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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