bool operator< issues

**William Putnam** · 09-26-2013

I'm currently working on a homework assignment where I have a vector of custom class objects. I want to sort these vector entries in alphabetical order. I've tried using my instructor's method of implementing the boolean operator for <, but it isn't working properly for me. Below is the header file involved with this issue:

Code:

class fileData{
    
private:
    char directName[256], *directType;
    int inodeNum;
    //---------
    int statNode, statMode, statLink, statUID, statGID, statMTime, statCTime;
    //---------
    char SHA1[20];
    
public:
    fileData(direntry d, stats s);
    ~fileData();
    bool operator<(fileData f) { return (strcmp(directName, f.directName) < 0);}
    void print(ostream& out);
    void serialize(ostream& out);
    
};

In the other class file, I've got:

Code:

class dirData{
    
private:
//...
    vector<fileData> fD;
    //...
    
public:
    dirData(char* name, stats s);
    ~dirData();
   //...
    void sortFD(){ std::sort(fD.begin(), fD.end());}
    //...
    
};

These are the errors I'm getting when trying to compile through the terminal:

Code:

/usr/include/c++/4.2.1/bits/stl_algo.h:91:15: error: invalid operands to binary
      expression ('const fileData' and 'const fileData')
      if (__a < __b)
          ~~~ ^ ~~~
/usr/include/c++/4.2.1/bits/stl_algo.h:2752:23: note: in instantiation of
      function template specialization 'std::__median<fileData>' requested here
                                       _ValueType(std::__median(*__first,
                                                  ^
/usr/include/c++/4.2.1/bits/stl_algo.h:2829:4: note: in instantiation of
      function template specialization
      'std::__introsort_loop<__gnu_cxx::__normal_iterator<fileData *,
      std::vector<fileData, std::allocator<fileData> > >, long>' requested here
          std::__introsort_loop(__first, __last,
          ^
./dirData.hpp:24:20: note: in instantiation of function template specialization
      'std::sort<__gnu_cxx::__normal_iterator<fileData *, std::vector<fileData,
      std::allocator<fileData> > > >' requested here
    void sortFD(){ std::sort(fD.begin(), fD.end());}
                   ^
./fileData.hpp:20:10: note: candidate function not viable: 'this' argument has
      type 'const fileData', but method is not marked const
    bool operator<(fileData f) { return (strcmp(directName, f.directName) < 0);}
         ^
/usr/include/c++/4.2.1/bits/stl_pair.h:102:5: note: candidate template ignored:
      failed template argument deduction
    operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
    ^
/usr/include/c++/4.2.1/bits/stl_iterator.h:288:5: note: candidate template
      ignored: failed template argument deduction
    operator<(const reverse_iterator<_Iterator>& __x,
    ^
/usr/include/c++/4.2.1/bits/stl_iterator.h:338:5: note: candidate template
      ignored: failed template argument deduction
    operator<(const reverse_iterator<_IteratorL>& __x,
    ^
/usr/include/c++/4.2.1/bits/stl_vector.h:959:5: note: candidate template
      ignored: failed template argument deduction
    operator<(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
    ^
/usr/include/c++/4.2.1/bits/basic_string.h:2225:5: note: candidate template
      ignored: failed template argument deduction
    operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/usr/include/c++/4.2.1/bits/basic_string.h:2237:5: note: candidate template
      ignored: failed template argument deduction
    operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/usr/include/c++/4.2.1/bits/basic_string.h:2249:5: note: candidate template
      ignored: failed template argument deduction
    operator<(const _CharT* __lhs,

Needless to say, I'm a little confused as to why it's not working, and would appreciate any explanation as to why.

A few things I should point out:

1) I have a header containing all the headers I need for the assignment. Especially included are <algorithm> and <sys/dir.h>. I've ruled out the headers in my issues, but may be wrong.

2) I've tried a few variations:

Code:

fileData::operator<(fileData f) { return (strcmp(directName, f.directName) < 0);}

inline operator<(fileData f) { return (strcmp(directName, f.directName) < 0);}

bool inline operator<(fileData f) { return (strcmp(directName, f.directName) < 0);}

bool inline operator<(fileData& f) { return (strcmp(directName, f.directName) < 0);}

bool inline operator<(const fileData& f) { return (strcmp(directName, f.directName) < 0);}

bool inline operator<(const fileData& f) const { return (strcmp(directName, f.directName) < 0);}

The first two need a function type, the middle three produce the same error, and the last one somehow manages to return "expected unqualified-id" error for when I'm initializing data objects in the constructor.

**oogabooga** · 09-26-2013

The last one of your variations is essentially correct, but you're missing the fileData:: to attach it to the class!

Code:

bool inline fileData::operator<(const fileData& f) const
{
    return strcmp(directName, f.directName) < 0;
}

**William Putnam** · 09-26-2013

Originally Posted by oogabooga

The last one of your variations is essentially correct, but you're missing the fileData:: to attach it to the class!

Code:

bool inline fileData::operator<(const fileData& f) const
{
    return strcmp(directName, f.directName) < 0;
}

I gave it a shot, but it still gives one error, although shorter:

Code:

./fileData.hpp:20:27: error: extra qualification on member 'operator<'
    bool inline fileData::operator<(const fileData& f) const { return...
                ~~~~~~~~~~^

**oogabooga** · 09-26-2013

I thought you were defining it outside of the class. But if it's in the class then get rid of both "inline" and "fileData::". Members defined in the class are automatically inline and connected to that class.

**William Putnam** · 09-26-2013

No, I'm defining this as an inline in the header.

I tried adjusting it again, and as I feared, I started getting unrelated error messages from the constructor.

**oogabooga** · 09-26-2013

Here's a little example that works.
If you still have problems, you may have to post more code.

Code:

// g++ -std=c++11 -Wall lessthan.cpp

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>

class A {
    static const size_t MAXSTR = 100;
    char cstr[MAXSTR];
public:
    A(const char *cstr) {
        std::strncpy(this->cstr, cstr, MAXSTR);
        this->cstr[MAXSTR-1] = '\0';
    }
     bool operator<(const A &a) const {
        return std::strcmp(cstr, a.cstr) < 0;
    }
    friend std::ostream& operator<<(std::ostream& os, const A& a);
};

std::ostream& operator<<(std::ostream& os, const A& a) {
    return os << a.cstr;
}

int main() {
    std::vector<A> v = {"def", "abc", "jkl", "pqr", "mno", "ghi"};
    std::sort(v.begin(), v.end());
    for (auto &x: v)
        std::cout << x << '\n';
    return 0;
}

**William Putnam** · 09-26-2013

oogabooga, your code did not compile on my computer, either for c++ or g++. g++ gave me these errors in the terminal:

Code:

d.cpp: In function ‘int main()’:
d.cpp:25: error: scalar object ‘v’ requires one element in initializer
d.cpp:27: error: a function-definition is not allowed here before ‘:’ token
d.cpp:29: error: expected primary-expression before ‘return’
d.cpp:29: error: expected `;' before ‘return’
d.cpp:29: error: expected primary-expression before ‘return’
d.cpp:29: error: expected `)' before ‘return’

**oogabooga** · 09-26-2013

It's C++11, so with g++:
g++ -std=c++11 -Wall lessthan.cpp

**William Putnam** · 09-26-2013

It wouldn't take the -std argument. "Unrecognized."

**oogabooga** · 09-26-2013

Here's an old-style C++ version.

Code:

// g++ -Wall lessthan.cpp

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>

class A {
    static const size_t MAXSTR = 100;
    char cstr[MAXSTR];
public:
    A(const char *cstr) {
        std::strncpy(this->cstr, cstr, MAXSTR);
        this->cstr[MAXSTR-1] = '\0';
    }
    bool operator<(const A &a) const {
        return std::strcmp(cstr, a.cstr) < 0;
    }
    friend std::ostream& operator<<(std::ostream& os, const A& a);
};

std::ostream& operator<<(std::ostream& os, const A& a) {
    return os << a.cstr;
}

int main() {
    std::vector<A> v;
    v.push_back("def");
    v.push_back("abc");
    v.push_back("pqr");
    v.push_back("mno");
    std::sort(v.begin(), v.end());
    for (size_t i = 0; i < v.size(); i++)
        std::cout << v[i] << '\n';
    return 0;
}

**laserlight** · 09-26-2013

Then try: -std=c++0x as I believe c++11 is only recognised in g++ 4.7+

**oogabooga** · 09-26-2013

Originally Posted by laserlight

Then try: -std=c++0x as I believe c++11 is only recognised in g++ 4.7+

c++0x sounds like they were hoping it would come out before 2010.

What is the pre-c++11 version? I don't know what to call it.

**William Putnam** · 09-26-2013

Nope. Not working.

(At this point I should mention I'm running OSX ML 10.8.5 and Xcode v.5.0 with command line tools extension.)

**laserlight** · 09-26-2013

Originally Posted by oogabooga

What is the pre-c++11 version? I don't know what to call it.

C++03, though because it was mainly a bunch of minor corrections to C++98, you would still use -std=c++98 with g++.

**oogabooga** · 09-26-2013

Originally Posted by laserlight

C++03, though because it was mainly a bunch of minor corrections to C++98, you would still use -std=c++98 with g++.

Okay, thanks. C++98 sounds familiar.

BTW, you're going to have to help this guy. It's kind of the blind leading the blind at this point.

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