The situation and solution of Link2019 in C++

1. Link2019 appears

• Error message: LNK2019 unresolved external symbol "void __cdecl Log(char const *)" (?Log@@YAXPEBD@Z), which is referenced in function main
• The reason is encountered, because during the linking process, the method body of the declared function cannot be found
• Code Situation:
  File A:

  #include <iostream>
  
  // 这里定义了一个Logr()函数，但是不存在Log()
  void Logr(const char* message) {
      std::cout << message << std::endl;
  }

  file B:
   

  #include <iostream>
   
  // 声明Log函数，但是没有方法体，链接会在编译后的obj文件里面，进行寻找
  // 如果没有找到，就会出现Link2019
  void Log(const char* message); 
  int main() {
      Log("Hello world!");
      std::cin.get();
  }
  
  

• Solution:
  1. In file B, stop calling the Log() function. The link will not look for functions that have not been called.
  2. Give the Log() function a method body, and change the Logr() function to Log()
• Supplement: return value function function name (parameter, parameter,...) definition must be the same, otherwise the link cannot be found
  

  void Log(const char* message); 
  void Log(const char* message,int num); 
  
  // 两个函数不一样，因为参数不同