C++

As C version:
test.cpp

extern "C"
{
	char* Test(char* RetString)
	{
		return RetString;
	}
}

test.bmx

SuperStrict

Import "test.cpp"

Extern
	Function Test:Byte Ptr(str:Byte Ptr)
EndExtern

Local Stringt:String = String.FromCString(Test("Testing..."))

Print Stringt

As C++ version:
test.cpp

#include <string>

using namespace std;

extern "C"
{
	string Test(string RetString)
	{
		return RetString;
	}
}

test.bmx

SuperStrict

Import "test.cpp"

Extern
	Function Test:String(str:String)
EndExtern

Local Stringt:String = String.FromCString(Test("Testing..."))

Print Stringt