Question

Inside of a class called Castle I have the following 2 functions:

vector<Location> Castle::getMoves(Location*** squares, Location* loc){
    int row = loc->getRow();
    int col = loc->getCol();

    vector<Location> moves;

    //problem is here!
    getVerticalMoves(squares, &moves, row, col);

    return moves;
}

void Castle::getVerticalMoves(Location*** squares, vector<Location*> * moves, int row, int col){
    //need to do something here, but can't call the function!
}

When I try to compile, I get an error that looks like this:

model/src/Castle.cpp: In member function ‘std::vector<Location> Castle::getMoves(Location***, Location*)’:
model/src/Castle.cpp:26:44: error: no matching function for call to ‘Castle::getVerticalMoves(Location***&, std::vector<Location>*, int&, int&)’
model/inc/Castle.h:38:8: note: candidate is: void Castle::getVerticalMoves(Location***, std::vector<Location*>*, int, int)
make: *** [model/obj/Castle.o] Error 1

I do not understand why this error is appearing. Why does it say that I am passing it references?

Answer 1

Do you want a vector of Locations? Or do you want a vector of pointers to Locations? You need to pick one and stick with it.

The reason it shows a reference is because if it showed just the value, that would suggest that you could not call a function that takes a reference, since a reference can appear on the left side of an equals sign and a value cannot. What you passed to the function can appear on the left side of an equals sign, so leaving off the & would make the error message less informative. (Suggesting perhaps your mistake was to pass a value where a modifiable reference was required.)

Consider:

int f1(int &j) { j++; return 3; }
int f2(int j) { return j+1; }
int q;

f1(3); // not legal, we are passing an 'int' value and need an 'int' reference
f2(3); // legal, passing an 'int' value function needs an 'int' value
f1(q); // legal, we are passing an 'int&', function need an 'int&'
f2(q); // legal, reference can be demoted to value

So when you pass an lvalue (something that can appear on the left side of an equal sign), you really are calling the function with a reference — something more powerful than a mere value.