链接错误。 我不知道我错过了什么

Question

I'm learning programming from Stroustrup's, Programming: Practices & Principles 2nd ed. From chapter. 6.3.4, I'm using the following code from the book and get the "Undefined symbols for architecture x86_64" error:

#include <iostream>
#include <vector>

using namespace std;

class Token{            //User-defined type with 2 members: Token.
    public:
        char kind;      //Member 'kind'. It has a char type.
        double value;   //Member 'value'. It has a double type.
};

Token get_token();  //Created a BLANK function of Token type to read cin input.
vector<Token> tok;  //Tokens will be placed inside vector 'tok'.    

int main() 
{
    while(cin){
        Token t= get_token();   //t from input.
        tok.push_back(t);   //Value in t is pushed back in the vector.
    }
    for(int i=0;i<tok.size();++i){  //i= 0 until less than size of vector, add 1.
        if(tok[i].kind=='*'){       //Finds '*'.
            double d=tok[i-1].value*tok[i+1].value; 
            //Evaluates object before '*', multiplied by object after it.
            //Now what?
        }
    }
}

The 'std_lib_facitilies.h' doesn't fix the problem. For function 'get_token();', am I missing any linking or is it that I can't leave the function blank (as written in the book)? I'm new to programming and any help will be appreciated. I'm using Clang set to: c++11, libc++. Error:

Undefined symbols for architecture x86_64:
  "get_token()", referenced from:
      _main in 6-5f59e7.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

You need to implement get_token(). In your case I'd try to do this within a constructor, like this:

class Token            //User-defined type with 2 members: Token.
{
    public:
        Token(std::istream& inputstream)
        {
            kind << inputstream;
            value << inputstream;
        }
        char kind;      //Member 'kind'. It has a char type.
        double value;   //Member 'value'. It has a double type.
};

Then you could write Token t(cin); instead of Token t = get_token();

But still I wouldn't like this implementation. To make vector<Token> work well, it should have a move constructor. That would enable you to write while(cin) tok.push_back(Token(cin)); .