error LNK 2019 Unresolved externals Build 在c++编译时出错

Question

我在 visual studio 2015 年尝试编译我的项目时不断收到此错误，我尝试了无数替代方案，但 none 似乎有效。

到目前为止我已经尝试过：

确保我是运行正确的 visual studio 项目（控制台应用程序）
检查了头文件并确保它们的顺序正确。

这是带有 main 函数的代码，main 函数是一个 hello world，我只是想让它先编译，所以我将 main 函数缩小到一个简单的 hello world 以便我可以专注于头文件。我真的需要多一双眼睛，这个错误一直是一场噩梦，欢迎任何调试想法。提前致谢。

代码：

KNNNode.h

#pragma once
#include <string>


using namespace std;


class KNNNode {
private:int index; 
        double distance;
        string c; 
public:
    KNNNode(int index, double distance, string c) {
        index = index;
        distance = distance;
        c = c;
    }

    int getIndex() {
        return index;
    }
    void setIndex(int lindex) {
        index = lindex;
    }
    double getDistance() {
        return distance;
    }
    void setDistance(double ldistance) {
        distance = ldistance;
    }
    string getC() {
        return c;
    }
    void setC(string lc) {
        c = lc;
    }
};

KNN.h

#pragma once
#include <vector>
#include <queue>
#include <unordered_map>
#include "KNNNode.h"




struct compareDistance {
    bool operator()(KNNNode lhs, KNNNode rhs) {
        return (lhs.getDistance() >= rhs.getDistance());
    }
};

class KNN {


    bool contains(vector<int> vect, int num) {
        for (int i = 0; i < vect.size(); i++) {
            if (vect[i] == num)
                return true;
        }
        return false;
    }

public:
    vector<int> getRandKNum(int k, int max) {
        vector<int> randomList = vector<int>(k);

        for (int i = 0; i < randomList.size(); i++) {
            int temp = static_cast<int>(rand() * max);
            if (!contains(randomList, temp)) {
                randomList.push_back(temp);
            }
            else {
                i--;
            }
        }
        return randomList;
    }

    double calDistance(vector<double> d1, vector<double> d2) {
        double distance = 0.00;
        for (int i = 0; i < (d1.size() - 1); i++) {
            distance += (d1[i] - d2[i]) * (d1[i] - d2[i]);
        }
        return distance;
    }

    string knn(vector<vector<double>> datas, vector<double> testData, int k) {
        priority_queue< KNNNode, vector<KNNNode>, compareDistance> pq;
        vector<int> randNum = getRandKNum(k, datas.size());
        for (int i = 0; i < k; i++) {
            int index = randNum[i];
            vector<double> currData = datas[index];
            string c = to_string(currData[currData.size() - 1]);
            KNNNode node = KNNNode(index, calDistance(testData, currData), c);
            pq.push(node);
        }
        for (int i = 0; i < datas.size(); i++) {
            vector<double> t = datas[i];
            double distance = calDistance(testData, t);
            KNNNode top = pq.top();
            if (top.getDistance() > distance) {
                pq.pop();
                pq.push(KNNNode(i, distance, to_string(t[t.size() - 1])));
            }
        }

        return getMostClass(pq);
    }

    string getMostClass(priority_queue<KNNNode, vector<KNNNode>, compareDistance> pq) {
        unordered_map<string, int> classCount;
        for (int i = 0; i < pq.size(); i++) {
            KNNNode node = pq.top();
            pq.pop();
            string c = node.getC();
            if (!(classCount.find(c) == classCount.end())) {
                int num = classCount.find(c)->second;

                classCount.insert({ { c, num + 1 } });
            }
            else {
                classCount.insert({ { c, 1 } });

            }
        }

        int maxCount = 0;
        string maxString;
        for (auto& x : classCount) {
            if (x.second > maxCount) {
                maxCount = x.second;
                maxString = x.first;
            }
        }


        return to_string(classCount.at(maxString));
    }
};

Main.cpp

#include "KNN.h"

#include <iostream>

class TestKNN {
public:
    int  main() {

        cout << "HELLO wORLD";

        system("PAUSE");
        return 0;
    }
};

错误：

1>MSVCRTD.lib(exe_main.obj) : error LNK2019: unresolved external symbol _main referenced in function "int __cdecl invoke_main(void)" (?invoke_main@@YAHXZ)
1>c:\users\underdog\documents\visual studio 2015\Projects\KNearestNeighbour\Debug\KNearestNeighbour.exe : fatal error LNK1120: 1 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

在 C++ 中，程序的主要函数必须是全局命名空间中名为 main 的函数，具有以下两个签名之一：

int main()
int main(int, char**)

你的程序没有这样的东西。它在 class TestKNN 中有一个名为 main 的成员函数，但那是完全不相关的东西。

Answer 2

与Java和C#不同，main()在C++中需要是全局自由函数。现在你有

class TestKNN {
public:
    int  main() {

        cout << "HELLO wORLD";

        system("PAUSE");
        return 0;
    }
};

这将不起作用，因为不考虑名称空间内的成员函数和函数。您需要将其从 class 中删除，只需要

int  main() {

    cout << "HELLO wORLD";

    system("PAUSE");
    return 0;
}

error LNK 2019 Unresolved externals Build 在c++编译时出错

error LNK 2019 Unresolved externals Build error during compilation in c++

c++

visual-studio-2012

visual-studio-2015

lnk2019

unresolved-external