将std :: map与std :: pair用作键，并将列表作为值

Question

So I have the following map parseTable

std::map<std::pair<Symbol, Symbol>, list<Symbol> > parseTable; 

I am confused on how to access to the list value if I have my map initialized this way:

std::map<std::pair<Symbol, Symbol>, list<Symbol> > parseTable = { 
        {{Symbol::Input, Symbol::OpenPar}, {Symbol::Expr, Symbol::Semicolon, Symbol::InputP}},
        {{Symbol::Input, Symbol::Ident}, {Symbol::Expr, Symbol::Semicolon, Symbol::InputP}},
        {{Symbol::Input, Symbol::Number}, {Symbol::Expr, Symbol::Semicolon, Symbol::InputP}}
};

I want to access to each of the values of my list individually when I use the find() function of my map.

This is what I have come up with but I am not able to get a reference to that index value:

 if (parseTable.find(std::pair(stack_symbol.top(), current_symbol)))

Answer 1

std::map::find will return an iterator to the found element, or to end if not found. That iterator will point to a std::pair<const Key, Value> , which in your case would translate to

std::pair< const std::pair<Symbol, Symbol>, list<Symbol> >

What you want is something like this

auto it = parseTable.find(std::pair(stack_symbol.top(), current_symbol));

if (it != parseTable.end()) { // A match was found
    //it->first is std::pair<Symbol, Symbol>
    //it->second is list<Symbol>
    for (auto& symbol : it->second) {
        //symbol is each individual value in the list
        ... do something with symbol
    }
}

Answer 2

This is not the greatest choice for map key, it won't let the map to be used efficiently.

std::map::find() return an iterator to the place where it found searched item or std::map::end() if not found. So, in your if statement, you need to check against that:

std::map<std::pair<Symbol, Symbol>, list<Symbol> >::iterator iter =
     parseTable.find(std::pair(stack_symbol.top(), current_symbol)) //or auto with C++11
if (iter != parseTable.end())

find returns an iterator, to access the object (which will be of type std::pair<std::pair<Symbol, Symbol>, list<Symbol>> , you'll need dereference operator *

Symbol currentSymbol = (*iter).first.second; //dummy example to show the use
std::list<Symbol> myList = (*iter).second'