[c++] What is this weird colon-member (" : ") syntax in the constructor?

This is an initialization list. It'll initialize the members before the constructor body is run. Consider

class Foo {
 public:
   string str;
   Foo(string &p)
   {
      str = p;
   };
 };

vs

class Foo {
public:
  string str;
  Foo(string &p): str(p) {};
};

In the first example, str will be initialized by its no-argument constructor

string();

before the body of the Foo constructor. Inside the foo constructor, the

string& operator=( const string& s );

will be called on 'str' as you do str = p;

Wheras in the second example, str will be initialized directly by calling its constructor

string( const string& s );

with 'p' as an argument.

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