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 } ... }; 1 2 T object = { . designator = arg1 , . designator { arg2 } . . . } ; T object { . designator = arg1 , . designator { arg2 } . . . } ;

direct initialization, that has the following form:

T object (arg1, arg2, ...); 1 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 }; 1 2 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; }; 1 2 3 4 5 6 7 8 9 10 11 12 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 1 2 3 4 5 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 1 2 3 4 5 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){} 1 2 3 4 5 6 7 8 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) 1 2 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 1 2 3 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; }; 1 2 3 4 5 6 7 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' 1 2 3 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 1 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.

Share this: Facebook

Twitter

Print

More

Email

Reddit



