c++visual-c++compiler-optimizationempty-class

Why is the empty base class optimization (EBO) is not working in MSVC?

Why is the empty base class optimization (EBO) not being fully applied in Visual C++?

If I have a lot of base classes, is there any way for me to help the compiler make this optimization?

#include <iostream>

struct T1 { };
struct T2 { };
struct T3 { };
struct T4 { };
struct T5 { };
struct T6 { };

struct Test : T1, T2, T3, T4, T5, T6 { };

int main() { std::cout << sizeof(Test); }   // Prints 5
Solution

  • This is a longstanding bug in the Visual C++ compiler. When a class derives from multiple empty base classes, only the initial empty base class will be optimized using the empty base optimization (EBO).

    This issue was reported on Microsoft Connect in 2006: Empty Base Optimization Not Working Properly. At the moment, old bugs are not visible on Microsoft Connect. I am told that this is a temporary issue, though I do not know when it will be resolved. In the meantime, the following is the response to the bug from Jonathan Caves, who is one of the developers on the Visual C++ compiler team:

    Hi: unfortunately even though this is a bug in the Visual C++ object model we are unable to fix it at this time given that fixing it would potentially break a lot of existing programs as the sizes of objects would change. Hopefully in the future we may be able to address this issue but not for the next release of the product.

    Thanks for reporting the issue.