找不到声明的函数

我正在尝试制作一个面向对象的菜单，但是在返回int函数的情况下初始化int变量时遇到了一些问题。

法典：

#include <Menu.h>
#include <iostream>
using namespace std;

void Menu::showMainMenu(){
    cout<<"""Main Menu"""<<endl;
    cout<<"1. Manage students"<<endl;
    cout<<"2. Manage assignments"<<endl;
    cout<<"3. Manage statistics"<<endl;
    cout<<"4. Print database"<<endl;
    cout<<"0. Exit"<<endl<<endl;
}

void Menu::showStudentMenu(){
    cout<<"""Student Menu"""<<endl;
    cout<<"1. Add student"<<endl;
    cout<<"2. Remove student"<<endl;
    cout<<"3. Edit student"<<endl;
    cout<<"4. Print student"<<endl;
    cout<<"5. Print students"<<endl<<endl;
}

void Menu::showAssignmentMenu(){
    cout<<"""Assignment Menu"""<<endl;
    cout<<"1. Add assignment"<<endl;
    cout<<"2. Remove assignment"<<endl;
    cout<<"3. Edit assignment"<<endl;
    cout<<"4. Print assignment"<<endl;
    cout<<"5. Print assignments"<<endl<<endl;
}

void Menu::showStatisticsMenu(){
    cout<<"""Assignment Menu"""<<endl;
    cout<<"1. Add assignment"<<endl;
    cout<<"2. Remove assignment"<<endl;
    cout<<"3. Edit assignment"<<endl;
    cout<<"4. Print assignment"<<endl;
    cout<<"5. Print assignments"<<endl<<endl;
}


string Menu::stringInputHandler(){
    string input;
    cout<<"enter:";  cin>>input; cout<<endl;
    if(!input.empty()){
        return input;
    }
    cout<<"Input cannot be empty"<<endl;
    stringInputHandler();
return "Error! (f:sIH)";
}
int intInputHandler(){
    int input;
    cout<<"enter:";  cin>>input; cout<<endl;
    if(input){
        return input;
    }
    cout<<"Input cannot be empty"<<endl;
    intInputHandler();
    return 0;
}

void Menu::CommandControllMain(){
    showMainMenu();
    int ret;
    ret = intInputHandler();
    while(ret){
        switch(ret){
        case 1:CommandControllStudent();break;
        case 2:CommandControllAssignment();break;
        case 3:break;
        case 4:break;
        case 0:break;
        default:cout<<"Wrong option selected!";break;
        }
    } 
}

void Menu::CommandControllStudent(){
    showStudentMenu();
    int ret = intInputHandler();

}

void Menu::CommandControllAssignment(){
    showAssignmentMenu();
    int ret =intInputHandler();
}
void Menu::CommandControllStatistics(){
    showStatisticsMenu();
    int ret =intInputHandler();
}

所以有：intInputHandler() 和一个 var int ret 我不能做 int ret = intInputHandler（）;

错误：

D:c++BeginLab6-8_UMLDebug/../Menu.cpp:121: undefined reference to `Menu::intInputHandler()'