Question

I write this code:

On running, an error occurs in the line

cout<<it2->first;

Unhandled exception at 0x00411edd in test3.exe: 0xC0000005: Access violation reading location 0x00000004.”

I have Visual Studio Express 2008 and Boost 1_47_0;

This is my full code:

#include "stdafx.h"
#include <iostream>
#include <boost/unordered_map.hpp>

using namespace std;

typedef boost::unordered_map<int,int > MAP;
        MAP map2;
        boost::unordered_map<int,int>::iterator it2;

void gen_random(char *s  ,char *p,int*  r,const int len);
void inline insert2(int i_key,int i_value);
void print();
//-----------main------------------------------------
void main()
{   
    char* s_key=new char[8];
    char* s_value=new char[8];
    int i_value=0, size_random=8;
    for(int i=0;i<12;i++)
    {
        gen_random(s_key,s_value,&i_value,size_random);
         insert2(i_value,i_value);
    }

        print();
    int a;
    std::cin>>a;
}
//-------------end main--------------------------------

//--------my function ---------------------------
//-------random--------------
void gen_random(char* s,char* p,int *r, const int len) {


    static const char alphanum[] =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";
     (*r)=rand();

    for (int i = 0; i < len; ++i) {
        s[i] = alphanum[rand() % (sizeof(alphanum) - 1)];
         p[i] = alphanum[rand() % (sizeof(alphanum) - 1)];
    }

    s[len] = 0;
    p[len] = 0;
}
//-----end random------------

    void inline insert2(int i_key,int i_value)
    {
        map2.insert(MAP::value_type(i_key,i_value));        
    }
    void print()
    {

    for(it2=map2.begin();it2 != map2.end();++it2 );
     {


         cout<<it2->first; 

     }

    }
//--------end function---------------------------

Answer 1

The line:

for(it2=map2.begin();it2 != map2.end();++it2 );

Should not end in a semicolon. The loop runs, but does not go into the block of code following it. When the loop completes, it2 is equal to map2.end() which is why you get an access violation when you try to use it.

This problem could have been avoided if you used a smaller scoped variable. For example, this would get you a compiler error (assuming it2 was no longer a global):

for(boost::unordered_map<int,int>::iterator it2 = map2.begin(); it2 != map2.end(); ++it2);
{
    cout<<it2->first;
}