11 Classes [class]

11.9 Initialization [class.init]

11.9.2 Explicit initialization [class.expl.init]

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 1: struct complex { complex(); complex(double); complex(double,double); }; complex sqrt(complex,complex); complex a(1); // initialized by calling complex(double) with argument 1 complex b = a; // initialized as a copy of a complex c = complex(1,2); // initialized by calling complex(double,double) with arguments 1 and 2 complex d = sqrt(b,c); // initialized by calling sqrt(complex,complex) with d as its result object complex e; // initialized by calling complex() complex f = 3; // initialized by calling complex(double) with argument 3 complex g = { 1, 2 }; // initialized by calling complex(double, double) with arguments 1 and 2 — end example]
[Note 1:
— end note]
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 2: 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 of 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 2:
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]
[Note 3:
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]
[Note 4:
The order in which objects with static or thread storage duration are initialized is described in [basic.start.dynamic] and [stmt.dcl].
— end note]