C++/CLI interop: unexpected behaviour of variadic function

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C++

Dear experts,

in a C++/CLI project I have a native Logging class with a virtual variadic member function 'Write'. Furthermore, I have a global __LOG variable that holds an instance of that class.

If I call '__LOG.Write(1,"%f",8.34)' from main, it displays the floating point value correctly.

However, if I call '__LOG.Write(1,"%f",8.34)' from a member function of a managed class, the function displays 0.00000. To get the expected output, I need to change the 'Write' function to a non-virtual function.

Can somebody explain this behavior?

// compiled with /clr
#include <cstdarg>
#include <iostream> 

#pragma unmanaged
class NativeLog
{
public:
	virtual void Write(int aLogType, const char* aFormat, ...) const
	{
		char str[128];

		std::va_list args;
		va_start(args, aFormat);
		vsprintf_s(str, 128, aFormat, args);
		va_end(args);

		std::cout << str << std::endl;
	}
};

// Global unmanaged log object
NativeLog __LOG;

#pragma managed

ref class ManagedCaller
{
public:
	void Call() { __LOG.Write(1, "%f", 8.34); }
};

int main()
{
	__LOG.Write(1, "%f", 8.34);
	(gcnew ManagedCaller())->Call();

	return 0;
}

What I have tried:

The issue came up in a larger project, and I have isolated it to the code above. I don't need help with the design. I just want to understand the behaviour.