The 3rd one is the default constructor of bar. The 4th one is the copy assignment operator of bar.
I think you need to instantiate the class first, in order to call the 2nd method. Otherwise the method would be called with an invalid 'this' that causes access violation.
The problem is how you instantiate it? If you can find a factory function that returns a bar in the DLL, you can try to use it.
If you don't see a factory function and you don't have the lib file, you can refer to answers here on how to create a lib from a DLL: How to make a .lib file when have a .dll file and a header file
You also need to create header file for the class, with the correct order and types of members. This way you don't have to use LoadLibrary and GetProcAddress, just use the class as normal.
You may still use LoadLibrary and GetProcAddress without the lib and header though, this blog shows how to manually allocate memory, call constructor, gets an object and pass that object to call a method: http://recxltd.blogspot.com/2012/02/working-with-c-dll-exports-without.html