error C2039: 'string' : is not a member of 'std', header file problem

问题

I am having problems with a class I am writing. I have split the class into a .h file that defines the class and an .cpp file that implements the class.

I receive this error in Visual Studio 2010 Express:

error C2039: 'string' : is not a member of 'std'

This is the header FMAT.h

class string;

class FMAT {
public:
    FMAT(); 

    ~FMAT(); 

    int session();              

private:
    int manualSession();    
    int autoSession();      

    int     mode;       
    std::string instructionFile;    

};

This is the implementation file FMAT.cpp

#include <iostream>
#include <string>
#include "FMAT.h"

FMAT::FMAT(){

    std::cout << "manually (1) or instruction file (2)\n\n";
    std::cin >> mode;
    if(mode == 2){
        std::cout << "Enter full path name of instruction file\n\n";
        std::cin >> instructionFile;
    }

}

int FMAT::session(){

    if(mode==1){
        manualSession();
    }else if(mode == 2){
        autoSession();
    }

    return 1;
}

int FMAT::manualSession(){
    //more code
    return 0;
}

this is the main file that uses this class

#include "FMAT.h"

int main(void)
{
    FMAT fmat;      //create instance of FMAT class

    fmat.session();     //this will branch to auto session or manual session

}

My inability to fix this error is probably a result of me not understanding how to properly structure a class into separate files. Feel free to provide some tips on how to handle multiple files in a c++ program.

回答1:

You need to have

#include <string>

in the header file too.The forward declaration on it's own doesn't do enough.

Also strongly consider header guards for your header files to avoid possible future problems as your project grows. So at the top do something like:

#ifndef THE_FILE_NAME_H
#define THE_FILE_NAME_H

/* header goes in here */

#endif

This will prevent the header file from being #included multiple times, if you don't have such a guard then you can have issues with multiple declarations.

回答2:

Your FMAT.h requires a definition of std::string in order to complete the definition of class FMAT. In FMAT.cpp, you've done this by #include <string> before #include "FMAT.h". You haven't done that in your main file.

Your attempt to forward declare string was incorrect on two levels. First you need a fully qualified name, std::string. Second this works only for pointers and references, not for variables of the declared type; a forward declaration doesn't give the compiler enough information about what to embed in the class you're defining.