如何修复vs2013上的C3848错误？

Question

我正在尝试在VS2013上使用C++实现最佳优先搜索.下面是代码.

    //node for tree
    struct Node
    {
        Node(std::string const& s, std::string const& p)
            : state(s), path(p)
        {}

        const std::string state;
        const std::string path;
    };

    //heuristic functor
    struct ManhattanDistance
    {
        std::size_t operator()(std::string const& state, std::string const& goal)
        {
            std::size_t ret = 0;
            for (int index = 0; index != goal.size(); ++index)
            {
                if ('0' == state[index])
                    continue;

                auto digit = state[index] - '0';
                ret += abs(index / 3 - digit / 3) + abs(index % 3 - digit % 3);// distance(row) plus distance(col)
            }

            return ret;
        }
    };

    //functor to compare nodes using the heuristic function.
    template<typename HeuristicFunc>
    struct GreaterThan
    {
        explicit GreaterThan(HeuristicFunc h, std::string const& g = "012345678")
            : goal(g), heuristic(h)
        {}

        bool operator()(Node const& lhs, Node const& rhs) const
        {
            return heuristic(lhs.state, goal) > heuristic(rhs.state, goal);
            return true;
        }

        const std::string goal;
        const HeuristicFunc heuristic;
    };

在单元测试中测试此代码时,编译器抱怨:

错误1错误C3848:类型为'const ai :: search :: ManhattanDistance'的表达式会丢失一些const-volatile限定符,以便调用'size_t ManhattanDistance :: operator()(const std :: string&,const std :: string &)"

如何理解这个错误？怎么解决？

Answer 1

您的方法std::size_t ManhattanDistance::operator()(std::string const& state, std::string const& goal)未声明const,但您尝试在const ManhattanDistance对象上调用它.编译器正确地拒绝了这个格式错误的程序.

更改定义行以声明方法const:

std::size_t operator()(std::string const& state, std::string const& goal) const
//                                                                        ^^^^^