為什麼Go中的常數初始化後不能修改？

Why Can't Constants Be Freely Modified?

At first glance, an error like this one may seem confusing:

const (
        paths = &map[string]*map[string]string {
            Smith: {
                "theFather": "John",
            },
        }
        paths["Smith"]["theSon"] = paths["Smith"]["theFather"] + " Junior"
)

Why can't a constant be assigned and modified after its initial definition?

The Nature of Constants

The answer lies in the very nature of constants. They are intended to remain immutable throughout the program's execution. In Go, the type system defines the operations permitted on a given value type.

Constant Types and Operations

Unfortunately, the map type is not constant in Go. Maps allow for dynamic key-value pair modifications after their creation, rendering them unsuitable for constant declarations.

Types Allowed as Constants

The Go specification carefully defines the types that can be declared as constants:

• Boolean constants
• Rune constants
• Integer constants
• Floating-point constants
• Complex constants
• String constants

Alternative to Constant Maps

If you require an immutable map, consider using a sync.Map, offering thread-safe read-only access to a data structure.

Conclusion

While it may initially seem counterintuitive, the restriction against modifying constant maps in Go is essential for maintaining the integrity and reliability of your code. By adhering to these rules, you ensure that your constants stay true to their name, providing a solid foundation for your programs.

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