为什么无参有返会"int（*）（）" 类型的值不能用于初始化 "int *" 类型的实体

#include<iostream>
using namespace std;

//1.无参无返
void test01()
{
    cout << "this is test01" << endl;
}
//2.有参无返
void test02(int a)
{
    cout << "this is test02 a=" << a << endl;
}
//3.无参有返
int test03()
{
    cout << "this is test03" << endl;
    return 1000;
}
//4.有参有返
int test04(int a)
{
    cout << "this is test04" << endl;
    return a;
}
int main()
{
    //1.无参无返
    test01();
    //2.有参无返
    test02(100);
    //3.无参有返
    int num1 = test03;
    cout << num1 << endl;
    //4.有参有返
    int num2 = test04(10000);
    cout << "num2=" << num2 << endl;
    system("pause");
    return 0;
}