0x00 Lesson
Optional strings might contain a string like “Hello” or they might be nil – nothing at all.
Consider this optional string:
vaf name: String? = nil
What happens if we use name.count? A real string has a count property that stores how many letters it has, but this is nil – it’s empty memory, not a string, so it doesn’t have a count.
Because of this, trying to read name.count is unsafe and Swift won’t allow it. Instead, we must look inside the optional and see what’s there – a process known as unwrapping.
A common way of unwrapping optionals is with if let syntax, which unwraps with a condition. If there was a value inside the optional then you can use it, but if there wasn’t the condition fails.
if let unwrapped = name {
print("(unwrapped.count) letters")
} else {
print("Missing name.")
}
if name holds a string, it will be put inside unwrapped as a regular String and we can read its count property inside the condition. Alternatively, if name is empty, the else code will be run.
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原文地址:https://blog.csdn.net/xjh093/article/details/128897323
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