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,
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T object = {arg1, arg2, ...}; |
In C++11 and above, we use without = as below,
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T object {arg1, arg2, ...}; |
In C++20, there are 3 new options that we can use,
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T object (arg1, arg2, ...); T object = { .designator = arg1 , .designator { arg2 } ... }; T object { .designator = arg1 , .designator { arg2 } ... }; |
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,
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struct st_xy { float x, y; }; struct st_xy xy{ 3.2f, 5.1f }; |
In modern C++, consider that we have a struct that has a, b, c members. We initialize first two members as below,
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struct st_x { short int a, b, c; }; struct st_x x{ .a = 10, .b = 20}; // x.c will be 0 |
We can directly initialize as below too,
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struct st_y { int a = 100, b = 200, c, d; } y; |
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,
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struct st_z : st_y { int e; }; struct st_z z{ 1, 2, 3, 4, 5}; |
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,
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#include <iostream> // Aggregate in C++14 struct st_xy { float a, b; }; struct st_xy xy{ 3.2f, 5.1f }; struct st_x { short int a, b, c; }; struct st_x x{ .a = 10, .b = 20}; // x.c will be 0 struct st_y { int a = 100, b = 200, c, d; } y; // Aggregate in C++17 struct st_z : st_y { int e; }; struct st_z z{ 1, 2, 3, 4, 5}; int main() { std::cout << "xy:" << xy.a << "," << xy.b << std::endl; std::cout << "x:" << x.a << "," << x.b << "," << x.c << std::endl; std::cout << "y:" << y.a << "," << y.b << "," << y.c << std::endl; std::cout << "z:" << z.a << "," << z.b << "," << z.c << "," << z.d << "," << z.e << std::endl; system("pause"); return 0; } |
and the output will be as below,
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xy:3.2,5.1 x:10,20,0 y:100,200,0 z:1,2,3,4,5 Press any key to continue . . . |
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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