go - 关于 Go 语法的困惑

Question

我在 golang.org 的 net 包源代码中看到了这一点。

c, err := dial(network, ra.toAddr(), dialer, d.deadline())
if d.KeepAlive > 0 && err == nil {
    if tc, ok := c.(*TCPConn); ok {
        tc.SetKeepAlive(true)
        tc.SetKeepAlivePeriod(d.KeepAlive)
        testHookSetKeepAlive()
    }
}
return c, err

在这种情况下，c.(*TCPConn) 到底在做什么？我最初以为这是某种类型转换，但它返回 2 个值给 tc 和 ok。

这让我很困惑。有人可以解释这段代码的作用吗？

source code here第 171 行。

Answer 1

The Go Programming Language Specification

Type assertions

For an expression x of interface type and a type T, the primary expression

x.(T)

asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

A type assertion used in an assignment or initialization of the special form

v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)

yields an additional untyped boolean value. The value of ok is true if the assertion holds. Otherwise it is false and the value of v is the zero value for type T.

如果接口(interface)类型 Conn 的 c 包含类型 *TCPConn 的值，则 ok为真并且 tc 被设置为 *TCPConn 类型的值存储在 c 中。 c 还可以包含 nil、*UDPConn、*UnixConn 等。在这种情况下，tc 将为 nil 而 ok 将为 false。

if tc, ok := c.(*TCPConn); ok {
    tc.SetKeepAlive(true)
    tc.SetKeepAlivePeriod(d.KeepAlive)
    testHookSetKeepAlive()
}