MSVC: what compiler switches affect the size of structs?

Question

I have two DLLs compiled separately, one is compiled from Visual Studio 2008 and one is a mex file compiled from matlab.
Both DLLs have a header file which they include. when I take the sizeof() the struct in one DLL it returns 48, and in the other it returns 64. I've checked the /Zp switch and in both compilations it is set to /Zp8.
What other compiler switches may affect the size of a struct?
The struct is a simple POCO with no inheritance and no virtual functions.

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Edit

The struct looks like this:

class LIBSPEC SGeometry
{
public:

    std::vector<IGeometry> m_i;
    uint N;
    uint n_im, n_s;
};

In debug it sizeof() returns 56 in both cases, in release, in the mex compilation it's 48, from VS it's 64.
I can tell matlab the exact compiler options to use when compiling the mex so it's not it.

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Edit

After checking with offsetof, it turns out that the difference is in the size of the std::vector. in one dll it's 32 and in the other it's 48.
Both dlls are x64.

Answer 1

Ok, so this is possibly the most obscure thing ever.
It turns out that matlab add /D_SECURE_SCL=0 to the compilation which disables something called 'secure iterators'
This in turn causes a difference of 16 bytes in the size of std::vector

Answer 2

• Check for differences in the include and library paths
• Check for differences in defined preprocessor symbols
• Check for any differences in the project settings (code generation, linkage etc.)
• Do compile both versions with the same IDE on the same machine?