@Crawl-W said in Add member variable to class in the shared library, will not break binary compatibility?:
My question is, why changing the layout of members of class in the DLL, will not break binary compatibility?
Changing the layout of members DOES break binary compatibility! Your example just doesn't show the effects of breaking compatibility.
Binary compatibility matters when you want to replace the DLL without recompiling the appplication.
Here's a simpler example:
// Header for DLL
class UNTITLED1_EXPORT SimpleClass
{
public:
void setC(double d) { c = d; }
double getC() { return c; }
char arrCh[100]; //Add later to update the DLL
double c;
};
int main(int argc, char *argv[])
{
...
SimpleClass sc;
sc.setC(50);
qDebug() << sc.c; // This line probably WON'T print "50" after you replace the DLL
qDebug() << sc.getC(); // This line WILL still print "50" after you replace the DLL
...
}
To see the effects, you must:
Build your original DLL
Build your original application using the original headers and link it to the original DLL.
Update the DLL code and build your new DLL
Put the DLL where your application expects to load it
DO NOT REBUILD YOUR APPLICATION. Just run the original application.
