我是编程新手。 编译包含 ah 文件中包含的 class 定义的代码后，我不断收到错误消息

Question

I am trying to call a member function of a custom class, but I keep getting error messages:

expected primary-expression before First_name 
expected primary-expression before Middle_name 
expected primary-expression before Surname

This is the code for main cpp :

#include <iostream >
#include <People.h>
auto NAME = P.Name_Input( std:string First_name, std::string Middle_name, std::string Surname) ;
std::cout<<NAME<<std::endl;

This is the header file:

class Person{
   public:
   std::string Name_Input(std::string First_name, std::string Middle_name, std::string Surname);
} ;

The header file compiles well, and has been link to a .cpp file to define the method, and that section also works perfectly. My problem is with the main.cpp file.

Answer 1

Neither main.cpp nor People.h are defining std::string , so the compiler doesn't know what it is.

You need to add #include <string> to People.h :

#ifndef People_H
#define People_H

#include <string> // <-- add this

class Person{
   public:
      std::string Name_Input(std::string First_name, std::string Middle_name, std::string Surname);
};

#endif

A general rule of thumb is that any source file that needs to use a type defined in another source file should #include that other file (except in cases where using forward declarations will suffice, such as when breaking circular references). SeeIs it good practice to rely on headers being included transitively? . Which means that main.cpp should have an #include <string> statement as well, even if People.h (or any other header main.cpp uses) already has its own #include <string> statement.

Also, your use of std::string in main() is all wrong. Don't include the type name when passing a variable to a function or class method. Use the type name only in the declaration of a variable or function parameter.

Try this instead:

#include <iostream>
#include <string> // <-- add this
#include "People.h"

int main()
{
    Person P;
    std::string First_name;
    std::string Middle_name;
    std::string Surname;

    ...

    auto NAME = P.Name_Input(First_name, Middle_name, Surname);
    std::cout << NAME << std::endl;

    ...

    return 0;
}