The 10-Man-Year Problem

For any modern programming language seeking adoption in C++-dominated ecosystems, interoperability represents a monumental challenge. As the D language specification candidly admits: "Being 100% compatible with C++ means more or less adding a fully functional C++ compiler front end... a minimum of a 10 man-year project." This harsh reality has forced language designers toward pragmatic compromises—and D's approach offers a fascinating case study in practical engineering trade-offs.

The Pragmatic Pathway

Rather than attempting full C++ emulation, D's extern(C++) attribute creates targeted bridges:

extern(C++) void foo(int i); // Declare C++ function
foo(42); // Call as native D function

Namespaces are supported through explicit scoping:

extern(C++, "mynamespace") {
    void namespaceFunc();
}

For classes, D mirrors C++ semantics with crucial distinctions. Structs and classes map directly but lose garbage collection integration:

extern(C++) class CppClass {
    void virtualMethod();
    int dataMember;
}

Critical Limitation: "D's new operator allocates memory using the D garbage collector, so no explicit delete is necessary... Attempting to allocate memory with D's new and deallocate with C++ ::operator delete will result in miserable failure."

Template Tango

D transparently binds C++ templates through explicit instantiation:

extern(C++) T max(T)(T a, T b);
writeln(max!int(3, 4)); // Explicit template instantiation

But the language acknowledges sharp edges—D and C++ follow different overloading rules, forcing developers to understand both systems.

The Elephant in the Room: Memory & Exceptions

Memory management reveals fundamental philosophical divides. D provides escape hatches like GC.addRoot to anchor GC-managed memory for C++ consumption, but warns:

"The garbage collector does not scan the stacks of threads not registered with the D runtime."

Exception handling remains a hard boundary: "At present, C++ exceptions cannot be caught in or thrown from D, and D exceptions cannot be caught in or thrown from C++."

Operator Overloads: A Glimpse of Harmony

Surprisingly, operator overloading bridges the gap effectively. This C++ class:

// C++
class Vector {
public:
    float& operator[](size_t i) { return elements[i]; }
};

Can be used naturally in D:

extern(C++) class Vector {
    float opIndex(size_t i);
}
float element = vec[3]; // Uses C++'s operator[]

The Road Not Taken

D's honest assessment of interoperability constraints—from empty base optimization workarounds to RTTI incompatibility—provides valuable lessons for language designers. Rather than chasing mythical seamless integration, it demonstrates how selective, well-documented bridges can deliver practical value while acknowledging the inherent complexities of marrying managed and unmanaged paradigms.

As systems grow increasingly polyglot, D's blueprint for pragmatic C++ coexistence offers both inspiration and cautionary wisdom for next-generation language ecosystems.

_Source: D Language Specification - C++ Interface_