C++20 introduced a restricted form of designated initializers for aggregates (i.e., arrays or classes that respect specific criteria). Designated
initializers enable the initialization of aggregates by naming their fields explicitly:
struct Point {
float x = 0.0;
float y = 0.0;
float z = 0.0;
};
Point p = {
.x = 1.0,
.y = 2.0,
// z will be 0.0
};
This initialization style is similar to designated initializers in C and many C++ compiler extensions predating C++20.
However, it is more restricted because some forms are not supported by the C++20 standard, namely:
- listing the fields out of order
- array initialization (including sparse array initialization)
- initialization of nested fields
- mixed initialization
This rule reports non-C++-compliant forms of designated initializers.
Noncompliant code example
struct A { int x, y; };
struct B { struct A a; };
struct A a = {.y = 1, .x = 2}; // Noncompliant: valid C, invalid C++ (out of order)
int arr[3] = {[1] = 5}; // Noncompliant: valid C, invalid C++ (array)
struct B b = {.a.x = 0}; // Noncompliant: valid C, invalid C++ (nested)
struct A c = {.x = 1, 2}; // Noncompliant: valid C, invalid C++ (mixed)
Compliant solution
struct A { int x, y; };
struct B { struct A a; };
struct A a = {.x = 2, .y = 1};
int arr[3] = {0, 5};
struct B b = {.a = {.x = 0}};
struct A c = {.x = 1, .y = 2};
