c++ - 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，编译器提示:

Error 1 error C3848: expression having type 'const ai::search::ManhattanDistance' would lose some const-volatile qualifiers in order to call 'size_t ManhattanDistance::operator ()(const std::string &,const std::string &)'

如何理解这个错误？如何解决？

Answer 1

您的方法 std::size_t ManhattanDistance::operator()(std::string const& state, std::string const& goal) 尚未声明为 const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
//                                                                        ^^^^^