I'm struggling to figure out a reason for this behavior, or maybe this is suppose to happen and I just wasn't aware.
For background, I'm using proto3, and am doing this in Go1.15, and I do know that packed is the default in proto3, and I'm relatively new to protobufs.
I defined the following message in a proto file:
message Response {
repeated uint32 points = 1 [packed=true];
}
Which will generate the following code using protoc-gen-go v1.25.0.
type Response struct {
state protoimpl.MessageState
sizeCache protoimpl.SizeCache
unknownFields protoimpl.UnknownFields
Points []uint32 `protobuf:"varint,3,rep,packed,name=points,json=points,proto3" json:"points,omitempty"`
}
I go to use the new struct, and it doesn't behave like I would normally expect a struct to behave. Here's some things I wrote, along with what was printed out.
newResponse := pb.Response{Points: []uint32{2,4,6,8}}
fmt.Println(newResponse)
//{{{} [] [] <nil>} 0 [] [2 4 6 8] --> I expect this
refToNewResponse := &newResponse
fmt.Println(refToNewResponse)
// points:2 points:4 points:6 points:8 --> not what I expected
Now you might be thinking, it's just formatting big deal.
But I expect a list... not numbers that each individually have a label. I've seen and used other protobufs... and when I see the response that they return, it doesn't look like this, it's one label to a list like:
points: [2 4 6 8]
I do need to use the reference version of this because I eventually want to expand and use a list of Responses which the generated code will spit out a slice of pointer Responses, but I can't understand why it's separating and labeling each element in the slice.
I'm hoping someone can point out something I'm doing or not doing that is causing this... thank you in advance.
This is indeed just formatting. Nothing has changed in the underlying data structure. You requested a repeated uint32 Points and it's literally printing them that way.
The marshaler in the protobuf implementation can really output whatever it likes, there is no reference version of the human-readable representation of a protobuf.
If you really must have a custom format for the .String() output, you can try a different proto library such as gogoprotobuf, or try various extensions. But ultimately, it's just human-readable output.
Note:
this has nothing to do with packed=true (which is indeed the default).
if you're confused about printing the pointer vs the basic type, it's because the String() method has a pointer receiver. See this question
Related
I want to know which is the best way to create instances of a certain struct based on a map[string]string
My app should process huge files in CSV format and should create an instance of a struct for each row of the file.
I'm already using the encoding/csv/Reader from golang to read the CSV file and create an instance of map[string]string for each row in the file.
So given this file:
columnA, columnB, columnC
a, b, c
My own reader implementation will return this map (each row values with the header):
myMap := map[string]string{
"columnA": "a",
"columnB": "b",
"columnC": "c",
}
(this is just an example in real life the file contains a lot of columns and rows)
so.. at this point I need to create an instance of the struct that is related with the row contents, let say:
type MyStruct struct {
AColumn string
BColumn string
CColumn string
}
My question is what could be the best way to create the instance of the struct using the given map, I have already implemented a version that just copy each value from the map to the struct but the code ended up being very long and tedious:
s := &MyStruct{}
s.AColumn := m["columnA"]
s.AColumn := m["columnB"]
s.AColumn := m["columnC"]
...
I also consider using this library https://github.com/mitchellh/mapstructure but I don't know if using reflection could be the best approach considering that the file is huge and will be using reflection for each row.
Maybe there is no other option but I'm asking just in case someone knows a better approach.
Thanks in advance.
I would say that the idiomatic Go way would be just populating the struct's fields from your map. Go favors explicitness this approach is the more direct and the easiest to read. In other words, your approach is correct.
You could make it slightly nicer by initializing the struct directly:
s := &MyStruct{
AColumn: m["columnA"],
BColumn: m["columnB"],
CColumn: m["columnC"],
}
Now, if your structure has 100s of fields (which is an odd design choice), you may want to leverage some code generation. Otherwise, just go with the straightforward code - it's the best approach in the long term.
I already posted a library that I made for some stuff I have needed sometimes, I've made a MapToStruct fews months ago, I pushed that today to share with you the full library. The library is based in the usage of reflect, I still testing and implementing stuff, you will find some odd comments and these kind of things.
https://github.com/FedeMFernandez/goscript
I Hope it is useful
The struct in the .pb.go file generated by .proto file has three additional fields and some other things.like this:
When converting this struct to json, if one field is empty, the field will not appear in json. Now I know it can be done using jsonpb.Marshaler.
m := jsonpb.Marshaler{EmitDefaults: true}
Now, I coverting struct to map[string]interface{}, put it in
InfluxDB. I have to convert struct to map[string]interface{}.The function NewPoint needs. like this:
I use structs.Map(value) function in go ,The transformed map has three additional fields, and running the program causes errors，like this:
{"error":"unable to parse 'txt,severity=1 CurrentValue=\"1002\",MetricAlias=\"CPU\",XXX_sizecache=0i,XXX_unrecognized= 1552551101': missing field value"}
When I remove these three fields, the program runs OK.These three fields are automatically generated, and I have a lot of structs.
What should I do?Thank you!
Protobuf generator adds some additional fields with names starting from XXX that are intended for optimizations. You can't change this behavior of protoc-gen-go.
The problem is in the way you convert struct to map[sting]interface{}. It's hard to figure out from which package exactly structs.Map comes from. Seems like it goes from here: https://github.com/fatih/structs/blob/master/structs.go#L89 - this code uses reflect to iterate through all fields of the structure and push them to map[sting]interface{}. You just need to write your own slightly modified version of FillMap routine that will omit XXX fields.
For testing, I often see go code read byte slices, which are parsed into structs using yaml, for example here:
https://github.com/kubernetes/kubernetes/blob/master/pkg/util/strategicpatch/patch_test.go#L74m
I just got bitten by not exporting my field names, resulting in an empty list which I iterated over in my test cases, thus assuming that all tests were passing (in hindsight, that should have been a red flag :)). There are other errors which are silently ignored by yaml unmarshaling, such as a key being misspelled and not matching a struct field exactly.
Is there a way to ensure that all the data in the byte slice was actually parsed into the struct returned by yaml.Unmarshal? If not, how do others handle this situation?
go-yaml/yaml
For anyone searching for a solution to this problem, the yaml.v2 library has an UnmarshalStrict method that returns an error if there are keys in the yaml document that have no corresponding fields in the go struct.
import yaml "gopkg.in/yaml.v2"
err := yaml.UnmarshalStrict(data, destinationStruct)
BurntSushi/toml
It's not part of the question, but I'd just like to document how to achieve something similar in toml:
You can find if there were any keys in the toml file that could not be decoded by using the metadata returned by the toml.decode function.
import "github.com/BurntSushi/toml"
metadata, err := toml.Decode(data, destinationStruct)
undecodedKeys := metadata.Undecoded()
Note that metadata.Undecoded() also returns keys that have not been decoded because of a Primitive value. You can read more about it here.
Json
The default go json library does not support this currently, but there is a proposal ready to be merged. It seems that it will be a part of go 1.10.
Now that we've reached Swift 2.0, I've decided to convert my, as yet unfinished, OS X app to Swift. Making progress but I've run into some issues with using termios and could use some clarification and advice.
The termios struct is treated as a struct in Swift, no surprise there, but what is surprising is that the array of control characters in the struct is now a tuple. I was expecting it to just be an array. As you might imagine it took me a while to figure this out. Working in a Playground if I do:
var settings:termios = termios()
print(settings)
then I get the correct details printed for the struct.
In Obj-C to set the control characters you would use, say,
cfmakeraw(&settings);
settings.c_cc[VMIN] = 1;
where VMIN is a #define equal to 16 in termios.h. In Swift I have to do
cfmakeraw(&settings)
settings.c_cc.16 = 1
which works, but is a bit more opaque. I would prefer to use something along the lines of
settings.c_cc.vim = 1
instead, but can't seem to find any documentation describing the Swift "version" of termios. Does anyone know if the tuple has pre-assigned names for it's elements, or if not, is there a way to assign names after the fact? Should I just create my own tuple with named elements and then assign it to settings.c_cc?
Interestingly, despite the fact that pre-processor directives are not supposed to work in Swift, if I do
print(VMIN)
print(VTIME)
then the correct values are printed and no compiler errors are produced. I'd be interested in any clarification or comments on that. Is it a bug?
The remaining issues have to do with further configuration of the termios.
The definition of cfsetspeed is given as
func cfsetspeed(_: UnsafeMutablePointer<termios>, _: speed_t) -> Int32
and speed_t is typedef'ed as an unsigned long. In Obj-C we'd do
cfsetspeed(&settings, B38400);
but since B38400 is a #define in termios.h we can no longer do that. Has Apple set up replacement global constants for things like this in Swift, and if so, can anyone tell me where they are documented. The alternative seems to be to just plug in the raw values and lose readability, or to create my own versions of the constants previously defined in termios.h. I'm happy to go that route if there isn't a better choice.
Let's start with your second problem, which is easier to solve.
B38400 is available in Swift, it just has the wrong type.
So you have to convert it explicitly:
var settings = termios()
cfsetspeed(&settings, speed_t(B38400))
Your first problem has no "nice" solution that I know of.
Fixed sized arrays are imported to Swift as tuples, and – as far as I know – you cannot address a tuple element with a variable.
However,Swift preserves the memory layout of structures imported from C, as
confirmed by Apple engineer Joe Groff:. Therefore you can take the address of the tuple and “rebind” it to a pointer to the element type:
var settings = termios()
withUnsafeMutablePointer(to: &settings.c_cc) { (tuplePtr) -> Void in
tuplePtr.withMemoryRebound(to: cc_t.self, capacity: MemoryLayout.size(ofValue: settings.c_cc)) {
$0[Int(VMIN)] = 1
}
}
(Code updated for Swift 4+.)
As you could probably tell from the below code I am working on a project which creates csv reports from data in mongoDB. After getting the data I need in, I need to structure the data into something more sensible then how it exists in the db, which is fairly horrendous (not my doing) and near impossible to print the way I need it. The structure that makes the most sense to me is a slice (for each document of data) of maps of the name of the data to a structure holding the data for that name. Then I would simply have to loop through the document and stuff values into the structs where they belong.
My implementation of this is
type mongo_essential_data_t struct {
caution string
citation string
caution_note string
}
mongo_rows_struct := make([]map[string]mongo_essential_data_t, len(mongodata_rows))
//setting the values goes like this
mongo_rows_struct[i][data_name].caution_note = fmt.Sprint(k)
//"i" being the document, "k" being the data I want to store
This doesn't work however. When doing "go run" it returns ./answerstest.go:140: cannot assign to mongo_rows_struct[i][data_name].caution_note. I am new to Go and not sure why I am not allowed to do this. I'm sure this is an invalid way to reference that particular data location, if it is even possible to reference it in Go. What is another way to accomplish this setting line? If it is too much work to accomplish this the way I want, I am willing to use a different type of data structure and am open to suggestions.
This is a known issue of Golang, known as issue 3117. You can use a temporary variable to get around it:
var tmp = mongo_rows_struct[i][data_name]
tmp.caution_note = fmt.Sprint(k)
mongo_rows_struct[i][data_name] = tmp
as per my understanding, when you write:
mongo_rows_struct[i][data_name]
compiler will generate code, which will return copy of mongo_essential_data_t struct(since struct in go is value type, not reference type), and
mongo_rows_struct[i][data_name].caution_note = fmt.Sprint(k)
will write new value to that copy. And after that copy will be discarded. Obviously, its not what you expect. So Go compiler generate error to prevent this misunderstanding.
In order to solve this problem you can:
1. Change definition of your data type to
[]map[string]*mongo_essential_data_t
2. Explicitly create copy of your struct, make changes in that copy and write it back to the map
data := mongo_rows_struct[i][data_name]
data.caution_note = fmt.Sprint(k)
mongo_rows_struct[i][data_name] = data
Of course, first solution is preferable because you will avoid unnecessary copying of data