Creating vector of vectors (2D array) inside a class - Error:C++ requires a type specifier for all declarations

Question

所以我在 C++ 中使用 Runge-Kutta 4(RK4) 方法程序化地编写了一种积分方程的方法。我目前正在尝试创建一个 RK4 class 来为我的程序添加更多功能。但是，当我在 class 中设置我的二维数组时，我收到 "C++ requires a type specifier for all declarations" 错误，这是我在原始程序中没有收到的。

起初，我使用的是 namespace std;，但是我遇到了一个回答，说那是糟糕的编程习惯，所以我删除了那行并改为调用 std::vector。我还确保包含矢量库，但无济于事。

#include <iostream>
#include <stdio.h>
#include <math.h>
#include <vector>

class RK4 {
public:
    RK4(double x_i, double y_i, double z_i, double vx_i, double vy_i, double vz_i);
    //time-related stuff
    double dt = 0.01;
    int numtimes = 1000;

    int numvar = 3;
    double mu = 398589590898200.0;

    //setting up variables
    std::vector<vector<double> > position(numtimes,vector<double>(numvar));
    std::vector<vector<double> > velocity(numtimes,vector<double>(numvar));
    std::vector<vector<double> > rk1(numtimes,vector<double>(numvar));
    std::vector<vector<double> > rk2(numtimes,vector<double>(numvar));
    std::vector<vector<double> > rk3(numtimes,vector<double>(numvar));
    std::vector<vector<double> > rk4(numtimes,vector<double>(numvar));
    std::vector<vector<double> > vk1(numtimes,vector<double>(numvar));
    std::vector<vector<double> > vk2(numtimes,vector<double>(numvar));
    std::vector<vector<double> > vk3(numtimes,vector<double>(numvar));
    std::vector<vector<double> > vk4(numtimes,vector<double>(numvar));

这是我在原始程序中构建代码的方式，没有问题：

#include <iostream>
#include <ostream>
#include <cmath>
#include <vector>

using namespace std;

int main(){
// time related things
    //double t_i = 0; // initial time
    double dt = 0.01; // time step
    int numtimes = 10000; // how many steps taken

    vector<vector<double> > position(numtimes,vector<double>(numvar));
    vector<vector<double> > velocity(numtimes,vector<double>(numvar));

    vector<vector<double> > rk1(numtimes,vector<double>(numvar));
    vector<vector<double> > rk2(numtimes,vector<double>(numvar));
    vector<vector<double> > rk3(numtimes,vector<double>(numvar));
    vector<vector<double> > rk4(numtimes,vector<double>(numvar));
    vector<vector<double> > vk1(numtimes,vector<double>(numvar));
    vector<vector<double> > vk2(numtimes,vector<double>(numvar));
    vector<vector<double> > vk3(numtimes,vector<double>(numvar));
    vector<vector<double> > vk4(numtimes,vector<double>(numvar));
}

我不太清楚为什么这个调用在我原来的（程序）程序中有效，但在我当前的程序中却无效。

Answer 1

在函数中，您可以使用括号将参数传递给变量的构造函数。这在为成员变量指定默认值时不起作用。你必须使用统一初始化.

 std::vector<std::vector<double>> position{numtimes, std::vector<double>(numvar)};

这种语法 - 使用大括号 - 也适用于声明变量，您可以在 main 中使用同一行来声明 position 变量。

统一初始化的另一件事是更强的类型安全性。您不能指定一个不适合您尝试存储它的位置的数字。您的代码中的一个问题是 numtimes 是 int，但是 vector 的构造函数的第一个参数是 size_t - [=18 的某种形式=].因此，您需要将 numtimes 的声明更改为

unsigned numtimes = 10000;

另请注意，您需要将 std:: 放在所有 vector 用法的前面。

Answer 2

使用类，您不能在函数外初始化成员。不过，您可以在构造函数中分配它们。

还有，你在一堆需要"std::vector"的地方写了"vector"。这应该可以解决它：

#include <math.h>
#include <stdio.h>
#include <iostream>
#include <vector>

class RK4 {
   public:
    RK4(double x_i, double y_i, double z_i, double vx_i, double vy_i, double vz_i) {
        double dt = 0.01;
        int numtimes = 1000;

        int numvar = 3;
        double mu = 398589590898200.0;

        //assigning the vectors
        position = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        velocity = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        rk1 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        rk2 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        rk3 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        rk4 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        vk1 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        vk2 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        vk3 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
        vk4 = std::vector<std::vector<double> >(numtimes, std::vector<double>(numvar));
    }
    //time-related stuff
    double dt = 0.01;
    int numtimes = 1000;

    int numvar = 3;
    double mu = 398589590898200.0;

    //declaring vectors
    std::vector<std::vector<double> > position;
    std::vector<std::vector<double> > velocity;
    std::vector<std::vector<double> > rk1;
    std::vector<std::vector<double> > rk2;
    std::vector<std::vector<double> > rk3;
    std::vector<std::vector<double> > rk4;
    std::vector<std::vector<double> > vk1;
    std::vector<std::vector<double> > vk2;
    std::vector<std::vector<double> > vk3;
    std::vector<std::vector<double> > vk4;
};

int main() {
    RK4 r(1, 1, 1, 1, 1, 1);

    return 0;
}

Answer 3

将那些 std::vector 初始化移动到 RK4 的构造函数并使用成员初始化列表语法来初始化它们：

#include <iostream>
#include <vector>

class RK4 
{
    double dt = 0.01;
    int numtimes = 1000;
    int numvar = 3;
    double mu = 398589590898200.0;

    //declaring vectors
    std::vector<std::vector<double> > position;
    std::vector<std::vector<double> > velocity;
    std::vector<std::vector<double> > rk1;
    std::vector<std::vector<double> > rk2;
    std::vector<std::vector<double> > rk3;
    std::vector<std::vector<double> > rk4;
    std::vector<std::vector<double> > vk1;
    std::vector<std::vector<double> > vk2;
    std::vector<std::vector<double> > vk3;
    std::vector<std::vector<double> > vk4;

   public:
    RK4(double x_i, double y_i, double z_i, double vx_i, double vy_i, double vz_i) : 
       position(numtimes, std::vector<double>(numvar)),
       velocity(numtimes, std::vector<double>(numvar)),
       rk1(numtimes, std::vector<double>(numvar)),
       rk2(numtimes, std::vector<double>(numvar)),
       rk3(numtimes, std::vector<double>(numvar)),
       rk4(numtimes, std::vector<double>(numvar)),
       vk1(numtimes, std::vector<double>(numvar)),
       vk2(numtimes, std::vector<double>(numvar)),
       vk3(numtimes, std::vector<double>(numvar)),
       vk4(numtimes, std::vector<double>(numvar))
    {}
};

给定成员变量的顺序，你也可以这样做（但这需要在任何其他向量之前声明position）：

#include <iostream>
#include <vector>

class RK4 
{
    double dt = 0.01;
    int numtimes = 1000;
    int numvar = 3;
    double mu = 398589590898200.0;

    //declaring vectors
    std::vector<std::vector<double> > position;
    std::vector<std::vector<double> > velocity;
    std::vector<std::vector<double> > rk1;
    std::vector<std::vector<double> > rk2;
    std::vector<std::vector<double> > rk3;
    std::vector<std::vector<double> > rk4;
    std::vector<std::vector<double> > vk1;
    std::vector<std::vector<double> > vk2;
    std::vector<std::vector<double> > vk3;
    std::vector<std::vector<double> > vk4;

   public:
    RK4(double x_i, double y_i, double z_i, double vx_i, double vy_i, double vz_i) : 
       position(numtimes, std::vector<double>(numvar)),
       velocity(position),
       rk1(position),
       rk2(position),
       rk3(position),
       rk4(position),
       vk1(position),
       vk2(position),
       vk3(position),
       vk4(position)
    {}
};

之所以可行，是因为 position 和所有其他向量的大小完全相同，并且以相同的值开始。所以用 position 初始化其他向量会变得更容易一些。但是同样，这需要在其他向量之前在 class 中声明 position 才能起作用。

Creating vector of vectors (2D array) inside a class - Error:C++ requires a type specifier for all declarations

Creating vector of vectors (2D array) inside a class - Error:C++ requires a type specifier for all declarations

c++

eclipse

class

stdvector

multidimensional-array