Question

I’m getting an odd error with parts of the NS-3 API. Here is my error message:

error: passing ‘const ns3::TopologyReader::Link’ as ‘this’ argument of ‘std::string ns3::TopologyReader::Link::GetAttribute(std::string)’ discards qualifiers

And here is the code causing the issue:

TopologyReader::ConstLinksIterator iter;
int num = 0;
for (iter = topologyReader->LinksBegin (); iter != topologyReader->LinksEnd(); iter++, num++)
  {
    std::istringstream fromName(iter->GetFromNodeName ());
    std::istringstream toName (iter->GetToNodeName ());
    iter->GetToNodeName();
    std::string w = "Weight";
    std::string weightAttr = (iter)->GetAttribute(w); // <- error
    /* snip */
  }

I think it might have to do with the fact that GetAttribute(std::string) is not a const function, according to the documentation for TopologyReader::Link, while the other functions, GetFromNodeName(void) and GetToNodeName(void) are declared as const functions. However, I’m not sure how to fix this problem.

Edit:
The function signatures are as shown (from the linked documentation):

std::string ns3::TopologyReader::Link::GetFromNodeName (void) const
std::string ns3::TopologyReader::Link::GetToNodeName (void) const
std::string ns3::TopologyReader::Link::GetAttribute (std::string name)  

Answer 1

Your analysis is correct. The obvious fix is to make GetAttribute be a const function. Its name suggests it should be const. It might not be in your power to change that code, though.

The alternative is to find some way of getting a non-const object to call the function on. Maybe you could declare iter as a LinksIterator instead of a ConstLinksIterator.

As a last resort, you could try using const_cast to tell the compiler that it’s really safe to call a non-const method on a supposedly const object.