如何解决头文件的问题？

Question

我决定处理模块化编程，通过头文件和cpp文件创建常规项目。

比如我拿这段代码 ///tabular.h 文件///

#pragma once
#include <iostream>
#include <stdio.h>
#include <math.h>

using namespace std;

float pi = 3.14159;
void print(float, float, float, float (*)(float), char[]);

///tabular.h 文件///

#include "tabular.h"

using namespace std;

void print(float a, float b, float h, float (*func)(float), char s[]) {
    float x;
    cout << "==================" << endl;
    cout << "  x  | "<< s << endl;
    cout << "==================" << endl;
    x = a;
    while (x<b) {
        printf("%6.2f |", x);
        printf("%8.4f\n", (*func)(x));
        x+=h;
    }
}

/// app.cpp 文件（主）///

#include "tabular.h"

using namespace std;

float lower, upper, step;

int main() {
    cout << "Enter lower, upper bounds and step, multiple of pi:";
    cin >> lower >> upper >> step;
    lower *= pi; upper *= pi; step *= pi;
    char name[] = "sin(x)";
    print(lower, upper, step, sin, name);
    char name2[] = "cos(x)";
    print(lower, upper, step, cos, name2);
}

仅当我编写这些命令时，这对我在 Visual Studio Code 中有效

PS C:\Users\User\Deskto