.h 和.cpp 文件分离时出错，但仅使用 ah 文件时没有错误。 我究竟做错了什么？

Question

I'm a C++ noob, so please go easy on me if my doubt seems too trivial.

#pragma once

class cube
{
public:
    double getVolume();
    double getSA();
    void setLength(double length);

private:
    double length_;
};

This is my header file cube.h ^^^

#include "cube.h"

double cube::getVolume()
{
    return length_ * length_ * length_;
}

double cube::getSA()
{
    return 6 * length_ * length_;
}

void cube::setLength(double length)
{
    length_ = length;
}

This is cube.cpp and the next is main.cpp

#include "cube.h"
#include <iostream>
int main()
{

    cube c;

    c.setLength(3.48);
    double vol = c.getVolume();
    std::cout << "Vol: " << vol;

    return 0;
}

Question 1: So when I include the implementation in.h file itself without using.cpp, the code runs as expected. But when I separate the.h and.cpp files, I get an error saying: 'undefined reference to cube::setLength(double)', 'undefined reference to cube::getVolume()'' and 'exited with code=1 in 0.47 seconds'.

I ran it on VS Code and just set it up, and just ran a simple cin/cout and a hello world program. Is there something wrong I'm doing with the code itself or is it an error due to VS Code? Do note that this problem occurs only when I separate the.h and.cpp files.

Question 2: Why even separate the.h and.cpp files?

Edit 1: As @KamilCuk pointed out, the problem might be in linking the main.cpp and cube.cpp files, but I have no clue how you 'link' the cpp files for compilation?

Edit 2: I just might be compiling it the wrong way. I usually just click the Code Runner enabled run button on the top right, but I have no clue how to compile two files at once.

Answer 1

Try to do this. This worked for me and gave desired result:

g++ main.cpp cube.cpp -I./ -o cube.out

You can implement in.h file inline as well and can be make your code work. But for more complex classes better to separate the implementation in.cpp and.h

Also its a good practice to guard header file for multiple definitions:

#ifndef _CUBE_H_
#define _CUBE_H_

class Cube
{
   public:
    double getVolume();
    double getSA();
    void setLength(double length);

   private:
    double length_;
 };

 #endif //_CUBE_H_

Below link provided details of how to build multiple C++ files using VSCode.

https://dev.to/talhabalaj/setup-visual-studio-code-for-multi-file-c-projects-1jpi