System information (version)

• OpenCV => 3.4.4 and later
• Operating System / Platform => Windows 64 bits
• Compiler => Visual Studio 2017

Detailed description

Starting with rev. 8a3c394 C structure constructors are disabled when building OpenCV but enabled when building a program that uses OpenCV. Relevant part of the diff is:

--- a/modules/core/include/opencv2/core/types_c.h
+++ b/modules/core/include/opencv2/core/types_c.h
@@ -44,9 +44,7 @@
 #ifndef OPENCV_CORE_TYPES_H
 #define OPENCV_CORE_TYPES_H
 
-#if !defined(__OPENCV_BUILD) && !defined(CV__DISABLE_C_API_CTORS)
 #define CV__ENABLE_C_API_CTORS // enable C API ctors (must be removed)
-#endif

This leads to an interesting problem when using, for instance, cvGetSize. According to https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=vs-2017 when a function returns a struct with a size less than 64 bits:

• If it's a POD, it is returned in RAX
• If not, the caller creates a temporary and passes a reference to it in RCX; function arguments are shifted

So here, when building OpenCV, CvSize is a POD, so the function assumes RCX contains the pointer to the CvArr argument. But when using the function, CvSize is not a POD any more, and the caller puts a reference to a temporary CvSize in RCX and the argument in RDX... As you can imagine, this doesn't work very well.

The workaround is obviously to define CV__DISABLE_C_API_CTORS when building my own code, but such a runtime failure with no warning is a bit disheartening.

Steps to reproduce

Use cvGetSize on an IplImage in a 64 bits Windows program. This leads to a "Bad argument (Array should be CvMat or IplImage)", unless by change your temporary CvSize has the right magic value packed in...