靜態方法

Question

我有一個C++「接口」類，並收到以下錯誤在編譯的時候：

Undefined symbols for architecture x86_64: 
     "Serializable::writeString(std::ostream&, std::string)", referenced from: 
      Person::writeObject(std::ostream&) in Person.o 
      Artist::writeObject(std::ostream&) in Artist.o 
     "Serializable::readString(std::istream&)", referenced from: 
      Person::readObject(std::istream&) in Person.o 
      Artist::readObject(std::istream&) in Artist.o 
     ld: symbol(s) not found for architecture x86_64 

clang: error: linker command failed with exit code 1 (use -v to see invocation) 

是否有可能實現在抽象類的靜態方法？

我的實現看起來像這樣。

.h文件中

#ifndef Ex04_Serializable_h 
#define Ex04_Serializable_h 

using namespace std; 

class Serializable { 

public: 
    virtual void writeObject(ostream &out) = 0; 
    virtual void readObject(istream &in) = 0; 

    static void writeString(ostream &out, string str); 
    static string readString(istream &in); 
}; 

#endif 

.cpp文件

#include <iostream> 
#include "Serializable.h" 

using namespace std; 

static void writeString(ostream &out, string str) { 

    int length = str.length(); 

    // write string length first 
    out.write((char*) &length, sizeof(int)); 

    // write string itself 
    out.write((char*) str.c_str(), length); 
} 

static string readString(istream &in) { 

    int length; 
    string s; 

    // read string length first 
    in.read((char*) &length, sizeof(int)); 
    s.resize(length); 

    // read string itself 
    in.read((char*) s.c_str(), length); 
    return s; 
} 

Answer 1

嘗試：

void Serializable::writeString (...) { 
// ... 
} 

（嘗試沒有靜態，並添加類名）