Fix:(issue_1197) Set destination field from altsrc for slice flags

[dearchap]

What type of PR is this?

(REQUIRED)

• bug

What this PR does / why we need it:

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Which issue(s) this PR fixes:

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Testing

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Tested with user provided code. Will add unit tests to confirm this.

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[Dokiys]

@dearchap Hi, I'm a little confused about Destination, I don't quite understand what it does

[dearchap]

Any flags value can be retrieved using ctx.XXXTypeName("flagName") or if the Destination field in the flag is set, that value. For example for

var bar int
ifg := IntFlag{Name: "foo", Destination: &bar}

So if the flag is set on cmdline or from env you could do ctx.Int(foo) of do *ifg.Destination to get the value.

[Dokiys]

Any flags value can be retrieved using ctx.XXXTypeName("flagName") or if the Destination field in the flag is set, that value. For example for

var bar int
ifg := IntFlag{Name: "foo", Destination: &bar}

So if the flag is set on cmdline or from env you could do ctx.Int(foo) of do *ifg.Destination to get the value.

Why not use Value? Is it because it is possible that the Value could be changed? Taking StringSlice as an example, in StringSliceFlag.Apply or StringSliceFlag.ApplyInputSourceValue, the value of Destination is the same as the value of Value:

cli/flag_string_slice.go

Lines 118 to 124 in 9120e06

	func (f *StringSliceFlag) Apply(set *flag.FlagSet) error {
	// apply any default
	if f.Destination != nil && f.Value != nil {
	f.Destination.slice = make([]string, len(f.Value.slice))
	copy(f.Destination.slice, f.Value.slice)
	}

[dearchap]

Value is supplied by the user and is supposed to represent the default value(s) etc. User could supply a destination or not. So if it is specified it should match the Value