Ask r/golang: what I do not understand about maps

<p>I stumbled on something that I do not understand. I hope somebody here cares to explain. I created a small piece of test code on play.golang but the essence is below. I expected both situations to be identical, but its not ?</p> <pre><code> // test 1 f := &#34;test1&#34; d := detailsStruct{details: map[string]string{}} d.add(&#34;{}&#34;) // this works todo.list[f] = d // test 2 f = &#34;test2&#34; todo.list[f] = detailsStruct{details: map[string]string{}} todo.list[f].add(&#34;{}&#34;) // this gives runtime error </code></pre> <p>Basically there is a small difference between test1 and test2 in the sample code. In test1 the .add() works fine. For test2 I get a runtime error:</p> <pre><code>prog.go:30: cannot call pointer method on todo.list[f] prog.go:30: cannot take the address of todo.list[f] </code></pre> <p>Full example: <a href="http://play.golang.org/p/8D3-72mzsK">http://play.golang.org/p/8D3-72mzsK</a></p> <hr/>**评论：**<br/><br/>bbrazil: <pre><p><a href="http://stackoverflow.com/questions/13101298/calling-a-pointer-method-on-a-struct-in-a-map" rel="nofollow">http://stackoverflow.com/questions/13101298/calling-a-pointer-method-on-a-struct-in-a-map</a></p></pre>1Gijs: <pre><p>Yes, so it has a very clear explanation:</p> <p>&#39;A map indexing operation is not addressable. This was decided so that map implementations would not need to guarantee that the addresses of values would not change. As more data is added to the map, it may relocate data for efficiency reasons.&#39;</p></pre>yiyus: <pre><p><a href="http://play.golang.org/p/zov9m5OrXZ" rel="nofollow">http://play.golang.org/p/zov9m5OrXZ</a></p></pre>1Gijs: <pre><p>Thank you :-)</p></pre>mdmd136: <pre><p>That&#39;s a compile time error, not a runtime error</p></pre>1Gijs: <pre><p>True. With all the tools running on every save I come to expect that everything works after that from a compiler point of view. On top of that I use a script to to build and run in one command.</p></pre>mimighost: <pre><p>the value type of todo is a detailStruct struct, but the add method is declared its pointer type.</p> <p><a href="https://golang.org/ref/spec#Method_sets" rel="nofollow">https://golang.org/ref/spec#Method_sets</a></p></pre>