Hi there,
I am facing a problem with the uClibc++ implementation of std::map,
which can be illustrated with the following code:
<-----------SNIP----------->
#include <cstdio>
#include <map>
class my_type
{
public:
my_type() : id(count++) { printf("my_type() %d\n", id); }
void test() { printf("test() %d\n", id); }
int id;
static int count;
};
int my_type::count = 0;
int main()
{
std::map<int, my_type> mymap;
mymap[1].test();
mymap[1].test();
}
<-----------SNIP----------->
I expect the output of the above code to be:
my_type() 0
test() 0
test() 0
i.e. a my_type object is constructed during the first call to
map::operator[]. The second call should just return a reference to the
object but not construct a new one. The above output is what I get
with libstdc++ and visual studio 2005. With uClibc++ however, I get:
my_type() 0
test() 0
my_type() 1
test() 0
i.e. a second my_type object is constructed during the second call to
operator[], but the correct instance (the one constructed during the
first call) is returned. I've had a look to uClibc++'s map::operator[]
and it looks like this:
<-----------SNIP----------->
reference operator[](const key_type& k){
//This is from the spec and is quite ugly.
return (*((insert(make_pair(k, T()))).first)).second;
}
<-----------SNIP----------->
The problem is, 'T()' will construct a new object regardless of
whether the key 'k' exists in the map or not.
In most situations, this behaviour does not pose any problem. But in
some cases, it can have side effects (as in the example above).
I've seen that this implementation of operator[] was introduced in
commit 9655, but the old code was working as expected, so IMO it
should be reverted.
In addition, is this code really in ISO/IEC 14882:1998 as the comment
suggests ? It is referenced at
http://www.sgi.com/tech/stl/Map.html
and
http://www.cplusplus.com/reference/stl/map/operator%5B%5D.html
but is this conform to what ISO says ?
Thanks,
Olivier Hochreutiner
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