12 Special member functions [special]

A constructor declared without the function-specifier explicit specifies a conversion from the types of its parameters to the type of its class. Such a constructor is called a converting constructor. [ Example:

struct X {
    X(int);
    X(const char*, int =0);
    X(int, int);
};

void f(X arg) {
  X a = 1;          // a = X(1)
  X b = "Jessie";   // b = X("Jessie",0)
  a = 2;            // a = X(2)
  f(3);             // f(X(3))
  f({1, 2});        // f(X(1,2))
}

 — end example ]

An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax ([dcl.init]) or where casts ([expr.static.cast], [expr.cast]) are explicitly used. A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization ([dcl.init]). [ Example:

struct Z {
  explicit Z();
  explicit Z(int);
  explicit Z(int, int);
};

Z a;                            // OK: default-initialization performed
Z a1 = 1;                       // error: no implicit conversion
Z a3 = Z(1);                    // OK: direct initialization syntax used
Z a2(1);                        // OK: direct initialization syntax used
Z* p = new Z(1);                // OK: direct initialization syntax used
Z a4 = (Z)1;                    // OK: explicit cast used
Z a5 = static_cast<Z>(1);       // OK: explicit cast used
Z a6 = { 3, 4 };                // error: no implicit conversion

 — end example ]

A non-explicit copy/move constructor ([class.copy]) is a converting constructor. An implicitly-declared copy/move constructor is not an explicit constructor; it may be called for implicit type conversions.