What are the semantics of _internal?

Question

I keep seeing references to _internal in examples like the following:

class Symbol {     final String name;     static Map<String, Symbol> _cache;      factory Symbol(String name) {         if (_cache == null) {         _cache = {};      }       if (_cache.containsKey(name)) {         return _cache[name];      } else {         final symbol = new Symbol._internal(name);         _cache[name] = symbol;         return symbol;       }     }    Symbol._internal(this.name); } 

I've gathered from the code that it's a privately accessible constructor. The last line Symbol._internal(this.name); seems a bit confusing because it appears to be a statement within the class body and not within a method body, leading me to believe it's actually the internal constructor definition without a method body.

Are my assumptions correct?

Answer 1

The _internal construction is just a name often given to constructors that are private to the class (the name is not required to be ._internal you can create a private constructor using any Class._someName construction).

For example the following code only allows you to create new persons from outside the class using a caching constructor:

class Person {      final String name;     static Map<String,Person> _cache;      factory Person(String name) {         if(_cache === null) {             _cache = new Map<String,Person>();          }          if(_cache[name] === null]) {             _cache[name] = new Person._internal(name);           }          return _cache[name];     }         Person._internal(this.name); } 

In general Dart treats any _construction as private to either the class or the library that contains it. For example you can define as a global function like this:

_globalToThisLibaryOnly() {     print("can only be called globally within this #library"); } 

which can be called from any file that is sourced within the library that defines it.