Why Do Arrays of Empty Structs in Go Sometimes Compare as Equal Despite Their Pointers Being Different?

Array of Empty Structs Comparison: Understanding Variable Behavior

When comparing arrays of empty structs in Go, you may encounter unexpected results. Let's explore this behavior through code examples and understand why it occurs.

Consider the following code:

var s, ss struct{} // two empty structs
arr1 := [6]*struct{}{&s} // array with empty struct pointer
arr2 := [6]*struct{}{&ss} // array with empty struct pointer
fmt.Println(&s == &ss, arr1 == arr2)  // false, true

Here, we define the empty structs s and ss, initialize arrays arr1 and arr2 with pointers to these structs, and print the comparison results. Surprisingly, while &s and &ss are not equal, arr1 and arr2 are equal.

To understand this, we need to refer to the Go specifications. The specification for pointer values states that they are equal if they point to the same variable or both have a value of nil. Crucially, it notes that pointers to distinct zero-size variables "may or may not be equal."

Moreover, structs with no fields have a size of zero. Therefore, &s and &ss point to distinct zero-size variables, and the comparison &s == &ss may evaluate to either true or false.

Similarly, the behavior of the array comparison arr1 == arr2 is not guaranteed. The specification for zero-size variables indicates that they "may" have the same address in memory. Thus, arr1 and arr2 may or may not be equal, and the result can vary across different runs of the program.

To summarize, comparing pointers to distinct zero-size variables, such as empty structs, can yield unpredictable results. While you may observe a particular outcome once, it may differ in subsequent runs due to the way the compiler optimizes memory allocation.

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