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6436066
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2011-06-22T06:56:45.277
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n.m. has already given a fine answer, but I found this question interesting so I decided to try to understand it a little better. It turns out then when you call <code>erase()</code> on the first item of an iterator (which we will call <code>item0</code>), here's what the iterator does: it uses the <code>=</code> operator of your class to do <code>item0 = item1</code>. Then it deletes <code>item1</code>. If you don't define your own <code>=</code> operator, I think it will simply copy the memory of your object over from <code>item1</code> to <code>item0</code>, so <code>item0</code> and <code>item1</code> will temporarily be pointing to the same string. Then when <code>item1</code> gets deleted, the string gets freed, leaving <code>item0</code> in an invalid state because it has a pointer to memory that has been freed. Here is some simple test code that reproduces and illuminates the problem: <pre><code>#include <cstring> #include <vector> #include <stdio.h> using namespace std; class Cube { public: char * str; Cube(const Cube &c) { set(c.str); } Cube(const char * s) { set(s); } ~Cube() { clear(); } // is "delete []" necessary? not sure #if 1 // change to 0 to cause a bug void operator=(const Cube &c) { clear(); // necessary to avoid memory leaks printf("operator=\n"); set(c.str); } #endif private: void set(const char * s) { str = new char[strlen(s) + 1]; printf("allocated %p for %s\n", str, s); strcpy(str, s); } void clear() { if (str) { printf("freeing %p: %s\n", str, str); delete str; } } }; int main(int argc, char ** argv) { printf("== CREATING VECTOR ==\n"); vector <Cube> vec; vec.push_back(Cube("octopus")); vec.push_back(Cube("squid")); printf("== BEGINNING ITERATION ==\n"); vector<Cube>::iterator it = vec.begin(); printf("First entry is %p %s\n", it->str, it->str); it = vec.erase(it); printf("Second entry is %p %s\n", it->str, it->str); // this prints garbage if Cube has no = operator return 0; } </code></pre> This code produces the following output: <pre><code>== CREATING VECTOR == allocated 00350F98 for octopus allocated 00350FB8 for octopus freeing 00350F98: octopus allocated 00350F98 for squid allocated 00350FD8 for squid allocated 00350FE8 for octopus freeing 00350FB8: octopus freeing 00350F98: squid == BEGINNING ITERATION == First entry is 00350FE8 octopus freeing 00350FE8: octopus operator= allocated 00350F98 for squid freeing 00350FD8: squid Second entry is 00350F98 squid freeing 00350F98: squid </code></pre> I compiled and ran this in Windows with MinGW. The command I used was <code>g++ -Wl,--enable-auto-import test.cpp && a.exe</code>.
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1. POC++ error: double free or corruption (fasttop)
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1. USDavid Grayson
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1. COthanks for your nice explanation !
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