[Interest] Whats the difference between compiling with /MD and /MT?

[Nuno Santos]

Hi,

Today I have built Qt statically with the option -openssl-linked. My 
first option was to link with the MD versions of libs because Qt is 
using /MD during the build process. After a successfull compilation of 
Qt I have built the a static plugin I'm working on but the host software 
couldn't find it. I couldn't tell why.

The other day I found someone who changed the msvc-desktop.conf to use 
/MT instead of /MD. From what I have read in Microsoft documentation and 
all around there, if I want to statically link with the MSVC runtime, 
not depending on the installation of the run time, I need to use the /MT 
flag.

I have decided to give it a try and after rebuilding Qt with /MT instead 
of /MD, and then rebuilding my plugin again, it is now detected by the 
host. However, when I try to compile it in Release Mode I have thousands 
of errors:

thousands of *unresolved external symbol __purecall*
thousands of *unresolved external symbol "void __cdecl operator 
delete(void *)" (??3 at YAXPAX@Z)*

What does this mean? Why is it able to compile in debug but not in 
release? Can someone kindly explain me the differences and implications 
of /MD and /MT?

Thanks,

Regards,

Nuno

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