Re: check for deleted map entry -> crash ?

From:

Rolf Magnus <ramagnus@t-online.de>

Newsgroups:

comp.lang.c++

Date:

Thu, 27 Apr 2006 10:58:39 +0200

Message-ID:

<e2q13v$if5$03$1@news.t-online.com>

bolnvhuis@wanadoo.nl wrote:

I'm using an STL map in my code.
My application sometimes tries to delete things twice from the map.
This leads to a crash in my current code. The problem is probably the
way I check whether it is necessary to delete something (line 44 and 52
in below code).

Yes.

In the below code the problem is demonstrated, line 65 will cause a
segmentation fault.

     1 #include <iostream>
     2 #include <cstdlib>
     3 #include <map>
     4
     5 using namespace std;
     6
     7 class MyClass
     8 {
     9 public:
    10 MyClass(int);
    11 ~MyClass();
    12 void show();
    13 private:
    14 int nr_;
    15 };
    16
    17 MyClass::MyClass(int nr) : nr_(nr)
    18 {
    19 cout << "MyClass: " << nr_ << " constructed" << endl;
    20 }
    21
    22 MyClass::~MyClass()
    23 {
    24 cout << "MyClass: " << nr_ << " destructed" << endl;
    25 }
    26
    27 void MyClass::show()
    28 {
    29 cout << "Show this MyClass: " << nr_ << endl;
    30 }
    31
    32 typedef map<int, MyClass *> RtdmCyclicTimerMap;
    33
    34 int main(void)
    35 {
    36 RtdmCyclicTimerMap cyclicRtdmTimer_;
    37 map<int, MyClass *>::const_iterator cyclicRtdmIt;
    38
    39 // create 1 & 2
    40 cyclicRtdmTimer_[1] = new MyClass(11);
    41 cyclicRtdmTimer_[2] = new MyClass(22);
    42
    43 // delete 2
    44 if (cyclicRtdmTimer_[2]) {
    45 delete cyclicRtdmTimer_[2];
    46 cyclicRtdmTimer_.erase(2);
    47 } else {
    48 cout << "delete 2 failed" << endl;
    49 }
    50
    51 // delete 2 AGAIN
    52 if (cyclicRtdmTimer_[2]) {
    53 delete cyclicRtdmTimer_[2];
    54 cyclicRtdmTimer_.erase(2);
    55 } else {
    56 cout << "delete 2[2] failed" << endl;
    57 }
    58
    59 // show map -> NOW IT WILL CRASH
    60 cout << endl << "going to show map entries" << endl;
    61 for

(cyclicRtdmIt=cyclicRtdmTimer_.begin();cyclicRtdmIt!=cyclicRtdmTimer_.end();

++cyclicRtdmIt) {
    62 int tKey = cyclicRtdmIt->first;
    63 MyClass *mcPtr = cyclicRtdmIt->second;
    64 cout << "map entry found: key=" << tKey << ", valP=" <<
mcPtr << ", val=";
    65 mcPtr->show();
    66 }
    67 }
    68

This is because line 52 somehow increases the internal map size from 1
to 2 entries.

If operator[] doesn't find the element you are searching for, it creates
that element. Otherwise, line 40 and 41 couldn't work the way they do.
Since you don't assign anything to it in line 52, the value is probably
indeterminate and cannot be used with delete.

Must it be done with an iterator ?

Yes. Use the find() member function instead of operator[]. This will never
create elements. If the element is found, you get an iterator to it,
otherwise you get end().