How to Properly Initialize Nested Structs in Golang?

Nested Struct Initialization in Golang

When working with nested structs in Golang, initializing the main struct can be tricky. This guide aims to provide a solution to the error encountered when trying to initialize a struct with an embedded anonymous struct as a field.

Error Encountered

type DetailsFilter struct {
  Filter struct {
    Name    string
    ID      int
  }
}

var M map[string]interface{}
M = make(map[string]interface{})
M["Filter"] = map[string]interface{}{"Name": "XYZ", "ID": 5}
var detailsFilter = DetailsFilter{Filter: M["Filter"]}}

This code attempts to initialize a DetailsFilter struct with a nested anonymous struct Filter. However, when trying to initialize the Filter field from a map, an error is encountered:

can not use (type interface {}) as type struct in field value : need type assertion

Solution

The recommended solution is to avoid initializing the nested anonymous struct during construction. Instead, initialize the zero-valued struct and then assign values to the nested fields:

df := DetailsFilter{}
df.Filter.Name = "myname"
df.Filter.ID = 123

Another alternative is to name the anonymous struct type and initialize it explicitly:

type Filter struct {
    Name string
    ID   int
}

type DetailsFilter struct {
    Filter Filter
}

df := DetailsFilter{Filter: Filter{Name: "myname", ID: 123}}

Additional Notes

• The error encountered is because the map contains an interface{} value, which cannot be directly assigned to a struct field.
• Naming the anonymous struct type allows for more explicit initialization.
• When working with nested structs, it's important to understand the limitations of different initialization approaches.

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