错误：****尚未声明

Question

In my Function.h file:

class Function{
  public:
    Function();
    int help();
};

In my Function.cpp file:

#include "Function.h"
int Function::help() //Error here
{
  using namespace std;
  cout << "Help";
  return 1;
}

In my Main.cpp

#include <iostream>
#include "Function.h"
using namespace std;

int menu(){
  Function fc;
  fc.help();
  return 1;
}

int main(int args, char**argv){
  return menu();
}

Error is : 'Function' has not been declared
Can anybody tell me why? Thank you.

I tried like this and the problem is solved, but I dont really understand why:
In Function.h file:
I use

class Function{
  public:
    int status;
    Function():status(1){}
    int help();
};

instead of the old one

class Function{
  public:
    Function();
    int help();
};

Answer 1

All your include statements are missing the # :

#include "Function.h"
^

Everything else looks fine, though you need to also #include <iostream> in Function.cpp since you're using cout .

Here is the Function.cpp that I got to compile and run:

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

int Function::help() // No error here
{
    using namespace std;
    cout << "Help";
    return 1;
}

Function::Function()
{
}

Answer 2

I had a similar problem. Make sure that you only have the required header files. I had two header files both including each other and it spit out this mistake.

Answer 3

In the first Function.h file you have declared the constructor but not defined it. In the second Function.h file (the one that works) you have defined and declared the Function constructor. You can either define and declare in the header or file, or declare in the header file and define in the Function.cpp file.

For example, declare in the header file "Function.h":

    class Function
    {
    Function();
    }

and define here in "Function.cpp":

    Function::Function(){}

Or the alternative is to declare and define in the header file "Function.h":

    Class Function
    {
    Function(){}
    }

The other thing that you have done in the second version of the header file is to initialise the member variable "status" in the "member initialisation list" which is a good thing to do (See Effective C++ by Scott Meyers, Item 4). Hope this helps :)

Answer 4

You have created a declaration for the constructor of the Function class without including it in your implementation (cpp file).

#include "Function.h"

Function::Function(){
    // construction stuff here
}

int Function::help() //Error here
{
using namespace std;
cout << "Help";
return 1;
}