Re: Getting access to std::map value (a struct)

From:

"Alex Blekhman" <xfkt@oohay.moc>

Newsgroups:

microsoft.public.vc.stl

Date:

Sun, 1 Oct 2006 18:52:11 +0200

Message-ID:

<u12rxnX5GHA.4832@TK2MSFTNGP06.phx.gbl>

"Angus" <nospam@gmail.com> wrote in message
news:uNuIaGV5GHA.1196@TK2MSFTNGP02.phx.gbl...

Hello

I am struggling here with a std::map like this:

struct agents
{
Socket* agentsocket; // ptr to socket for client
DWORD dwDeviceID; // (DeviceID)
char szLogon[100]; // logon
};
agents thisagent;
thisagent.agentsocket = 0;
thisagent.dwDeviceID = 6;
lstrcpy(thisagent.szLogon, "Angus");
agents thatagent;
thatagent.agentsocket = 0;
thatagent.dwDeviceID = 8;
lstrcpy(thatagent.szLogon, "Lisa");
// std::map<Key,Val>:
std::map<int, agents> m_AgentsList;
m_AgentsList[4] = thisagent;
m_AgentsList[8] = thatagent;

So if I want to get access to the agents in
m_AgentsList[4] I would do this:
agents myagent = m_AgentsList[4]; // compile error line
std::cout << myagent.szLogon << std::endl;
Easy enough. This compiles in my test program. But if I
compile in my main
program I get this:
SocketServer.cpp(410) :
error C2678:
binary '[' : no operator defined which takes a left-hand
operand of type
'const class std::map<int,struct CSocketServe
r::agents,struct std::less<int>,class
std::allocator<struct
CSocketServer::agents> >' (or there is no acceptable
conversion)

I have exactly the same code in SocketServer.cpp as my
test code in a really
small test cpp file.

You cannot call `std::map::operator[]' on constant instance
of map. It happens because `std::map::operator[]'

    a) can change map instance
    b) returns non-const reference to contained object.

If you need to search for value on constant map instance,
then use `std::map::find' method:

std::map<int, agents>::const_iterator it =
    m_AgentsList.find(4);

if(it != m_AgentsList.end())
{
    const agents& a = it->second;

    std::cout << a.szLogon << std::endl;
}