 2016 at 10:31 AM, Alex Shaver <alexs...@gmail.com> wrote:

Suppose I have a message:

message Foo{
  oneof b_message {
    Bar = 16;
    Baz = 17;
  }
}

and I have some C++ handler that is a templated function:

template<class T>
void b_handler(const T&);
template void b_handler(const Bar& bar){...}
template void b_handler(const Baz& baz){...}  

What I'd like to be able to do is:

void foo_handler(const Foo& foo){
  ...
  b_handler(foo.b_message());
}  

Essentially, if protoc generated some kind of function like

template<class T>
const T& Foo::b_message() const;
const Bar& Foo::b_message() const;
const Baz& Foo::b_message() const;

that would make for automatic handling of whatever kind of b_message Foo had carried, rather than doing a switch(foo.b_message_case()) type decision tree.

It's pretty easy to write a protoc plugin to generate additional code. What I don't understand though is how can you have two methods with the same signature return different types? Is that supported by C++?

 

I'm sure there are a lot of edge cases that I'm not thinking of, or cross-language considerations, but is anything like this possible?

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[Alex Shaver]

So, the cpprefernce page on template type deduction seemingly suggests you can: http://en.cppreference.com/w/cpp/language/template_argument_deduction

[Adam Cozzette]

I don't think this kind of type deduction is possible here, because the type of the oneof can't be known until runtime and so the compiler has no way of knowing which function to select. Alex, I believe the convert example at the top of your link works but only because the caller explicitly specifies the return type, using e.g. convert<int>(d). Ultimately I would guess that you either need a switch statement or something like a std::map that maps each enum value to the right handler function taking const Foo& as its argument.