15 Special member functions [special]

15.3 Conversions [class.conv]

15.3.1 Conversion by constructor [class.conv.ctor]

1
#
A constructor declared without the function-specifier explicit specifies a conversion from the types of its parameters (if any) 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
 ]
2
#
[ Note
:
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; see also [over.match.copy].
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 b{};                          // OK: direct initialization syntax used
Z c = {};                       // error: copy-list-initialization
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
 ]
— end note
 ]
3
#
A non-explicit copy/move constructor ([class.copy]) is a converting constructor.
[ Note
:
An implicitly-declared copy/move constructor is not an explicit constructor; it may be called for implicit type conversions.
— end note
 ]