String concatenation

There are several ways to concatenate strings in Go. Some examples include:

The “+” operator
fmt.Sprintf
strings.Builder
text/template
safehtml/template

Though there is no one-size-fits-all rule for which to choose, the following guidance outlines when each method is preferred.

Prefer “+” for simple cases

Prefer using “+” when concatenating few strings. This method is syntactically the simplest and requires no import.

// Good:
key := "projectid: " + p

Prefer `fmt.Sprintf` when formatting

Prefer using fmt.Sprintf when building a complex string with formatting. Using many “+” operators may obscure the end result.

// Good:
str := fmt.Sprintf("%s [%s:%d]-> %s", src, qos, mtu, dst)

// Bad:
bad := src.String() + " [" + qos.String() + ":" + strconv.Itoa(mtu) + "]-> " + dst.String()

Best Practice: When the output of the string-building operation is an io.Writer, don’t construct a temporary string with fmt.Sprintf just to send it to the Writer. Instead, use fmt.Fprintf to emit to the Writer directly.

When the formatting is even more complex, prefer text/template or safehtml/template as appropriate.

Prefer `strings.Builder` for constructing a string piecemeal

Prefer using strings.Builder when building a string bit-by-bit. strings.Builder takes amortized linear time, whereas “+” and fmt.Sprintf take quadratic time when called sequentially to form a larger string.

// Good:
b := new(strings.Builder)
for i, d := range digitsOfPi {
    fmt.Fprintf(b, "the %d digit of pi is: %d\n", i, d)
}
str := b.String()

Note: For more discussion, see GoTip #29: Building Strings Efficiently.

Constant strings

Prefer to use backticks (`) when constructing constant, multi-line string literals.

// Good:
usage := `Usage:

custom_tool [args]`

// Bad:
usage := "" +
  "Usage:\n" +
  "\n" +
  "custom_tool [args]"

String concatenation