Stroustrup Chapter 6 class Token Example Link Error

Hello everyone, I'm having a linking error with:

Code:

#include "../../../std_lib_facilities.h"

//------------------------------------------------------------------------------

class Token {
public:
    char kind;        // what kind of token
    double value;     // for numbers: a value 
    Token(char ch)    // make a Token from a char
        :kind(ch), value(0) { }    
    Token(char ch, double val)     // make a Token from a char and a double
        :kind(ch), value(val) { }
};

//------------------------------------------------------------------------------

 Token get_token();  // PROBLEM MAY BE HERE

//------------------------------------------------------------------------------

vector<Token> tok;    // we'll put the tokens here

//------------------------------------------------------------------------------

int main()
{

    while (cin) {
        Token t = get_token();
        tok.push_back(t);
    }
    // ...
    

    for (int i = 0; i<tok.size(); ++i) {
        if (tok[i].kind=='*') {    // we found a multiply!
            double d = tok[i-1].value*tok[i+1].value;
            // now what?
        }
    }
}

Getting:
1>D:\Documentos\Projectos\Programming P & P w C++\Libro\6.3 Example Tokenizing\Debug\6.3 Example Tokenizing.exe : fatal error LNK1120: 1 unresolved externals
1>
1>Build FAILED.

Pleas advise

It should have given you a list of the unresolved symbols.

And yes, you will need to implement get_token().
It isn't enough just to state that it exists.

Hello there!

It results that the entire chapter leads you to inminent error!

He does a lot of unexplained implementations because he is not WRITING the program, he is thinking it. STILL, I did not get that until the last part of the chapter, where STILL I don't get the entire flow of the program.

Whoever read the chapter, will agree that the grammar implementation with the tokenize input (which all that implies - returning the token to the stream after unused evaluations & stuff) is complicated to grasp at first glance.

Basically, at the end of the chapter we have:

class Token - tokenizer
class ts - fetches and returns tokens from i/o streams
double expression();
double term();
double primary();
int main();

The process of how to achieve "correctesness" is impressively weel explained, but the program itself it may not be the best example just because of the classes implementations with constructors which I don't fully understand.

Thanks a bunch!