Why is explicit allowed for default constructors and constructors with 2 or more (non-default) parameters?

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One reason certainly is because it doesn’t hurt.

One reason where it’s needed is, if you have default arguments for the first parameter. The constructor becomes a default constructor, but can still be used as converting constructor

struct A {
  explicit A(int = 0); // added it to a default constructor
};

C++0x makes actual use of it for multi parameter constructors. In C++0x, an initializer list can be used to initialize a class object. The philosophy is

  • if you use = { ... }, then you initialize the object with a sort of “compound value” that conceptually represents the abstract value of the object, and that you want to have converted to the type.

  • if you use a { ... } initializer, you directly call the constructors of the object, not necessarily wanting to specify a conversion.

Consider this example

struct String {
    // this is a non-converting constructor
    explicit String(int initialLength, int capacity);
};

struct Address {
    // converting constructor
    Address(string name, string street, string city);
};

String s = { 10, 15 }; // error!
String s1{10, 15}; // fine

Address a = { "litb", "nerdsway", "frankfurt" }; // fine

In this way, C++0x shows that the decision of C++03, to allow explicit on other constructors, wasn’t a bad idea at all.

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