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c++ - error C2678: no operator found which takes a left-hand operand of type 'const _Ty'

问题描述

I get the error:

1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.28.29910\include\xstddef(127,1): error C2678: binary '<': no operator found which takes a left-hand operand of type 'const _Ty' (or there is no acceptable conversion) when i run my code:

#include <iostream>
#include <algorithm>
#include <vector>
#include <stack>
#include <string>
#include <iomanip>
#include <map>
#include <bitset>
#include <cstring>
#include <queue>
#include <math.h>

using namespace std;

class grid {
private:
public:
    vector<vector<short> > tiles;

    grid()
    {
        tiles.resize(4);
        for (int i = 0; i < 4; i++)
            tiles[i].resize(4);
    }

    inline bool operator==(grid& b)
    {
        for (int i = 0; i < 4; i++)
            for (int j = 0; j < 4; j++)
                if (tiles[i][j] != b.tiles[i][j])
                    return false;
        return true;
    }

    inline bool operator<(const grid& b)
    {
        return false;
    }

    void shiftUp()
    {
        for (int k = 0; k < 4; k++)
            for (int i = 0; i < 4; i++)
                for (int j = 0; j < 3; j++)
                    if (tiles[j][i] == 0)
                        swap(tiles[j][i], tiles[j + 1][i]);

        for (int i = 0; i < 4; i++)
        {
            for (int j = 0; j < 3; j++)
            {
                if (tiles[j][i] == tiles[j + 1][i])
                {
                    tiles[j][i] *= 2, tiles[j + 1][i] = 0;
                    break;
                }
            }
        }

        for (int k = 0; k < 4; k++)
            for (int i = 0; i < 4; i++)
                for (int j = 0; j < 3; j++)
                    if (tiles[j][i] == 0)
                        swap(tiles[j][i], tiles[j + 1][i]);
    }
};

map<grid, bool> used;

bool exists(grid x)
{
    return used[x];
}

int main()
{
    grid x;
    for (int i = 0; i < 4; i++)
        for (int j = 0; j < 4; j++)
            cin >> x.tiles[i][j];

    x.shiftUp();
    for (int i = 0; i < 4; i++)
    {
        for (int j = 0; j < 4; j++)
            cout << x.tiles[i][j] << " ";
        cout << endl;
    }
}

I'm using Visual Studio Community 2019. I've tried adding dummy '<' operators to the class and haven't found any other threads that worked for me.

标签： c++c++17

解决方案

std::map默认情况下std::less用于比较，默认情况下使用operator <. 所以你实现那个操作符是正确的。

但是，该错误表明没有operator <左手边const。你需要一个 const 版本：

class grid {
    // ...
    bool operator < (const grid &o) const { // Note *const* at end
        // Do something meaningful here
    }
}

阅读关于 const-correctness 的信息（例如这里）。

另请注意，运算符 < 需要提供严格的弱排序，因此如果a < b == true，它必须是b < a == false

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