Idiomatic way to cast a string to any other primitive type

Question

I'm trying to create a function that can cast a given string to the given reflect type.

I'm using the cast package:

package main

import (
    "fmt"
    "reflect"
    "strings"

    "github.com/spf13/cast"
)

type functions struct{}

func (f functions) Float64(v string) float64 {
    return cast.ToFloat64(v)
}

func toTarget(v string, target reflect.Kind) interface{} {
    n := strings.Title(fmt.Sprintf("%s", target))
    method := reflect.ValueOf(functions{}).MethodByName(n)
    // Call.
    return method.Call([]reflect.Value{reflect.ValueOf(v)})[0].Interface()
}

func main() {
    originalValue := "10.0"
    floatingValue := toTarget(originalValue, reflect.Float64)

    fmt.Println(floatingValue)
}

In the above example I'm keeping simple (it only works for string -> float64 conversion), but on my code, it will work for all the other primitives as well.

I prefer using this solution over a giant and ugly switch statement, but as a new go developer, I'm not sure if there is a better approach.

Thank you for your help.

There is no typecasting in Go at all, idiomatic or otherwise. Only type conversion. — Jonathan Hall, Jan 07 '19 at 13:12
Why not just use `n, err := fmt.Sscan("someString", &variable)`? This will handle most types you'll need. For the rest, you need custom parsing anyway... — icza, Jan 07 '19 at 13:14
Hey @icza, could you provide a working example of using the `fmt.Sscan`? I'm trying but it keeps returning error `expected integer`. — ubaxibuna, Jan 07 '19 at 13:21
Why the downvote? The question is valid and provides what I'm trying to accomplish. — ubaxibuna, Jan 07 '19 at 13:22
@ubaxibuna Using `fmt.Sscan()`, you have to pass the address of a variable, not a `reflect.Kind`, e.g. `var i int; fmt.Sscan("12", &i)` — icza, Jan 07 '19 at 13:23
Do you mind to properly answer the question, you deserve points for helping me haha — ubaxibuna, Jan 07 '19 at 13:26

icza · Accepted Answer · 2019-01-07T13:55:55.883

The "big switch" statement you want to avoid, that is already written in the standard library. Use the fmt package to easily parse primitive values from strings, specifically the fmt.Sscan() and fmt.Sscanf() functions.

fmt.Sscan() requires a string value to parse something out from, and the address of a variable to put the parsed value into. The type of the pointed variable also determines what and how to parse out of the string. fmt.Sscan() will return the number of successfully parsed values, and an optional error (if something goes wrong).

A simple example:

var i int
if _, err := fmt.Sscan("12", &i); err != nil {
    fmt.Println("Error:", err)
}
fmt.Println(i)

var f float32
if _, err := fmt.Sscan("12.2", &f); err != nil {
    fmt.Println("Error:", err)
}
fmt.Println(f)

Output (try it on the Go Playground):

12
12.2

Also note that you can parse multiple values in one step with fmt.Sscan(), for example:

var i int
var f float32

fmt.Println(fmt.Sscan("12 12.2", &i, &f))
fmt.Println(i, f)

This will print (try it on the Go Playground):

2 <nil>
12 12.2

The first line contains the return values of fmt.Sscan(): tells it parsed 2 values, and returned no error (nil error). The second line contains the parsed values of i and f.

For more options, read: Convert string to integer type in Go?

Perfect! Thank you very much icza! – ubaxibuna Jan 07 '19 at 14:58 — ubaxibuna, Jan 07 '19 at 14:58

Idiomatic way to cast a string to any other primitive type

1 Answers1