Error LNK2019 and dont know what is happening

I have this class:

    #ifndef String_H
#define String_H


class String
{
    public:
        String();
        String(const char*, ...);
        String(const String&);
        ~String();

        const String& operator= (const char*);      


        const int capacity();
        void clear(){ string[0] = '\0'; }

        const char* getString()const { return string; } 
        const int lenght()const { return length; }

    private:
        int length;
        char *string;
        void alloc(const int);

};


#endif

And in the implementation i have this:

#include <wtypes.h>
#include "String.h"
#include "Log.h"
#include <stdio.h>

String::String()
{
    alloc(1);
    clear();
}

String::String(const char* _string, ...)
{
    //length = 0;
    if (_string != NULL)
    {
        static char buff1[4096];
        va_list args;

        va_start(args, _string);
        int res = vsprintf_s(buff1, 4096, _string, args);
        va_end(args);

        if (res > 0)
        {
            alloc(res + 1);
            strcpy_s(string, length, buff1);
        }
    }
    else
    {
        alloc(1);
        clear();
    }
}

String::String(const String& _string)
{
    if (&_string != NULL)
    {
        alloc(_string.lenght());
        strcpy_s(string, length, _string.getString());
    }
    else
    {
        alloc(1);
        clear();
    }
}

String::~String()
{
    delete[]string;
}

const String& String::operator= (const char* str)
{
    if (strlen(str) > sizeof(string) + 1)
    {       
        delete[] str;
        alloc(strlen(str) + 1);
        strcpy_s(string, length, str);

    }
    else
    {
        strcpy_s(string, length, str);
    }
    return (*this);
}


void String::alloc(const int size)
{
    length = size;
    string = new char[size];
}

And when in main I do:

String a;
String b("hi");
a = b;

The compiler says me that:

Error 2 error LNK2019: unresolved external symbol "public: class String const & __thiscall String::operator=(class String const &)" (??4String@@QAEABV0@ABV0@@Z) referenced in function _main C:..\main.obj

AND

Error 3 error LNK1120: 1 unresolved externals C:..\MyLibrary.exe

This is making me crazy. Please help me. I can't see what i'm doing wrong.