C++ Compiling issue with class and strings

Question

I am quite new to c++ and I really don't understand what this error is or why it is occuring. I am receiving no useful information on why it is not compiling except for some unreferenced things. I have followed tutorials online for this and am getting errors nobody else has when doing this? Maybe I shouldn't use the tutorials I have found, have you got any?

Anyway the error is as follows:

main.o -> main.cpp:(.text+0x16e): undefined reference to 'validate::set_string(std::string)'
collect2.exe -> [Error] Id returned 1 exit status
Makefile.win -> recipe for target 'Math++.exe' failed

The code is as follows:

#include <iostream>
#include <cstdlib>
#include <string>

using namespace std;

string getName();
string name;

class validate {
    public:
        string item;
        void set_string (string);
        bool vInput() {

        }
        bool vName() {
            cout << item;
            system("pause");
            return true;
        }
};

int main() {
    name = getName();
}

string getName() {
    validate val;
    cout << "Enter your name: ";
    cin >> name;
    val.set_string(name);
    bool result = val.vName();
    return name;
}

I have tried moving the "validate val;" around to main, outside all functions and nothing happened with that. I also tried removing the vInput function as it has no return but that also made no difference.

After a comment that told me to declare the functions, I tried doing so but still got the compilation error. I declared them like this which may be incorrect for classes:

bool vName();
bool vInput();
void set_string (string);

The code after people have tried to help me:

#include <iostream>
#include <cstdlib>
#include <string>

using namespace std;

string getName();
string name;

bool vName();
bool vInput();
void set_string (string);

class validate {
    public:
        string item;
        void set_string (string);
        bool vInput() {
            return true;
        }
        bool vName() {
            cout << item;
            system("pause");
            return true;
        }
};

int main() {
    name = getName();

}

string getName() {
    validate val;
    cout << "Enter your name: ";
    cin >> name;
    val.set_string(name);
    bool result = val.vName();
    return name;
}

Nobody seems to understand why my set_string does nothing. Well its not suppose to! I am following an example on how to do this and they dont do any function?

// classes example
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
    public:
        void set_values (int,int);
        int area() {return width*height;}
 };

void Rectangle::set_values (int x, int y) {
    width = x;
    height = y;
}

int main () {
    Rectangle rect;
    rect.set_values (3,4);
    cout << "area: " << rect.area();
    return 0;
}

Answer 1

You declared a function called set_string, so the compiler now knows that there exists a function called like this. This means that if it encounters a call to this function, it knows that it's valid. The problem is, you never defined it.

Here's basic logic: a function must do something, right? Here you don't even have a function, so how should the compiler guess what it does? That's why it generates no object code for it. Then the linker comes and sees that lack of object code and raises an error.

Answer 2

As far as the compiler is concerned, it's not an error not to define the body of a method that you have declared within a class. That's what allows us to put possibly very complex method bodies in a different source file, which is often advantageous to writing clearer code. But the linker will object if it can't find the body for a method that you call.

Somewhere in your program, not necessarily in the same source file, you should have something like

void validate::set_string(string parameter)
{
 // your code goes here
}