为什么在printfl中使用+号？

Question

What's the difference between:

var x float64 = 3.141592
fmt.Println("the value is" + x)

and

var x float64 = 3.141592
fmt.Println("the value is", x)

What does the + means?

Why is the first one wrong and the second correct?

Answer 1

fmt.Println is a variadic function whose arguments are generic interfaces. Any type can fulfill this interfere, including strings and floats. The second example works for this reason.

The first example, however, involves the binary operator +. As https://golang.org/ref/spec#Operators says, binary operators work in identical types. This means you can't "add" a float to a string without first explicitly casting to a string.

In general, this is a decision the golang inventors made. If you read the design tenets of go, I think you'll find this aligns well. But for the purposes of your question, it's sufficient to say, that's how it was made to work.