This can be done by simply make the type itself unexported. You can export the function NewSomething and even the fields Text and DefaultText, but just don't export the struct type something. Another way to customize it for you own module is by using a Config struct to set default values Option 5 in the link Not a good way though. Learn more. Asked 1 year, 2 months ago. Active 1 year, 2 months ago.
Viewed 1k times. Valy Valy 23 4 4 bronze badges. Active Oldest Votes. You can export the function NewSomething and even the fields Text and DefaultText, but just don't export the struct type something Another way to customize it for you own module is by using a Config struct to set default values Option 5 in the link Not a good way though.
Original Post. Ceus Ceus 13 5 5 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.My Go courses are discounted for the next few weeks to help out anyone who may need or want access to them. I'm also going to try to help out anyone who can't afford a course, and I will be writing posts about working from home over the next week in an attempt to help anyone new to WFH.
Assign Default Value for Struct Field
Read more here. I was recently skimming my Twitter feed for new Go articles, libraries, etc and I stumbled across a tweet with this message:. Regardless, this struck me as a big cry for help. In JSON there are effectively two types of null.
Unfortunately or fortunately? That field will always be present, and the value of that field will be nil or a valid value. This leaves us with two options for PublishedAt - either it will reference an integer, or it will be nil.
Where this does come into play is when we are doing partial updates. But what happens when the user does want to update this field? So how to we handle this in Go? The first thing to note is that we can customize how JSON gets unmarshalled for any type by implementing the Unmarshaler interface. That means we need to make sure we DO NOT use a pointer if we need to determine if a key has been set.
Putting those two tidbits together, we can create a custom JSONInt type that will be used to determine if a value has been set, and whether or not it is null. Specifically, we can set a few flags to dictate whether or not the key was provided. If you want to see this in action, we can test it out with a few JSON strings using the code below, or you can run it on the Go Playground.
While there are likely better API designs to avoid this issue, if you do ever run into a situation where you need to support both types of null for a JSON key you can leverage this technique to make it happen. Simply replace the Value field with whatever type you need to support and you should be set. The sample includes three chapters from the book, and over 2.
You will also receive notifications when I release new articles, along with other freebies that I only share with my mailing list.
Jon Calhoun is a full stack web developer who also teaches about Go, web development, algorithms, and anything programming related. He also consults for other companies who have development needs.
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. All-in-all, I have a feeling this looks like an XY problem. From your question, it appears you want to have a default value on one of your fields if nothing was unmarshaled.
If so, just post-process the values of this type unmarshaled from JSON and if nothing was unmarshaled to Encodning set it to whatever default you want. Since your FileData isn't too complex, you can easily make use of json.
Unmarshaler interface.Basic JSON in Go
Declare Encoding as a separate type and set the default value in the unmarshal method:. Now when you encode a json with empty Encoding value, it'll be set to base64 :. Learn more. Ask Question. Asked 3 years, 7 months ago. Active 3 years, 7 months ago.
Viewed 20k times.
How to use JSON with Go [best practices]
How to set default value for Encoding as "base64"? JagsSparrow JagsSparrow 4, 5 5 gold badges 23 23 silver badges 39 39 bronze badges. Active Oldest Votes. You can't because in Go types do not have constructors. Alternatively you might consider this approach: Have a custom type for your field.
Something like type EncodingMethod string should do. Have a custom JSON unmarshaling method for this type which would do whatever handling it wants.
Son Bui Son Bui 5 5 bronze badges. Println f, f. Sign up or log in Sign up using Google. Sign up using Facebook.
Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. Featured on Meta.This Golang tutorial help to understand marshalling and un-marshalling of data using Golang.
We can encode and decode struct data using marshal and unmarshal. JSON is a language-independent data format. Marshal method help to convert Struct into Byte dataThis method takes object as a param and returned Bytes code. We will create simple employee struct, that have name, age and salary property. We will convert Employee struct data into json using json. Marshal method. The below code use to convert Struct data into Byte code using golang marshaling.
Line 1: Creating object using Employee Struct. Line 2: Convert Struct Object into Byte data. Line 3: Convert Byte data into json string to display data. Unmarshal method help to convert json Byte data into Struct ObjectThis method takes json byte data as a param and returned struct object. I am using same above json string and convert into byte data, Finally convert json byte data into Employee struct. The Employee struct have Name, Age and Salary property. The string keys in the JSON are matched to the field names in the structs.
We will convert above json string byte data and convert into Employee struct using json. Unmarshal method. We will create Response Struct that will use to match byte code using unmarshal. Line 1: Creating json string into byte code. Line 2: Create empty Response struct and assign res variable.
Line 3: Unmarshal by passing a pointer to an empty structs. Line 3: Print the Struct Name value. Table of Contents. Name string. Age int. Println string emp.
Cyclic data structures are not supported; they will cause Marshal to go into an infinite loop.
Pointers will be encoded as the values they point to or 'null' if the pointer is nil. The json package only accesses the exported fields of struct types those that begin with an uppercase letter. Therefore only the the exported fields of a struct will be present in the JSON output.
Unmarshalpassing it a byte of JSON data and a pointer to m. If b contains valid JSON that fits in mafter the call err will be nil and the data from b will have been stored in the struct mas if by an assignment like:. How does Unmarshal identify the fields in which to store the decoded data? For a given JSON key "Foo"Unmarshal will look through the destination struct's fields to find in order of preference :.
An exported field with a tag of "Foo" see the Go spec for more on struct tags. Unmarshal will decode only the fields that it can find in the destination type. In this case, only the Name field of m will be populated, and the Food field will be ignored. This behavior is particularly useful when you wish to pick only a few specific fields out of a large JSON blob.
A struct type rectangle is declared that has three data fields of different data-types. Here, struct used without instantiate a new instance of that type. You can also add methods to struct types using a method receiver. A method EmpInfo is added to the Employee struct.
Struct can be nested by creating a Struct type using other Struct types as the type for the fields of Struct. Nesting one struct within another can be a useful way to model more complex structures. Creates an instance of rectangle struct by using a struct literal and assigning values to the fields of the struct.
An instance of a struct can also be created with the new keyword. It is then possible to assign data values to the data fields using dot notation. Method of assigning a custom default value can be achieve by using constructor function. Instead of creating a struct directly, the Info function can be used to create an Employee struct with a custom default value for the Name and Age field. A struct short for "structure" is a collection of data fields with declared data types. Golang has the ability to declare and create own data types by combining one or more types, including both built-in and user-defined types.
Each data field in a struct is declared with a known type, which could be a built-in type or another user-defined type. Structs are the only way to create concrete user-defined types in Golang. Struct types are declared by composing a fixed set of unique fields. Structs can improve modularity and allow to create and pass complex data structures around the system. You can also consider Structs as a template for creating a data record, like an employee record or an e-commerce product.
The declaration starts with the keyword type, then a name for the new struct, and finally the keyword struct. Within the curly brackets, a series of data fields are specified with a name and a type. During the definition of a struct type, optional string values may be added to each field declaration. Any changes made to r2 will not be applied to r1 and vice versa. When r3 is updated, the underlying memory that is assigned to r1 is updated.
Marshal emp if err! Solutions You Must Read. The reflect package support to check the underlying type of a struct.
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. How can I print in the console the IdTitleNameetc. This Go by example page provides another technique:.
If you don't have any instance, then you need to use reflection to display the name of the field of a given struct, as in this example. I want to recommend go-spewwhich according to their github "Implements a deep pretty printer for Go data structures to aid in debugging". MarshalIndent -- surprised this isn't suggested, as it is the most straightforward. I think it would be better to implement a custom stringer if you want some kind of formatted output of a struct.
To use just take your struct, initialize it in main or whatever package you are in and pass it into PrettyPrint. Play around with the code here. I recommend to use Pretty Printer Library. In that you can print any struct very easily. Also you can get difference between component through this library and so more.
You can also have a look on library Docs here. Visit here to see the complete code. Below is the program snippet that only prints the field names. There's also go-renderwhich handles pointer recursion and lots of key sorting for string and int maps. Another way is, create a func called toString that takes struct, format the fields as you wish. Learn more. How to print struct variables in console?
Subscribe to RSS
Ask Question. Asked 5 years, 9 months ago. Active 24 days ago. Viewed k times. All of them, for debugging? Try fmt. Active Oldest Votes.
To print the name of the fields in a struct: fmt.