passing as 'this' argument of discards qualifiers

Hi , I have a problem when compile following code:

Code:

---

for(int i=0;i<curConnections.size();++i)
    {
        if (curConnections.at(i).Descriptor()==connName)
        {
            curConnections.at(i).sendCmd(cmd);
            break;
        }
    }

---

Compiler complains:
error: passing 'const ConnConnection' as 'this' argument of 'void ConnConnection::sendCmd(const QString&)' discards qualifiers

Then I use following it compiles OK.

Code:

---

curConnections[i].sendCmd(cmd);

---

I am not quite get what's the difference of these 2 lines of code.

at() is a const function and returns a const object. operator[]() is non-const.

many thanks