The following code produces the following output and ends up in kind of an endless loop with 100% cpu load.
#include <iostream>
#include <set>
class Foo{};
void delete_object_from_set(std::set<Foo *>& my_set, Foo* ob)
{
std::set< Foo *>::iterator setIt;
std::cout << "Try to delete object '" << ob << "'..." << std::endl;
for(setIt = my_set.begin(); setIt != my_set.end(); ++setIt)
{
Foo * tmp_ob = *setIt;
std::cout << "Check object '" << tmp_ob << "'..." << std::endl;
// compare the objects
//
if(ob == tmp_ob)
{
// if the objects are equal, delete this object from the set
//
std::cout << "Delete object '" << tmp_ob << " from set..." << std::endl;
setIt = my_set.erase(setIt);
std::cout << "Deleted object '" << tmp_ob << " from set..." << std::endl;
}
}
std::cout << "loop finished." << std::endl;
};
int main()
{
Foo* ob = new Foo();
std::set< Foo * > my_set;
my_set.insert(ob);
delete_object_from_set(my_set, ob);
std::cout << "finished" << std::endl;
return 0;
}
The output:
Try to delete object '0x563811ffce70'...
Check object '0x563811ffce70'...
Delete object '0x563811ffce70 from set...
Deleted object '0x563811ffce70 from set...
so it does not finish, having 100% cpu load.
I know how to do it correctly (see below), but I cannot understand what is going on here. It's not an endless loop, since then it should output something continuously, but it just keeps doing something. Any idea what?
How to do it the right way: Deleting elements from std::set while iterating and How to remove elements from an std::set while iterating over it
Hokay, so you asked how this could loop infinitely without continuously triggering the "Check object" print.
The quick answer (that you already got from others) is that calling operator++ on my_set.end() is UB, and thus able to do anything.
A deeper dive into GCC specifically (since #appleapple could reproduce on GCC, while my test in MSVC found no infinite loop) revealed some interesting stuff:
The operator++ call is implemented as a call to _M_node = _Rb_tree_increment(_M_node); and that one looks as follows:
static _Rb_tree_node_base*
local_Rb_tree_increment(_Rb_tree_node_base* __x) throw ()
{
if (__x->_M_right != 0)
{
__x = __x->_M_right;
while (__x->_M_left != 0)
__x = __x->_M_left;
}
else
{
_Rb_tree_node_base* __y = __x->_M_parent;
while (__x == __y->_M_right)
{
__x = __y;
__y = __y->_M_parent;
}
if (__x->_M_right != __y)
__x = __y;
}
return __x;
}
So, it defaults to finding the "next" node by taking the first right, and then running all the way to the left. But! a look in the debugger at the my_set.end() node reveals the following:
(gdb) s
366 _M_node = _Rb_tree_increment(_M_node);
(gdb) p _M_node
$1 = (std::_Rb_tree_const_iterator<Foo*>::_Base_ptr) 0x7fffffffe2b8
(gdb) p _M_node->_M_right
$2 = (std::_Rb_tree_node_base::_Base_ptr) 0x7fffffffe2b8
(gdb) p _M_node->_M_left
$3 = (std::_Rb_tree_node_base::_Base_ptr) 0x7fffffffe2b8
Both the left and right of the end() node apparently points at itself. Why? Ask the implementer, but probably because it makes something else easier or more optimizable. But it does mean that in your case the UB you run into is an infinite loop on essentially:
__x->_M_left = __x;
while (__x->_M_left != 0)
__x = __x->_M_left; // __x = __x;
Again, this is the case for GCC, on MSVC it did not loop (debug threw an exception, release just ignored it; finished the loop and printed "loop finished." and "finished" as if nothing strange had happened). But that is the "fun" part about UB - anything could happen...
The code is equivalent of:
{
setIt = my_set.begin();
while(setIt != my_set.end())
{
Foo * tmp_ob = *setIt;
std::cout << "Check object '" << tmp_ob << "'..." << std::endl;
if(ob == tmp_ob)
{
std::cout << "Delete object '" << tmp_ob << " from set..." << std::endl;
setIt = my_set.erase(setIt);
std::cout << "Deleted object '" << tmp_ob << " from set..." << std::endl;
}
++setIt;
}
}
The call to my_set.erase(setIt); may return end() if last element of container was deleted. Consequently increments happens, which is UB. Would it trigger exception or not depends on implementation, but at any following point setIt never will be equal to my_set.end(), thus an infinite loop is possible.
for(setIt = my_set.begin(); setIt != my_set.end();)
{
Foo * tmp_ob = *setIt;
std::cout << "Check object '" << tmp_ob << "'..." << std::endl;
// compare the objects
//
if(ob == tmp_ob)
{
// if the objects are equal, delete this object from the set
//
std::cout << "Delete object '" << tmp_ob << " from set..." << std::endl;
setIt = my_set.erase(setIt);
std::cout << "Deleted object '" << tmp_ob << " from set..." << std::endl;
}
else
setIt++;
}
Related
I'm new in c++ so my problem could be very simple but I cannot solve it.
In my constructor I want to start detached thread which will be looping with class variable and removing old data.
userCache.hpp
struct UserCacheItem {
long m_expires;
oatpp::Object<User> m_user;
UserCacheItem(const long expires, const oatpp::Object<User> &user);
};
class UserCache {
private:
std::map<std::string, std::shared_ptr<UserCacheItem> > m_userCacheItem;
public:
UserCache();
void cacheUser(std::string payload, std::shared_ptr<UserCacheItem> &userCacheItem);
};
userCache.cpp
UserCache::UserCache()
{
std::thread thread([this]() mutable {
while (true){
auto curTimestamp = std::chrono::seconds(std::chrono::seconds(std::time(nullptr))).count();
for(auto &elem : m_userCacheItem){
if (curTimestamp > elem.second->m_expires){
std::cout << "Erasing element: " << elem.second->m_expires << std::endl;
m_userCacheItem.clear();
}
}
std::cout << "Cache size: " << m_userCacheItem.size() << " Current timestamp: " << curTimestamp << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(10));
};
});
thread.detach();
}
When I reach line m_userCacheItem.clear(); I get segmentation fault. Ofcourse if if block is false line with cout cache sizie is printed properly.
So I can read my variable but I cannot modify it :(
Where I'm making error?
You cannot modify the map while you're iterating it
for(auto &elem : m_userCacheItem){
if (curTimestamp > elem.second->m_expires){
std::cout << "Erasing element: " << elem.second->m_expires << std::endl;
m_userCacheItem.clear();
}
}
If you want to erase an element, use std::map::erase:
for(auto & it = m_userCacheItem.begin(); it != m_userCacheItem.end();) {
if(condition) {
it = m_userCacheItem.erase(it);
} else {
++it;
}
}
In question about scope of exception it is stated by Aj. that throw and catch clauses will create copies of the exception (unless reference is used I guess)
I tried myself a small toy code and I don't understand the result. here :
//g++ 7.4.0
#include <iostream>
using namespace std;
struct Some_error {
Some_error(float code):err_code(code){ cout << "Some_error(" << err_code << ")\n"; }
~Some_error() { cout << "~Some_error(" << err_code << ")\n"; }
Some_error(const Some_error& o):err_code(o.err_code+0.1) { cout << "Some_error(copy::" << err_code << ")\n"; }
Some_error(Some_error&& o):err_code(std::move(o.err_code)+.01){ cout << "Some_error(move::" << err_code << ")\n"; }
int get_code() const { return err_code; }
private : float err_code;
};
int do_task() {
if ( false ) return 42; else throw Some_error {1};
cout << "end do_task\n" ;
}
void taskmaster(){
try { auto result = do_task(); cout << "the answer is " << result << "\n" ; }
catch (Some_error e) { cout << "catch Some_error : " << e.get_code() << "\n" ; }
cout << "end taskmaster\n" ;
}
int main() { taskmaster(); }
the trace I get is as follows :
Some_error(1)
Some_error(copy::1.1)
catch Some_error : 1
~Some_error(1.1)
~Some_error(1)
end taskmaster
Now first, as I used no reference here, according to Aj., I would expect 2 copies to happen.
And second, there was a copy, that set err_code to 1.1, but the display is still 1.
Remark: just to be complete, I changed the catch to : catch(Some_error& e),
and then the trace looks fine to me :
Some_error(1)
catch Some_error : 1
~Some_error(1)
end taskmaster
I would expect 2 copies to happen.
Why? Only one copy is made by the catch block. Where would the second copy happen?
set err_code to 1.1, but the display is still 1.
Because get_code returns an int, so the floating point value gets truncated.
Hello I'm trying to rewrite my own memory manager and STL (nothing fancy, just some basic vector and string features) and I'm getting a strange behaviour. I'm trying to get experience in the memory management field because I'm a high school student with time to spare. The problem is, when I create my first variable everything goes perfectly but after creating the second variable, the program crashes while creating the first variable.
String.h/.cpp
class String {
char* pointer_toBuffer = nullptr;
size_t buffer_length = 0;
IAllocator* Allocator;
public:
String(const char* text, IAllocator* Allocator);}
String::String(const char* text, TuranAPI::MemoryManagement::IAllocator* MemoryAllocator) : Allocator(MemoryAllocator) {
std::cout << "String creation has started: " << text << std::endl;
unsigned int i = 0;
while (text[i] != 0) {
i++;
}
buffer_length = i + 1;
pointer_toBuffer = (char*)Allocator->Allocate_MemoryBlock(buffer_length * sizeof(char));//When I write the Second String part, FirstString crashes directly. I use VSDebug and it says access violation here while creating FirstString. It is successful if I delete the SecondString part.
for (unsigned int letterindex = 0; letterindex < i; letterindex++) {
pointer_toBuffer[letterindex] = text[letterindex];
}
pointer_toBuffer[i] = 0;
}
MemoryManagement.h/cpp
TAPIMemoryAllocator::TAPIMemoryAllocator(MemoryBlockInfo MemoryPool_toUse){
std::cout << "TAPIMemoryAllocator is created!\n";
std::cout << "MemoryPool's start pointer: " << MemoryPool_toUse.address << std::endl;
MemoryPool.address = MemoryPool_toUse.address;
MemoryPool.size = MemoryPool_toUse.size;
SELF = this;
}
void* TAPIMemoryAllocator::Allocate_MemoryBlock(size_t size) {
std::cout << "MemoryPool's start pointer: " << MemoryPool.address << std::endl;
std::cout << "A buffer of " << size << " bytes allocation request found in TAPIMemoryAllocator!\n";
if (SELF == nullptr) {
TMemoryManager First(1024 * 1024 * 1024 * 1);
MemoryBlockInfo FirstMemoryBlock;
FirstMemoryBlock.address = SELF->MemoryPool.address;
FirstMemoryBlock.size = size;
Allocated_MemoryBlocks[0] = FirstMemoryBlock;
return (char*)SELF->MemoryPool.address;
}
void* finaladdress = SELF->MemoryPool.address;
for (unsigned int blockindex = 0; blockindex < MAX_MEMORYBLOCKNUMBER; blockindex++) {
MemoryBlockInfo& MemoryBlock = Allocated_MemoryBlocks[blockindex];
finaladdress = (char*)finaladdress + MemoryBlock.size;
if (size <= MemoryBlock.size && MemoryBlock.address == nullptr) {
std::cout << "Intended block's size is less than found memory block!\n";
MemoryBlock.address = finaladdress;
//You shouldn't change Memory Block's size because all of the allocations before this are based upon the previous size!
//You should move all the previous allocated memory to set the size (which is not ideal!)
//If I'd want to find memory leaks causing this, I could write code here to log the leaks!
return MemoryBlock.address;
}
else if (MemoryBlock.size == 0 && MemoryBlock.address == nullptr) {
std::cout << "An empty block is created for intended block! Block's Array index is: " << blockindex << "\n";
std::cout << "MemoryPool's start pointer: " << MemoryPool.address << std::endl << "MemoryBlock's pointer: " << finaladdress << std::endl;
//This means this index in the Allocated_MemoryBlocks has never been used, so we can add the data here!
MemoryBlock.address = finaladdress;
MemoryBlock.size = size;
return MemoryBlock.address;
}
}
//If you arrive here, that means there is no empty memory block in the Allocated_MemoryBlocks array!
std::cout << "There is no empty memory block in the Allocated_MemoryBlocks array, so nullptr is returned!\n";
return nullptr;
}
TMemoryManager::TMemoryManager(size_t Main_MemoryBlockSize) {
if (SELF != nullptr) {
std::cout << "You shouldn't create a MemoryManager!";
return;
}
std::cout << "TMemoryManager is created!\n";
MainMemoryBlock.address = malloc(Main_MemoryBlockSize);
MainMemoryBlock.size = Main_MemoryBlockSize;
SELF = this;
std::cout << "Main Memory Block's start pointer: " << MainMemoryBlock.address << std::endl;
MemoryBlockInfo TuranAPI_MemoryPool;
TuranAPI_MemoryPool.address = MainMemoryBlock.address;
std::cout << "TuranAPI_MemoryPool.address: " << TuranAPI_MemoryPool.address << std::endl;
TuranAPI_MemoryPool.size = 1024 * 1024 * 10;
TAPIMemoryAllocator Create(TuranAPI_MemoryPool);
}
TMemoryManager* TMemoryManager::SELF = nullptr;
TMemoryManager First(1024 * 1024 * 1024 * 1);
Main.cpp
String FirstString("How are you?", TAPIMemoryAllocator::SELF);
std::cout << FirstString << std::endl; //If I delete the below, it prints "How are you?" as expected
String SecondString("I'm fine, thanks!", TAPIMemoryAllocator::SELF);
std::cout << SecondString << std::endl;
Solved: The problem was in Allocator. When allocator goes out of scope, it's Allocate_MemoryBlock function (it's a virtual function, not static) is deleted. I don't know why it doesn't occur when only one String is created (maybe a compiler optimization) but storing Allocator's itself (All of variables was static already) and assinging SELF as stored one's pointer solved the problem.
My question is that of safety. I've searched cplusplus.com and cppreference.com and they seem to be lacking on iterator safety during std::move. Specifically: is it safe to call std::unordered_map::erase(iterator) with an iterator whose object has been moved? Sample code:
#include <unordered_map>
#include <string>
#include <vector>
#include <iostream>
#include <memory>
class A {
public:
A() : name("default ctored"), value(-1) {}
A(const std::string& name, int value) : name(name), value(value) { }
std::string name;
int value;
};
typedef std::shared_ptr<const A> ConstAPtr;
int main(int argc, char **argv) {
// containers keyed by shared_ptr are keyed by the raw pointer address
std::unordered_map<ConstAPtr, int> valued_objects;
for ( int i = 0; i < 10; ++i ) {
// creates 5 objects named "name 0", and 5 named "name 1"
std::string name("name ");
name += std::to_string(i % 2);
valued_objects[std::make_shared<A>(std::move(name), i)] = i * 5;
}
// Later somewhere else we need to transform the map to be keyed differently
// while retaining the values for each object
typedef std::pair<ConstAPtr, int> ObjValue;
std::unordered_map<std::string, std::vector<ObjValue> > named_objects;
std::cout << "moving..." << std::endl;
// No increment since we're using .erase() and don't want to skip objects.
for ( auto it = valued_objects.begin(); it != valued_objects.end(); ) {
std::cout << it->first->name << "\t" << it->first.value << "\t" << it->second << std::endl;
// Get named_vec.
std::vector<ObjValue>& v = named_objects[it->first->name];
// move object :: IS THIS SAFE??
v.push_back(std::move(*it));
// And then... is this also safe???
it = valued_objects.erase(it);
}
std::cout << "checking... " << named_objects.size() << std::endl;
for ( auto it = named_objects.begin(); it != named_objects.end(); ++it ) {
std::cout << it->first << " (" << it->second.size() << ")" << std::endl;
for ( auto pair : it->second ) {
std::cout << "\t" << pair.first->name << "\t" << pair.first->value << "\t" << pair.second << std::endl;
}
}
std::cout << "double check... " << valued_objects.size() << std::endl;
for ( auto it : valued_objects ) {
std::cout << it.first->name << " (" << it.second << ")" << std::endl;
}
return 0;
}
The reason I ask is that it strikes me that moving the pair from the unordered_map's iterator may (?) therefore *re*move the iterator's stored key value and therefore invalidate its hash; therefore any operations on it afterward could result in undefined behavior. Unless that's not so?
I do think it's worth noting that the above appears to successfully work as intended in GCC 4.8.2 so I'm looking to see if I missed documentation supporting or explicitly not supporting the behavior.
// move object :: IS THIS SAFE??
v.push_back(std::move(*it));
Yes, it is safe, because this doesn't actually modify the key. It cannot, because the key is const. The type of *it is std::pair<const ConstAPtr, int>. When it is moved, the first member (the const ConstAPtr) is not actually moved. It is converted to an r-value by std::move, and becomes const ConstAPtr&&. But that doesn't match the move constructor, which expects a non-const ConstAPtr&&. So the copy constructor is called instead.
I'm trying to write a copy constructor for an object managing a STL map containing pointers, where the key is a string. However, when I attempt to insert new values in the map, the pointers are set to NULL:
// ...
for(std::map<std::string, data_base*, order>::const_iterator it = other.elements.begin();
it != other.elements.end(); ++it){
data_base *t = it->second->clone();
std::cout << "CLONE: " << std::hex << t << std::endl;
elements[it->first] = t;
std::cout << "INSERTED: " << std::hex << elements[it->first] << std::endl;
}
// ...
other is the object being copied and elements the map. The clone() method returns a pointer to a new object (via new).
Running the code above I get something like:
CLONE: 0xcfbbc0
INSERTED: 0
I'm not a very experienced programmer and this issue is probably simple to fix, but I didnt find any solution to it searching around.
Thanks a lot for your time.
I don't see any problem with this code, other than maybe
std::map<std::string, data_base*, order>::const_iterator it
Here order gives the key comparator to use to sort the pairs contained in the map (often implemented as a tree).
Maybe you're doing something wrong in it, making your [] operator don't find the right ke, making your last line logging a new pair with a null ptr.
First, try without that order, using the default key-comparator (std::less), then if it don't work, post your order definition and the map declaration. If it's not enough, just provide a simple complete program that reproduce the problem.
I just wrote a simple similar test, using the default key-comparator :
#include <map>
#include <string>
#include <iostream>
struct Data
{
int k;
Data* clone() { return new Data(); }
};
typedef std::map< std::string, Data* > DataMap;
DataMap data_map;
int main()
{
data_map[ "hello" ] = new Data();
data_map[ "world" ] = new Data();
DataMap other_map;
for( DataMap::const_iterator it = data_map.begin(); it != data_map.end(); ++it)
{
Data*t = it->second->clone();
std::cout << "CLONE: " << std::hex << t << std::endl;
other_map[it->first] = t;
std::cout << "INSERTED: " << std::hex << other_map[it->first] << std::endl;
}
std::cin.ignore();
return 0;
}
On VS2010SP1, this outputs :
CLONE: 00034DD0
INSERTED: 00034DD0
CLONE: 00035098
INSERTED: 00035098
So it should be the problem, or maybe you're doing something wrong before.
Try this out, to help debug the issue. I'd recommend double-checking that the order function is correct. You can remove it to use std::less<T>, which is known to work.
// ...
typedef std::map<std::string, data_base*, order> string_db_map;
for(string_db_map::const_iterator it = other.elements.begin();
it != other.elements.end();
++it)
{
data_base *t = it->second->clone();
std::cout << "CLONE: " << std::hex << t << std::endl;
std::pair<string_db_map::iterator, bool) result = elements.insert(
string_db_map::value_type( it->first, t));
if ( !result.second )
{
std::cout << "element['" << it->first << "'] was already present, and replaced." << std::endl;
}
std::coud << "INSERTED [iterator]: " << std::hex << (*result.first).second << std::endl;
std::cout << "INSERTED [indexed]: " << std::hex << elements[it->first] << std::endl;
}
// ...