使库在其他语言中可用

This might seem like a silly question, but is it possible to write a library in GO that can be called from other languages, e.g. C++?

That's unfortunately not directly ("can be called") possible. There are some issues wrt what is defined by the platform's C implementation (for most/all of the officially supported platforms):

• Calling convention is not the same: For example, Go functions/methods don't use any registers for return values (if any).
• Execution model is different: Split stacks are used.
• Garbage collector may get confused by memory owned by the process but not "registered" by the GC as either "non-collectable" or specially marked (for precise collection).
• Initialization of the Go runtime is a problem. It expects to be done before anything else in the process. If you would link with more than one Go .so, no ready made mechanism for coordination of the initialization exists.

All of the above applies to 'gc'. The same is to certain extent relaxed in 'gccgo'. More info about this in C C_Interoperability.

Your best bet is JSON-RPC. I've been looking around for ways to integrate legacy Python code with Go, to no success, until I found this. If your data structures can be converted to JSON, you are good to go. Here's a silly example:

Go JSON-RPC server

import (
    "log"
    "net"
    "net/rpc"
    "net/rpc/jsonrpc"
)

type Experiment int

func (e *Experiment) Test(i *string, reply *string) error {
    s := "Hello, " + *i
    *reply = s
    log.Println(s, reply)
    return nil
}

func main() {
    exp := new(Experiment)
    server := rpc.NewServer()
    server.Register(exp)
    l, err := net.Listen("tcp", ":1234")
    if err != nil {
        log.Fatal("listen error:", err)
    }
    for {
        conn, err := l.Accept()
        if err != nil {
            log.Fatal(err)
        }
        server.ServeCodec(jsonrpc.NewServerCodec(conn))
    }
}

Python client

import json
import socket
s = socket.create_connection(("127.0.0.1", 1234))
s.sendall(json.dumps(({"id": 1, "method": "Experiment.Test", "params": ["World"]})))
print s.recv(4096)

Response

{"id":1,"result":"Hello, World","error":null}