内容简介:The C++20 standard provides new ways to initialize aggregates. These are:In this article, we will see how list initialization with designated initializers work.The designated initialization syntax allows to initialize non-static direct data members of a ty
The C++20 standard provides new ways to initialize aggregates. These are:
-
list initialization with designated initializers, that has the following forms:
T object = { .designator = arg1 , .designator { arg2 } ... }; T object { .designator = arg1 , .designator { arg2 } ... };
-
direct initialization, that has the following form:
T object (arg1, arg2, ...);
In this article, we will see how list initialization with designated initializers work.
The designated initialization syntax allows to initialize non-static direct data members of a type T. Here is an example:
struct foo { int a; char c = 'a'; } foo f { .a = 42 };
The class foo has two non-static data members, a and c . When initializing the object f , the member a is initialized with the syntax .a = 42 . In this context, .a is called a designator .
The following rules apply to designated initializers:
- a designator must refer to a non-static direct data member
- all the designator used in the initialization expression must follow the order of the declaration of the data members in the class
- not all data members must have a designator, but those that do must follow the rule above
- it is not possible to mix designated and non-designated initialization
- desginators of the same data member cannot appear multiple times
- designators cannot be nested
Let us see several examples to understand it better. Consider the following classes:
struct bar { int x; }; struct foo { int a; bar b; char c = 'a'; double d; };
The following inialization is allowed:
foo f1{}; // OK: a = 0, b = {x = 0}, c = 'a', d = 0.0 foo f2{ .a = 42 }; // OK: a = 42, b = {x = 0}, c = 'a', d = 0.0 foo f3{ .a = 42, .c = 'b' }; // OK: a = 42, b = {x = 0}, c = 'b', d = 0.0 foo f4{ .a = 42, .b = {.x = 5} }; // OK: a = 42, b = {x = 5}, c = 'a', d = 0.0 foo f5{ .a = 42, .b = {5} }; // OK: a = 42, b = {x = 5}, c = 'a', d = 0.0
However, the following forms of initialization are illegal:
foo f6{ .d = 1, .a = 42 }; // ERROR: out-of-order foo f7{ .a = 42, true, 'b', 1 }; // ERROR: mixed designated and non-designated foo f8{ .a = 42, .a = 0 }; // ERROR: duplicate designator foo f9{ .b.x = 42 }; // ERROR: nested initializer int arr[5] = { [0] = 42 }; // ERROR: array designators not allowed
Here are several more examples. Consider the following classes and functions:
struct A { int a, b; }; struct B { int b, a; }; struct C { int a, c; }; void f(A){} void f(B){} void f(C){} void g(B){}
The following calls are permitted:
f({ .a = 1, .c = 2 }); // OK: calls f(C) g({ .b = 1, .a = 2 }); // OK: calls g(B)
However, the following calls are, on the other hand, erroneous:
f({.a = 1, .b = 2}); // ERROR: ambiguous between f(A) and f(B) f({.a = 1}); // ERROR: ambiguous call, f(A), f(B), or f(C) g({.a = 1, .b = 2}); // ERROR: g(B) but designators are in the wrong order
A designated initializer, and only one, can be used to initialize a union. Let us consider the following union type:
union Foo { int a; bool b; char c; double d; };
The following forms of initialization are correct:
Foo u1{}; // OK: a = 0 Foo u2{ .a = 1 }; // OK: a = 1 Foo u3{ .c = 'b' }; // OK: c = 'b'
However, having more than one designator is not allowed:
Foo u4{ .a = 1, .b = true }; // ERROR: cannot have more than one element
Designated initialization is a feature that is also available in the C programming language. However, it is more relaxed than in C++. In C, it is possible to perform out-of-order designated initialization, to mix designated initializers and regular initializers, to nest designators, and to initialize arrays with designators. Therefore, in this aspect, C and C++ are not fully compatible.
Designated initializers are supported in VC++ 2019 169.1, GCC 8 and Clang 10.
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