Demystifying Compiler Error: Understanding "Default Member Initializer Required Before End of Enclosing Class"

This error message, encountered by GCC and Clang compilers, signals a specific issue in C code. To comprehend this issue, let's analyze a sample code snippet:

class Downloader {
public:
    struct Hints {
        int32_t numOfMaxEasyHandles = 8;
    };

    static Downloader *Create(const Hints &hints = Hints());
};

When compiling this code using GCC and Clang (while it compiles successfully in MSVC), an error message appears:

error: default member initializer for 'Downloader::Hints::numOfMaxEasyHandles' required before the end of its enclosing class

The crux of this error lies in the default constructor for the Hints struct. When commented out, the code compiles seamlessly across all three compilers. However, uncommenting Hints() or Hints() = default; triggers the error in GCC and Clang.

To grasp this behavior, it's crucial to understand that GCC and Clang implement a feature called "delayed template parsing." This feature postpones the parsing and evaluation of certain parts of the code, such as default member initializers, until they are encountered during code generation.

In cases where the compiler encounters a member function that uses a default argument involving an uninitialized data member, it may struggle to generate code, leading to the error message in question. This occurs because the default member initializer must be parsed and evaluated before the function definition is complete.

This issue can be resolved by explicitly initializing the data member within the struct, ensuring it has a value before the function definition:

class Downloader {
public:
    struct Hints {
        int32_t numOfMaxEasyHandles = 8;  // Explicit initialization
    };

    static Downloader *Create(const Hints &hints = Hints());
};

In conclusion, the error message "default member initializer required before the end of its enclosing class" signifies that GCC and Clang require a default member initializer to be explicitly defined within the struct declaration, particularly when the struct is used as a default argument in a function. By ensuring proper initialization, developers can avoid this compiler error and ensure seamless compilation across different compiler implementations.