Why Does Go's `==` Operator Fail to Compare `time.Time` Structs Correctly, and How Can `Time.Equal()` and `Time.In()` Methods Resolve This?-Golang-php.cn

Why Does Go's `==` Operator Fail to Compare `time.Time` Structs Correctly, and How Can `Time.Equal()` and `Time.In()` Methods Resolve This?

Linda Hamilton

Release： 2024-12-24 08:50:18

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Why Does Go's `==` Operator Fail to Compare `time.Time` Structs Correctly, and How Can `Time.Equal()` and `Time.In()` Methods Resolve This?

Time Struct Comparison Anomaly

In Go, the == operator for struct comparison evaluates whether all fields match. This principle applies to time.Time, which includes a location field. Consequently, when comparing two time.Time instances with identical date and time but potentially different location pointers, the == operator yields false.

Consider the following example:

import (
    "fmt"
    "time"
)

func main() {
    // Date 2016-04-14, 01:30:30.222 with UTC location
    t1 := time.Date(2016, 4, 14, 1, 30, 30, 222000000, time.UTC)

    // Calculate nanoseconds from 1970-01-01 to t1 and construct t2
    base := time.Date(1970, 1, 1, 0, 0, 0, 0, t1.Location())
    nsFrom1970 := t1.Sub(base).Nanoseconds()
    t2 := time.Unix(0, nsFrom1970)

    // Print comparison results
    fmt.Println("Time t1:", t1)
    fmt.Println("Time t2:", t2)
    fmt.Println("t1 == t2:", t1 == t2)
    fmt.Println("t1.Equal(t2):", t1.Equal(t2))

    // Construct a new time t3 with the same values as t1
    t3 := time.Date(2016, 4, 14, 1, 30, 30, 222000000, time.UTC)
    fmt.Println("t1 == t3:", t1 == t3)
}

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Output:

Time t1: 2016-04-14 01:30:30.222 +0000 UTC
Time t2: 2016-04-14 01:30:30.222 +0000 UTC
t1 == t2: false
t1.Equal(t2): true
t1 == t3: true

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As evident from the output, t1 == t2 is false despite t1.Equal(t2) returning true. This discrepancy stems from the different location pointers in t1 and t2, as demonstrated by:

fmt.Println("Locations:", t1.Location(), t2.Location())
fmt.Printf("Location pointers: %p %p", t1.Location(), t2.Location())

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Output:

Locations: UTC UTC
Location pointers: 0x1e2100 0x1e6de0

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Different location pointers indicate that these times refer to the same instant, but observed from different locations.

To ensure consistency when comparing times, consider using the Time.In() method to set the same location:

t2 = t2.In(t1.Location())
fmt.Println("t1 == t2:", t1 == t2)

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Output:

t1 == t2: true

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