15 Special member functions [special]

15.6 Initialization [class.init]

15.6.1 Explicit initialization [class.expl.init]

1
#
An object of class type can be initialized with a parenthesized expression-list, where the expression-list is construed as an argument list for a constructor that is called to initialize the object.
Alternatively, a single assignment-expression can be specified as an initializer using the = form of initialization.
Either direct-initialization semantics or copy-initialization semantics apply; see [dcl.init].
[ Example
: 
struct complex {
  complex();
  complex(double);
  complex(double,double);
};

complex sqrt(complex,complex);

complex a(1);                   // initialize by a call of complex(double)
complex b = a;                  // initialize by a copy of a
complex c = complex(1,2);       // construct complex(1,2) using complex(double,double),
                                // copy/move it into c
complex d = sqrt(b,c);          // call sqrt(complex,complex) and copy/move the result into d
complex e;                      // initialize by a call of complex()
complex f = 3;                  // construct complex(3) using complex(double), copy/move it into f
complex g = { 1, 2 };           // initialize by a call of complex(double, double)
— end example
 ]
[ Note
:
Overloading of the assignment operator ([over.ass]) has no effect on initialization.
— end note
 ]
2
#
An object of class type can also be initialized by a braced-init-list.
List-initialization semantics apply; see [dcl.init] and [dcl.init.list].
[ Example
: 
complex v[6] = { 1, complex(1,2), complex(), 2 };
Here, complex​::​complex(double) is called for the initialization of v[0] and v[3], complex​::​complex(​double, double) is called for the initialization of v[1], complex​::​complex() is called for the initialization v[2], v[4], and v[5].
For another example,
struct X {
  int i;
  float f;
  complex c;
} x = { 99, 88.8, 77.7 };
Here, x.i is initialized with 99, x.f is initialized with 88.8, and complex​::​complex(double) is called for the initialization of x.c.
— end example
 ]
[ Note
:
Braces can be elided in the initializer-list for any aggregate, even if the aggregate has members of a class type with user-defined type conversions; see [dcl.init.aggr].
— end note
 ]
3
#
[ Note
:
If T is a class type with no default constructor, any declaration of an object of type T (or array thereof) is ill-formed if no initializer is explicitly specified (see [class.init] and [dcl.init]).
— end note
 ]
4
#
[ Note
:
The order in which objects with static or thread storage duration are initialized is described in [basic.start.dynamic] and [stmt.dcl].
— end note
 ]