encoding/json: Marshal silently ignores custom implementations on pass by value

[mcandre]

What version of Go are you using (go version)?

$ go version
go version go1.11 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/tkmamhf/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/tkmamhf/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/tkmamhf/.gvm/gos/go1.11"
GOTMPDIR=""
GOTOOLDIR="/Users/tkmamhf/.gvm/gos/go1.11/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/y6/dbk1vvs179nbpfv38j__10b5rsnf87/T/go-build193800130=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Carefully implemented json.Marshaler and json.Unmarshaler interfaces, and passed my struct directly to json.Marshal().

What did you expect to see?

The resulting JSON byte array uses my custom json.Marshaler interface method.

What did you see instead?

The resulting JSON byte array ignores my custom json.Marshaler interface method, instead presenting the keys and values of the plain Go struct declaration. This causes downstream breakages, as my custom JSON marshaling is needed to offset some issues with the data I happen to be handling.

Workaround

As a workaround, I am making sure that I pass a reference (&) of my object to the json.Marshal() method. This is quite dangerous, as it is a really easy mistake to make, across a large codebase. And currently no way to validate this with the Go type system on the existing API. With other Go API calls, I at least get an annoying compile about needing a receiver pointer.

In the future, can we improve the json API to prevent this from happening?

[ericlagergren]

FWIW: you only need to pass a pointer to json.Unmarshal if you implement (*T).MarshalJSON. And if you always pass a pointer, you can implement either (*T).MarshalJSON or (T).MarshalJSON.

See: https://play.golang.org/p/4i6MZABdBa7

[antong]

If at all possible, please include in the "what did you do" section the actual code, and in the "what did you see instead" the actual result and errors if any. This would make it so much easier to analyze and help.

My guess is that the value you pass to json.Marshal(yourval) does not actually implement the json.Marshal interface at all, probably because the method had a pointer receiver e.g., func (x *MyType) MarshalJSON() ([]byte, error and you passed a MyType value, not a pointer.

Example: https://play.golang.org/p/K2M4Ap6uO-C

[gopherbot]

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