Re: app crash when there's more than 3000 entries in std::map ?
On Aug 12, 6:49 pm, Jonathan Lee <jonathan.lee....@gmail.com> wrote:
On Aug 12, 12:33 pm, mast4as <mast...@yahoo.com> wrote:
Hi everyone
I have this strange behaviour happening with this code which I can't
explain. On my computer when I set nt with a value greater than 3000
it crashes. Is there a max number of keys I can use with a std::map ?
It's because this doesn't do what you think it does:
string tmp = "test" + i;
It does *not* make a string named "test1", "test2", etc.
Instead, it does pointer arithmetic on the const char*
that points to "test".
So
("test" + 0) is a pointer to the string "test"
("test" + 1) is a pointer to the string "est"
("test" + 2) is a pointer to the string "st"
etc.
Once i passes the null byte in the string literal, though,
you're into some random chunk of memory. All bets are off.
Eventually you're reading outside of memory that belongs
to your application and you get a segfault.
--Jonathan
Thanks a lot yes ... I just figured (long day at work ;-(
Thank you so much
std::string createRandomString()
{
char alpha[ 6 ] = { 'a', 'b', 'c', 'd', 'e', 'f' };
char str[ 10 ];
for ( int i = 0 ; i < 10 ; i++ )
{
int a = int(drand48()*5);
str[ i ] = a;
}
return std::string( str );
}
int main()
{
float t = clock();
int nt = 10000;
#if 1
hash_map<std::string, int, hash<std::string> > mymap;
for ( int i = 0; i < nt; ++i )
{
string tmp = createRandomString();
mymap[ tmp ] = i;
}
for ( int i = 0; i < 10e5; ++i )
{
int a = (int)(drand48() * nt);
string tmp = createRandomString();
hash_map<std::string, int, hash<std::string> >::iterator it =
mymap.find( tmp );
if ( it != mymap.end() )
{
}
else
{
//printf("not found\n");
}
}
unsigned max_size = mymap.max_size();
printf("%d\n", max_size );
#else
std::map<std::string, int> mymap;
for ( int i = 0; i < nt; ++i )
{
string tmp = createRandomString();
mymap[ tmp ] = i;
}
for ( int i = 0; i < 10e5; ++i )
{
int a = (int)(drand48() * nt);
string tmp = createRandomString();
std::map<std::string, int>::iterator it = mymap.find( tmp );
if ( it != mymap.end() )
{
}
else
{
//printf("not found\n");
}
}
unsigned max_size = mymap.max_size();
printf("%d\n", max_size );
#endif
printf("time %f\n", (clock() - t ) / float( CLOCKS_PER_SEC ) );
return 0;
}
#endif