什么会导致 Visual Studio 中的 LNK2019 错误？

Posted 2023-04-13

【中文标题】什么会导致 Visual Studio 中的 LNK2019 错误？

【英文标题】：What can cause LNK2019 error in VisualStudio?

【发布时间】：2020-06-14 19:21:56

【问题描述】：

我在 Visual Studio 中创建了一个简单的 cpp 项目，但遇到了以下链接错误：

LNK2019: 未解析的外部符号“public: class std::vector > __thiscall solver::euler(class point_3d (__cdecl*)(class point_3d,double),double,int,class point_3d,double)” (?euler@ ?$solver@Vpoint_3d@@@@QAE?AV?$vector@Vpoint_3d@@V?$allocator@Vpoint_3d@@@std@@@std@@P6A?AVpoint_3d@@V4@N@ZNH0N@Z

我不知道是什么原因造成的，如果有任何建议，我将不胜感激。 构成项目的文件：

main.cpp

#include "point_3d.h"
#include "solver.h"
point_3d lorentz(point_3d val, double t) 
    double x = 10*(val.get_y()-val.get_x());
    double y = val.get_x()*(28.0- val.get_z())- val.get_y();
    double z = val.get_x()* val.get_y()-(8.0/3)* val.get_z();

    return point_3d(x, y, z);


int main()
    auto file = std::ofstream("data/results.json");
    solver<point_3d> sol;
    std::vector<point_3d> result = sol.euler(lorentz, 0.01, 50000, point_3d(1, 1, 1));

求解器.h

template<typename T>
class solver
public:
     std::vector<T> euler(T (*f)(T val, double s),double h, int steps, T val, double t0 = 0.0);
     std::vector<T> back_euler(T (*f)(T val, double s), double h, int steps, T val, double t0 = 0.0,int n=50);
     std::vector<T> rk_2(T (*f)(T val, double s), double h, int steps, T val, double t0 = 0.0);
     std::vector<T> rk_4(T (*f)(T val, double s), double h, int steps, T val, double t0 = 0.0);
;

求解器.cpp

#include "solver.h"

template<typename T>
std::vector<T> solver<T>::euler(T (*f)(T val, double s), double h, int steps, T val, double t0 ) 
    std::vector<T>res;
    res.resize(steps + 1);
    res[0] = val;
    double t = t0;
    for (int i = 0; i < steps;i++) 
        res[i + 1] = res[i] + h * f(res[i], t);
        t += h;
    
    return res;


template<typename T>
std::vector<T> solver<T>::back_euler(T (*f)(T val, double s), double h, int steps, T val, double t0, int n) 
    std::vector<T>res;
    res.resize(steps + 1);
    res[0] = val;
    double t = t0;
    for (int i = 0; i < steps; i++) 
        t += h;
        res[i + 1] = res[i];
        for (int j = 0; j < n; j++) 
            res[i + 1] += res[i] + h * f(res[i + 1],t);
        
    
    return res;

 

template<typename T>
std::vector<T> solver<T>::rk_2(T (*f)(T val, double s), double h, int steps, T val, double t0) 
    std::vector<T>res;
    res.resize(steps + 1);
    res[0] = val;
    double t = t0;
    for (int i = 0; i < steps; i++) 

        auto k = h * f(res[i], t) / 2.0;
        res[i + 1] = res[i] + h * f(k+res[i],t+h/2.0);
        t += h;
    
    return res;


template<typename T>
std::vector<T> solver<T>::rk_4(T (*f)(T val, double s), double h, int steps, T val, double t0) 
    std::vector<T>res;
    res.resize(steps + 1);
    res[0] = val;
    double t = t0;
    for (int i = 0; i < steps; i++) 
        auto k1 = f(res[i],t);
        auto k2 = f(res[i]+h/2.0*k1, t+h/3.0);
        auto k3 = f(res[i] + h / 2.0 * k2, t + 2*h / 3.0);
        auto k4 = f(res[i] + h * k3, t + h);
        res[i + 1] = res[i] + (k1 / 6 + k2 / 3 + k3 / 3 + k4 / 6) * h;
        t += h;
    
    return res;

point_3d.h

class point_3d
    double x;
    double y;
    double z;
public:
    point_3d(double x, double y, double z): x(x), y(y), z(z) ;
    double get_x();
    double get_y();
    double get_z();
    point_3d operator+(point_3d other);
    point_3d& operator=(point_3d other);
    friend point_3d operator*(double k, point_3d p);
;

point_3d.cpp

#include "point_3d.h"

double point_3d::get_x() 
    return x;


double point_3d::get_y() 
    return y;


double point_3d::get_z() 
    return z;


point_3d point_3d::operator+(point_3d other) 
    double x = get_x() + other.get_x();
    double y = get_y() + other.get_y();
    double z = get_z() + other.get_z();
    return point_3d(x, y, z);


point_3d& point_3d::operator=(point_3d other) 
    this->x = other.get_x();
    this->y = other.get_y();
    this->z = other.get_z();
    return *this;


point_3d operator*(double k, point_3d p) 
    double x = p.get_x();
    double y = p.get_y();
    double z = p.get_z();
    return point_3d(k * x, k * y, k * z);
 

【问题讨论】：

solver.cpp 包含通用 template 的实现代码，但没有其特化。见Why can templates only be implemented in the header file?。

【参考方案1】：

您需要包含向量库才能使用 std::vector。

#include <vector>

“未解析的符号”表示有一个类或函数已声明但在项目的任何地方（或它包含的库​​中）都找不到。