c ++中奇怪的分段错误

Question

i have the following method :

string Company::cheap(list<Candidate*>& candidates) {
    candidates.sort(candidateSalaryCompare);
    for (std::list<Candidate*>::iterator iter = candidates.begin(); iter
    != candidates.end(); ++iter) {
    }
    int m(candidates.front()->getExpectedSalary());
    list<Candidate*> potentialList;
    for (std::list<Candidate*>::iterator iter = candidates.begin(); (*iter)->getExpectedSalary()
    == m && iter != candidates.end(); ++iter)
        potentialList.push_back(*iter);
    if (potentialList.size() > 0)
        potentialList.sort(candidateIdCompare);
    return potentialList.front()->getId();
}

running it as is and my program works, but if i remove the empty FOR loop in the beginning (which doesn't do anything) i get a segmentation fault. any clues ?

EDIT

candidate class, and actually i'm not sure at what line i'm getting the segfault, i'm using eclipse and the debugger doesn't seem to work

#include "../include/Candidate.h"
#include <iostream>
#include "../include/AppLogger.h"
#include <sstream>

Candidate::Candidate(string id, list<Skill> skills, list<
        string> desiredJobs, double expectedSalary) :
        id_(id), dateJoined_(), skills_(skills),
        desiredJobs_(desiredJobs), expectedSalary_(expectedSalary),
        originalSalary_(expectedSalary), gotJob_(0) {
}


void Candidate::compromise(const DateTime& currentDate) {
    double salaryAfter30(0.9*this->originalSalary_);
    double salaryAfter60(0.8*this->originalSalary_);
    double salaryAfter90(0.7*this->originalSalary_);
    Timespan duration = currentDate - this->dateJoined_;
    if (duration.days() == 30 || duration.days() == 60 || duration.days() == 90) {
        if (duration.days() == 30 && (this->expectedSalary_
                == this->originalSalary_)) {
            this->expectedSalary_ = salaryAfter30;
            std::stringstream sstm;
            sstm << "Candidate "<< this->getId() <<" is willing to compromise, and his expected salary is " <<this->expectedSalary_ << ".";
            CAppLogger::Instance().Log(sstm.str(),
                    Poco::Message::PRIO_WARNING);
            return;
        }
        else if (duration.days()==30)
            poco_bugcheck_msg("Error, 30 days passed, worker already compromised");
        if (duration.days() == 60 && (this->expectedSalary_ == salaryAfter30)) {
            this->expectedSalary_ = salaryAfter60;
            std::stringstream sstm;

            sstm << "Candidate "<< this->getId() <<" is willing to compromise, and his expected salary is " <<this->expectedSalary_ << ".";
            CAppLogger::Instance().Log(sstm.str(),
                    Poco::Message::PRIO_WARNING);
            return;
        }
        else if (duration.days()==60)
            poco_bugcheck_msg("Error, 60 days passed, worker already compromised");

        if ((duration.days() == 90) && (this->expectedSalary_ == salaryAfter60)) {
            this->expectedSalary_ = salaryAfter90;
            std::stringstream sstm;
            sstm << "Candidate "<< this->getId() <<" is willing to compromise, and his expected salary is " <<this->expectedSalary_ << ".";
            CAppLogger::Instance().Log(sstm.str(),
                    Poco::Message::PRIO_WARNING);
            return;
        }
        else if (duration.days()==90)
            poco_bugcheck_msg("Error, 90 days passed, worker already compromised");

    }
    else poco_bugcheck_msg("Error, worker told to compromise when not needed");


}

list<Skill> Candidate::getSkills() const {
    return this->skills_;
}

list<string> Candidate::getDesiredJobs() const {
    return this->desiredJobs_;

}
double Candidate::getExpectedSalary() const {
    return this->expectedSalary_;
}
DateTime Candidate::getDateJoined() const {
    return this->dateJoined_;
}
DateTime Candidate::getDateLeft() const {
    return this->dateLeft_;
}
void Candidate::setDateLeft(const DateTime& date) {
    this->dateLeft_ = date;
}
string Candidate::getId() const {
    return this->id_;
}

void Candidate::setDateJoined(const DateTime& date) {
    this->dateJoined_=date;
    this->setGotJob();
}

void Candidate::setGotJob() {
    if (this->gotJob_==1)
        std::cerr<<"error, setting gotJob while already has job"<<std::endl;
    this->gotJob_=1;
}
bool Candidate::gotJob() const {
    return this->gotJob_;
}
void Candidate::setQl(double ql){
        jobQl_=ql;
}

int Candidate::getQl() const{
    return this->jobQl_;
}

after applying the supplied solutions i get the following error :

assignment2(48823) malloc: *** mmap(size=140734799806464) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc

EDIT

changed int m to double m and now it seems to work because get expected salary returns a double but why did it cause that error ?

Answer 1

Please, look at Cris Hopman answer here , as his answer is much more complete and detailed than mine pointing to other issues that can/will cause undefined behavior (segmentation fault) in the original code.

This condition is wrong:

for (std::list<Candidate*>::iterator iter = candidates.begin(); (*iter)->getExpectedSalary()
== m && iter != candidates.end(); ++iter)

You should reorder so that the test for end() comes before the dereference and keep the short circuit evaluation. As it is, you are dereferencing a pointer past the end of the container:

for (std::list<Candidate*>::iterator iter = candidates.begin(); 
     iter != candidates.end() && (*iter)->getExpectedSalary() == m; ++iter)

Answer 2

Let me break this up a little...

...
    int m(candidates.front()->getExpectedSalary());

You assume that candidates is nonempty, this is fine if it is documented but I would add

assert(!candidates.empty())

at the beginning of the function. This both enforces the constraint and makes the constraint clear to anyone reading the code.

Second, getExpectedSalary() returns a double that you are converting to an int... more on this later.

    list<Candidate*> potentialList;
    for (std::list<Candidate*>::iterator iter = candidates.begin(); 
        (*iter)->getExpectedSalary() == m && iter != candidates.end(); ++iter)
       potentialList.push_back(*iter);

Two problems here: iter != candidates.end() should be the first part of the conditional, otherwise you may dereference an invalid iterator in the first part. Second, the equality check is bad. It may fail for even the first item in the list because if d is a double then it is not necessarily the case that (int)d == d . The point is that m should be a double. With that change, the test may still fail unexpectedly since it is a floating point comparison, see here .

    if (potentialList.size() > 0)
        potentialList.sort(candidateIdCompare);
    return potentialList.front()->getId();

This is inconsistent. In the if you assume that potentialList might be empty, but in the return you assume that it can't be empty. In fact, potentialList should not be empty (as it should at the very least contain candidates.front()) and so this should be changed to:

    assert(!potentialList.empty());
    potentialList.sort(candidateIdCompare);
    return potentialList.front()->getId();

If you make only that change, you will find that the assert will fail. You will then realize that potentialList is empty because of the conversion from double to int discussed above.

Finally, it appears that you are storing monetary values as floating point. This can cause problems, be careful. It is often much safer to use only fixed-point for such, but is not necessary.

As a last note, this function itself is overly complex. The below should work (no guarantees)...

string Company::cheap(list<Candidate*>& candidates) {
    typedef list<Candidate*> list_t;
    assert(!candidates.empty());

    candidates.sort(candidateSalaryCompare);
    pair<list_t, list_t> rng = equal_range(candidates.begin(), candidates.end(),
        candidates.front(), candidateSalaryCompare);

    assert(rng.first != rng.second);
    return (*min_element(rng.first, rng.second, candidateIdCompare))->getId();
}

This also conveniently offloads all the salary comparison to one other function and so it only has to be done correctly once.

Answer 3

Your second for loop, specifically the condition, could be dangerous:

(*iter)->getExpectedSalary() == m && iter != candidates.end()

This is because it deferences iter to compare against m before actually checking if the iterator is valid. You need to invert the order of the checks so the condition becomes:

iter != candidates.end() && (*iter)->getExpectedSalary() == m