C++ Windows DLL: Exception Handling in Different Function - c++

Question

This is more of a design suggestion I need. Below code is from a C++ dll that am working on and this dll is called by many client executables. InputParams is a pointer to client exe.
The dll calls functions in the client to get certain values and the client doesn't have any exception build into it. So i need to implement exception handling in dll code.
for e.g., in the line input->getName, if it returns a NULL pointer or if status == ERROR, then I need to throw exception and catch it in GetNumbers function.
void Metrics::GetNumbers(InputParams* input, OutputParams* output)
{
int i, x, k;
int status = 0;
char* mesg;
try{
const char* name = input->getName(&status, &mesg); //this returns a name string
//and status will contain whether success or failure and mesg will have any error
//messages.
const char* age = input->getAge(&status, &mesg);
//Many other similar calls to client
vector <int> myVectorNumers* = input->getNumberArray(&status, &mesg);
vector <int> myVectorInfos* = input->getInfoArray(&status, &mesg);
//Many other similar calls to client;
}
catch (std::exception e&)
{
// TODO
}
catch (string msg)
{
// TODO
}
I came up with a way of doing it. And this is the snippet of code.
string errorMsg = "";
const char* name = input->getName(&status, &mesg);
if (name == NULL || status == ERROR) // ERROR == 0
{
errorMsg = mesg;
throw errorMsg;
}
const char* age = input->getAge(&status, &mesg);
if (age== NULL || status == ERROR)
{
errorMsg = mesg;
throw errorMsg;
}
//Same for other calls to client using **input** pointer
Now as you see, I have to duplicate almost the same code in every place were there should be an exception check.
What am looking for is?
string errorMsg = "";
const char* name = input->getName(&status, &mesg);
CheckException(name, status, mesg, &errorMsg); // this function should do the error check and then throw the exception. And that exception should be caught by catch block in **GetNumbers** function.
const char* age = input->getAge(&status, &mesg);
CheckException(age, status, mesg, &errorMsg);
I have no idea whether even this is possible.
So a dummy implementation of CheckException will look like......
std::exception CheckException (/*1st parameter needs to be template type*/T* var, int status, string mesg, string *errorMsg)
{
if (var == NULL || status == ERROR)
{
errorMsg = mesg;
return /*TODO*/; //Not sure how to return a throw
}
}
1st of all, is this possible? Or is there a better way of doing this?
If you can provide me some sample code, that will be great!

Answer 1

Exceptions are not allowed to pass over the DLL boundary into the calling executable. The caller is not guaranteed to know how to handle an exception properly, especially in regards to cleanup, even if the executable and DLL are compiled with the same compiler. Or, the caller may not even understand what an exception is at all, if the executable and DLL are compiled in different languages.
If you need to convey error information to the caller, you must do so using a return value, an output parameter, a separate function call, etc. Things that are more-or-less universally supported across compilers/languages.
DO NOT throw exceptions over the DLL boundary into the caller!
UPDATE
You are already catching std::exception& in GetNumbers() (though, it really should be const std::exception & instead), so simply have CheckException() actually throw a std::exception object, don't try to return it, eg:
template<typename T>
void CheckException (const T *var, int status, const string &mesg)
{
if (var == NULL || status == ERROR) {
// or make a custom class derived from std::exception, if you want...
throw std::runtime_error(mesg);
}
}
...
try
{
const char* name = input->getName(&status, &mesg);
CheckException(name, status, mesg);
const char* age = input->getAge(&status, &mesg);
CheckException(age, status, mesg);
// Many other similar calls to client...
vector<int> *myVectorNumers = input->getNumberArray(&status, &mesg);
CheckException(myVectorNumers, status, mesg);
vector<int> *myVectorInfos = input->getInfoArray(&status, &mesg);
CheckException(myVectorInfos, status, mesg);
// Many other similar calls to client...
}
catch (const std::exception e&)
{
// TODO
}