[英]Kotlin: Default value for the argument(s) of generic method
Why this is correct and the below one is wrong?为什么这是正确的,而下面的错误?
fun main () { AppModule().provideHttpClient(CIO) }
fun <T: HttpClientEngineConfig> provideHttpClient(engineFactory: HttpClientEngineFactory<T> = CIO): HttpClient
Type mismatch.
类型不匹配。
Required:HttpClientEngineFactory必需:HttpClientEngineFactory
Found: CIO发现:首席信息官
With CIO being defined as: CIO被定义为:
public object CIO: HttpClientEngineFactory<CIOEngineConfig> { init { addToLoader() } override fun create(block: CIOEngineConfig.() -> Unit): HttpClientEngine = CIOEngine(CIOEngineConfig().apply(block)) override fun toString(): String = "CIO" }
The semantics of a generic method is that "I work with any type", so the type parameters of a generic method is specified by the caller - the caller gets to decide what T
is.泛型方法的语义是“我使用任何类型”,因此泛型方法的类型参数由调用者指定 - 调用者决定
T
是什么。 Therefore, the default value that you specify as the callee must be compatible with HttpClientEngineFactory<T>
, for any T
that the caller might pass in.因此,对于调用者可能传入的任何
T
,您指定为被调用者的默认值必须与HttpClientEngineFactory<T>
兼容。
Using CIO
doesn't work here, because you are forcing T
to be CIOEngineConfig
.在这里使用
CIO
不起作用,因为您强制T
成为CIOEngineConfig
。
Imagine what would happen if the default value of CIO
was allowed, and the caller did:想象一下,如果允许
CIO
的默认值,并且调用者这样做了,会发生什么:
AppModule().provideHttpClient<SomeOtherEngineConfig>()
From the way that provideHttpClient
is declared, this should be possible - I'm passing SomeOtherEngineConfig
for the generic type parameter T
, and since the engineFactory
parameter has a default value, I don't need to pass any other parameters.从声明
provideHttpClient
的方式来看,这应该是可能的 - 我为泛型类型参数T
传递SomeOtherEngineConfig
,并且由于engineFactory
参数具有默认值,因此我不需要传递任何其他参数。 But when this actually gets run, the default value of CIO
is used for a parameter of type HttpClientEngineFactory<SomeOtherEngineConfig>
!但是当它实际运行时,
CIO
的默认值用于HttpClientEngineFactory<SomeOtherEngineConfig>
类型的参数!
It's easy to workaround this: simply declare another overload of provideHttpClient
that is not generic:解决这个问题很容易:只需声明另一个非通用的
provideHttpClient
重载:
fun provideHttpClient(): HttpClient = provideHttpClient(CIO)
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