Why Does Using `% v` in a Nested Struct\'s `String()` Method Lead to a Stack Overflow?

Recursive Printing Error in Nested Struct String() Method

This question explores the issue of stack overflow when attempting to print a nested struct using the String() method with the % v format specifier.

Problem:

A user attempts to return nested struct elements in the String() method using the following code:

<code class="go">func (c ConfigOne) String() string {
    return fmt.Sprintf("%+v\n", c)
}</code>

When the user tries to print the struct using Logger.Infoln(c.String()), they encounter the error:

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

Explanation:

The %v and % v formats use the value of String() if the type implements it. Therefore, using % v on a type within the String() function for that type causes infinite recursion.

In this case, the String() method calls fmt.Sprintf("% v", c), which calls the String() method recursively on the embedded Daemon struct, and so on. This leads to an infinite loop and stack overflow.

Solution:

To overcome this issue, the user should not use % v in the String() function. Instead, they should construct their own string, showing the contents of the structure in whatever way they see fit. For example:

<code class="go">func (c ConfigOne) String() string {
    return fmt.Sprintf("Loglevel: %d, Logfile: %s\n", c.Daemon.Loglevel, c.Daemon.Logfile)
}</code>

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