未解决的外部符号热度

Question

I discovered a novel approach to Java-like enumeration through a class implementation right here on StackOverflow.

Symbols class declared in its' own header

class Symbols {

public:

    Symbols() {}

    static const Symbols CREATE;
              . . .
private:

    std::string the_symbol;

    // Symbols constructor
    Symbols (std::string symbol){
        this->the_symbol = symbol;
    }

public:

    std::string get_symbol(void) {
        return the_symbol;
    }

    void set_symbol(std::string new_value) { the_symbol = new_value; }  

    void initialize(void){
        const Symbols CREATE = Symbols("create");
                     . . .
    }
};

Used in classes in several ways

bool RecursiveDescentParser::type_name(){
if (accept(Symbols::STRING) || accept(Symbols::INTEGER)){
    return true;
}
else
    return false;

}

and

symbol_ = Symbols::WHERE;

if (!boost::iequals(tokenizer_->peekAtNext(), symbol_.get_symbol(), loc_)){
    // complete any tokens
    std::string temp = tokenizer_->completeToken();                     
}

The Symbols header is included in several classes. There are many compiler errors of the type

1>recursive_descent_parser.obj : error LNK2001: unresolved external symbol "public: static class Symbols const Symbols::VALUES" (?VALUES@Symbols@@2V1@B)

What gives? I have received in all 39 unresolved externals and some of which are unrelated to this question. I am properly defining header files to ensure no duplicate declarations and all classes compile properly.

Answer 1

You've declared the static member; but it also needs to be defined and initialised in exactly one source file.

const Symbols Symbols::CREATE("create");

I guess you think that's what the initialize function is doing; but it's just creating a local object of type Symbols , then destroying it when the function returns.

Answer 2

With static const Symbols CREATE; you only declare that a static const object of type Symbols is somewhere, which makes the compiler happy.

But there needs to be some place where the CREATE object is actually defined, that's why the linker can't find it.

You'll have to put

const Symbols Symbols::CREATE;

in one of your cpp files. That way a place of storage will actually be allocated so that the linker can find it.