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); complex b = a; complex c = complex(1,2); complex d = sqrt(b,c); complex e; complex f = 3; complex g = { 1, 2 };
— end example ] [ Note: overloading of the assignment operator ([over.ass]) has no effect on initialization. — 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:
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 ]
[ 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 ]
[ Note: the order in which objects with static or thread storage duration are initialized is described in [basic.start.init] and [stmt.dcl]. — end note ]
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