Why Does C 20 Prohibit Aggregate Initialization for Structs with User-Declared Constructors?

Breaking Aggregate Initialization: Impact of C 20 Modifications

Since the transition from C 17 to C 20, aggregate initialization has undergone a significant change when explicit constructors are defaulted or deleted. In this article, we delve into the reasoning behind this modification and explore its implications.

Origin of the Change

Previously, in C 17, aggregate initialization was allowed for structs with explicitly defaulted or deleted constructors. However, with the adoption of C 20, the C standard was revised to disallow aggregate initialization for structs with any user-declared constructors, including defaulted or deleted ones.

Justification

The key motivation behind this change lies in addressing potential inconsistencies and confusion in aggregate initialization. The standard committee recognized that allowing aggregate initialization for structs with user-declared constructors, even defaulted or deleted ones, led to unexpected and often erroneous behavior.

Consider the following example:

<code class="cpp">struct Foo
{
    Foo() = default;
    int bar;
};

int main()
{
    Foo test = { 0 }; // Error: Cannot initialize due to defaulted constructor
}</code>

In this case, the default constructor is explicitly provided, even though it essentially has no effect on the struct's initialization. However, allowing aggregate initialization in this scenario could bypass the constructor, potentially leading to undefined behavior or inconsistencies.

Impact on Developer Practice

The change in aggregate initialization practices has shifted the recommended approach for structuring and initializing classes and structs. Previously, it was common practice to explicitly default constructors to improve code readability and to ensure cleaner syntax. However, with the current standard, it is no longer advisable to default constructors for classes or structs that require initialization.

Conclusion

The modification to aggregate initialization in C 20 was made to enhance code clarity, consistency, and safety. By disallowing aggregate initialization for structs with user-declared constructors, the standard aims to eliminate potential pitfalls and improve the overall reliability of codebases. Developers should adjust their practices accordingly to align with these changes, leading to more robust and maintainable C applications.

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