The Aggregate Member Initialization is one of the features of C++. This feature is improved and modernized with C++11, C++14 and C++20. With this feature, objects can initialize an aggregate member from braced-init-list. In this post, we explain what the aggregate member initialization is and what were the changes to it in modern C++ standards.
Table of Contents
What is aggregate member initialization in modern C++?
Aggregate initialization initializes aggregates. Since C++11, aggregates are a form of listed initializations. Since C++20 they are direct initializations. An aggregate could be an array or class type (a class, a struct, or a union).
Here is the general syntax,
In C++11 and above, we use without = as below,
In C++20, there are 3 new options that we can use,
How to use aggregate member initialization in modern C++?
C++14 provides a solution to problems in C++11 and above, for example in C++14, consider we have x, y coordinates in a struct, we can initialize them as below in a new xy object,
In modern C++, consider that we have a struct that has a, b, c members. We initialize first two members as below,
We can directly initialize as below too,
In C++17 and above, we can use this st_y as a base and we can add a new member to a new struct, then we can initialize as below,
What restrictions are there for the aggregate member initialization in C++?
If we consider the C++17 standard, an aggregate initialization can NOT be applied to a class type if it has one of the below,
- private or protected non-static data members,
- a constructor that is user-provided, inherited, or explicit constructors (explicitly defaulted or deleted constructors are allowed),
- base class or classes (virtual, private, protected),
- virtual member functions
If we consider the C++20 standard, an aggregate initialization can NOT be applied to a class type if it has one of the below,
- private or protected non-static data members,
- user-declared or inherited constructors,
- base class or classes (virtual, private, protected),
- virtual member functions
Is there a full example of aggregate member initialization in C++?
Here is a full example that explains simply most used features of aggregate member initialization,
and the output will be as below,
For more information about this feature, please see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3651.pdf
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