声明友元函数时的变量范围错误

Question

Friend functions can't access variables of the classes

I'm having a problem with several friend functions not being able to access the variables in classes where they have been declared as friends.

The actual error text is: error: 'fid' was not declared in this scope. this repeats for the other private variables. The same error is given for three functions, read, negative, and write.

A couple of notes: 1) This lab requires that I write the code so that the functions can be used by both classes.

I'm compiling this in windows with code::blocks using g++ and I've also tried compiling my code in ubuntu using g++ from the terminal using the -g flag and I get the same error both times.

Any suggestions you have would be greatly appreciated.

Header File

#ifndef PXMUTILS_H
#define PXMUTILS_H

#include <cstdio>
#include <cstdlib>
#include <string>
#include <sstream>
#include <vector>
#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string.h>

using namespace std;
typedef unsigned char uchar;

class pgm
{
public:
    pgm();
    ~pgm();
    void read(string &);
    void negative();
    void write(string);
    friend void read (const string &);
    friend void write(string);
    friend void negative();
private:
    int nr;
    int nc;
    int mval;
    int ftyp;
    string fid;
    uchar **img;
};

class ppm
{
public:
    ppm();
    ~ppm();
    void read(string &);
    void negative();
    void write(string);
    friend void read (const string &);
    friend void write (string);
    friend void negative ();
private:
    int nr;
    int nc;
    int mval;
    int ftyp;
    string fid;
    uchar **img;
};

#endif

C++ program

#include <cstdio>
#include <cstdlib>
#include <string>
#include <sstream>
#include <vector>
#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string.h>
#include "pxmutils.h"

using namespace std;
typedef unsigned char uchar;

uchar ** newimg(int nr, int nc, int ftyp)
{
uchar **img=new uchar *[nr];
img[0]=new uchar [nr*nc*ftyp];
for(int i=1; i<nr; i++)
    {
        img[i]=img[i-1]+nc*ftyp;
    }
    return img;
}

void deleteimg(uchar **img)
{
    if(img)
    {
        if(img[0])
        {
            delete [] img[0];
        }
        delete [] img;
    }
}
void read (const string &fname)
{
    ifstream fin(fname.c_str(), ios::in);
    if(!fin.is_open())
    {
        cerr<<"Could not open "<<fname<<endl;
        exit(0);
    }
    fin >>fid
        >>nc
        >>nr
        >>mval;
        while (fin.get() != '\n') { /*skip to EOL */ }

    img=newimg(nr, nc);
    fin.read((char *)img[0], nr*nc);
    fin.close();
    }

void set_cmap(string mname)
{
}

void negative()
{
    for(int i=0; i<nr; i++)
    {
        for(int j=0; j<nc; j++)
        {
           int t=img[i][j];
           img[i][j]=(255-t);
        }
    }
}

void write(string fname)
{
        ofstream fout (fname.c_str(), ios::out);
        size_t dp;
    if ((dp = fname.rfind(".pgm")) != string::npos)
        {
            fout<<"P5"<<endl;
        }
        if((dp= fname.rfind(".ppm")) != string::npos)
        {
            fout<<"P6"<<endl;
        }
        fout<<nc<<" "<<nr<<endl;
        fout<<mval<<endl;

    for(int i=0; i <nr; i++)
    { 
        for (int j=0; j<nc; j++)
        {
            fout<<img[i][j]<<" ";
        }
        fout<<endl;
    }

    fout.close();
}

pgm::pgm()
{
    nr=0;
    nc=0;
    mval=0;
    ftyp=1;
    fid="";
    img=NULL;
}

pgm::~pgm()
{
    deleteimg(img);
}

ppm::ppm()
{
    nr=0;
    nc=0;
    mval=0;
    ftyp=1;
    fid="";
    img=NULL;
}

ppm::~ppm()
{
    deleteimg(img);
}

Program to test functions

#include <cstdlib>
#include <iostream>
#include <string>
using namespace std;

#include "pxmutils.h"

int main(int argc, char *argv[])
{
    if (argc == 1) {
        cerr << "No input file specified!\n";
        exit(0);
    }

    string fname = argv[1];
    size_t dp;

    if ((dp = fname.rfind(".pgm")) == string::npos) {
        cout << "PGM error: file suffix " << fname
             << " not recognized\n";
        exit(0);
    }

    fname.erase(dp);

pgm img_g;
    ppm img_c;

    img_g.read(fname+".pgm");

    if (argc == 3)    
    img_c.set_cmap(argv[2]);

    img_c = img_g;

    img_g.negative();

    img_g.write(fname+"_n.pgm");
    img_c.write(fname+"_c.ppm");
}

Answer 1

fin >>fid
    >>nc
    >>nr
    >>mval;
    while (fin.get() != '\n') { /*skip to EOL */ }

In this code, fid, nc, nr etc are undefined. You need to use the class instance to be able to access them, they don't exist by themselves.

Your functions don't accept the class objects as parameters, so how are you going to read into them?

Answer 2

You should have another think of your design. It is best to avoid friend functions if possible,

Answer 3

You need to go a bit back to basics. When you define non-static members of a class you are defining attributes or operations of the objects of the class, but those attributes don't exist by themselves, only as part of the instances of the class.

This concept is orthogonal to access and access specifiers , that is, this is so regardless of the members being public , protected or private . Once you have an instance, when your try to access those members the access specifiers come into play, and there is where friendship comes into play: it will grant your code access to members that would otherwise be inaccessible ( private or protected outside of the inheritance hierarchy).

The problem in your code is that you don't have an object, and thus cannot access the members of the object. You will need to either create or pass an object of the appropriate type to the functions.

There are other problems in the code, like for example, the memory allocations inside newimg look a little suspicious (what were you intending to allocate?) but that is outside of the scope of this question.