__value classes in Unmanaged C++

[Katamneni]

Hi All i have defined a __value type class.How can i declare and create it in UnmanagedC++ class using new operator on the C runtime heap? For example public __value MyClasss { public: int a; } //my unmanaged classs class MyUnmanagedCPP { void SomeMethod() { MyClasss *pMyClass = new MyClasss();// here i'm getting error C3828 } }

[Tom Archer]

This is explained in depth in my book, Extending MFC Applications with the .NET Framework, but the main issue is that managed objects are dynamically allocated on the CLR heap (not the C++ free store). In your particular situation, you're probably getting C3828 because you're writing an MFC applications and running it in Debug mode where the new operator is being #define'd as a placement new operator. You need to do the following where 1) the new operator is being undefined and 2) __nogc is forcing a GC heap allocation instead of a C++ heap allocation:

void Cdeletethis3Dlg::OnBnClickedOk()
{
#pragma push_macro("new")
#undef new
MyClasss *pMyClass = __nogc new MyClasss();
#pragma pop_macro("new")
}

Cheers, Tom Archer * Inside C# -Second Edition * Visual C++.NET Bible * Extending MFC Applications with the .NET Framework