C ++ Pass by value / reference

Question

I have the following code snippet:

vector<DEMData>* dems = new vector<DEMData>();
ConsumeXMLFile(dems);

if(!udp_open(2500))
{

}        

I want the ConsumeXMLFile method to populate the vector with DEMData objects built from reading an XML file. When ConsumeXMLFile returns, the dems vector is empty. I think I'm running into a pass by value problem.

Answer 1

通过这看起来ConsumeXMLFile采用指向向量的指针，所以我怀疑这是一个值传递值问题。

Answer 2

Are you at any point reassigning the pointer that is passed into the function? In other words, does your function look anything like this:

void ConsumeXMLFile(vector<DEMData>* dems) 
{
   // ... some code ...

   dems = new vector<DEMData>();

   // ...more code...
}

This is a common mistake that I see beginning C++ programmers (and C programmers) make. What is going on here is that a pointer to the dems vector is being passed by value , which means that if you modify the pointed-to vector, that will affect the the vector possessed by the caller. However if you modify the pointer (which is passed by value ) this will not affect the pointer possessed by the caller. After the re-assignment, the dems pointer in ConsumeXMLFile will point to a totally different vector than the dems pointer that the caller holds.

One of the things that makes me suspect that you might be doing this is that this is C++ and there's no clear reason why you would want to pass a pointer to the vector instead of a reference to the vector otherwise.