Why Do I Get a \'Default Member Initializer Required\' Error, and What Can I Do About It?

Understanding Compiler Error Message: "Default Member Initializer Required Before the End of its Enclosing Class"

When faced with the compiler error message "default member initializer required before the end of its enclosing class," it is essential to delve into the code and understand the underlying issue. This error arises in scenarios where a default member initializer for a class member is missing within the class definition.

Compiler Behavior: A Tale of Three

The code in question, featuring the Downloader class and its Hints nested struct, exhibits different behavior among compilers. While MSVC2017 compiles it successfully, GCC8.2 and Clang7.0 raise the aforementioned error. This discrepancy stems from the absence of a default member initializer for the Hints struct.

Unveiling the Root Cause

Uncommenting the line Hints(){} within the Hints struct resolves the issue for all three compilers. This line essentially provides an empty constructor, which fulfills the requirement for a default member initializer. However, uncommenting Hints() = default; leads to a compilation error with Clang and GCC, but not with MSVC.

A Peek into the Bug

This behavior can be attributed to a bug in Clang and GCC. A bug report (https://bugs.llvm.org/show_bug.cgi?id=41303) demonstrates a similar issue, showcasing a similar error message. The report suggests that the issue lies in the delayed parsing of default member initializers in certain contexts, causing the compilers to misinterpret the code.

Conclusion

To avoid such compiler errors, it is crucial to ensure that all class members with default values have explicit default member initializers defined within the class definition. This practice ensures consistent compilation behavior across different compilers and helps prevent unexpected errors during code development.

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