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• m.find(...) == m.end() - which is used, iterator or const_iterator

  - by leeeroy

  std::map find/end both provides const_iterator and iterator, e.g. iterator end (); const_iterator end () const Out of curiosity,if I have a std::map , which will be called/compared here, an iterator or a const_iterator ? : if(m.find(key) != m.end()) { ... } And should I care ?

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• calling resize on std vector of pointers crashed

  - by user11869

  The problem can be reproduced using VS 2013 Express. It crashed when internal vector implementation tried to deallocate the original vector. However, the problem can solved by using 'new' instead of 'malloc'. Anyone can shed some light on this? struct UndirectedGraphNode { int label; vector<UndirectedGraphNode *> neighbors; UndirectedGraphNode(int x) : label(x) {}; }; int main(int argc, char** argv) { UndirectedGraphNode* node1 = (UndirectedGraphNode*)malloc(sizeof(UndirectedGraphNode)); node1->label = 0; node1->neighbors.resize(2); return 0; }

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• sort std::list case sensitive elements

  - by Dave18

  #include <list> #include <string> using std::string; using std::list; int main() { list <string> list_; list_.push_back("C"); list_.push_back("a"); list_.push_back("b"); list_.sort(); } does sort() function sort the elements according to their character codes? I want the result here to be a b C after the sorting is done.

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• C++: Trouble with dependent types in templates

  - by Rosarch

  I'm having trouble with templates and dependent types: namespace Utils { void PrintLine(const string& line, int tabLevel = 0); string getTabs(int tabLevel); template<class result_t, class Predicate> set<result_t> findAll_if(typename set<result_t>::iterator begin, set<result_t>::iterator end, Predicate pred) // warning C4346 { set<result_t> result; return findAll_if_rec(begin, end, pred, result); } } namespace detail { template<class result_t, class Predicate> set<result_t> findAll_if_rec(set<result_t>::iterator begin, set<result_t>::iterator end, Predicate pred, set<result_t> result) { typename set<result_t>::iterator nextResultElem = find_if(begin, end, pred); if (nextResultElem == end) { return result; } result.add(*nextResultElem); return findAll_if_rec(++nextResultElem, end, pred, result); } } Compiler complaints, from the location noted above: warning C4346: 'std::set<result_t>::iterator' : dependent name is not a type. prefix with 'typename' to indicate a type error C2061: syntax error : identifier 'iterator' What am I doing wrong?

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• How to read arbitrary number of values using std::copy?

  - by Miro Kropacek

  Hi, I'm trying to code opposite action to this: std::ostream outs; // properly initialized of course std::set<int> my_set; // ditto outs << my_set.size(); std::copy( my_set.begin(), my_set.end(), std::ostream_iterator<int>( outs ) ); it should be something like this: std::istream ins; std::set<int>::size_type size; ins >> size; std::copy( std::istream_iterator<int>( ins ), std::istream_iterator<int>( ins ) ???, std::inserter( my_set, my_set.end() ) ); But I'm stuck with the 'end' iterator -- input interators can't use std::advance and neither I can use two streams with the same source... Is there any elegant way how to solve this? Of course I can use for loop, but maybe there's something nicer :)

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• Access Voilation in std::pair

  - by sameer karjatkar

  I have an application which is trying to populate a pair . Out of no where the application crashes . The Windbg analysis on the crash dump suggest PRIMARY_PROBLEM_CLASS: INVALID_POINTER_READ DEFAULT_BUCKET_ID: INVALID_POINTER_READ STACK_TEXT: 0389f1dc EPFilter32!std::vector,std::allocator ::size+0xc INVALID_POINTER_READ_c0000005_Test.DLL!std::vector_std::pair_unsigned_int, unsigned_int_,std::allocator_std::pair_unsigned_int,unsigned_int___::size Following is the statement in the code where it fails const branch_info& b1 = en1.m_branches[i1]; where branch_info is std::pair and the en1.m_branches[i1] fetches me a pair value

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• C++ std::equal -- rationale behind not testing for the 2 ranges having equal size?

  - by ShaChris23

  I just wrote some code to test the behavior of std::equal, and came away surprised: int main() { try { std::list<int> lst1; std::list<int> lst2; if(!std::equal(lst1.begin(), lst1.end(), lst2.begin())) throw std::logic_error("Error: 2 empty lists should always be equal"); lst2.push_back(5); if(std::equal(lst1.begin(), lst1.end(), lst2.begin())) throw std::logic_error("Error: comparing 2 lists where one is not empty should not be equal"); } catch(std::exception& e) { std::cerr << e.what(); } } The output (a surprise to me): Error: comparing 2 lists where one is not empty should not be equal Observation: why is it the std::equal does not first check if the 2 containers have the same size() ? Was there a legitimate reason?

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• Would vector of vectors be contiguous?

  - by user1150989

  I need to allocate a vector of rows where row contains a vector of rows. I know that a vector would be contiguous. I wanted to know whether a vector of vectors would also be contiguous. Example code is given below vector<long> firstRow; firstRow.push_back(0); firstRow.push_back(1); vector<long> secondRow; secondRow.push_back(0); secondRow.push_back(1); vector< vector < long> > data; data.push_back(firstRow); data.push_back(secondRow); Would the sequence in memory be 0 1 0 1?

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• Why does string::find return size_type and not an iterator?

  - by dehmann

  In C++, why does string::find return size_type and not an iterator? It would make sense because functions like string::replace or string::insert take iterators as input, so you could find some character and immediately pass the returned iterator to replace, etc. Also, std::find returns an iterator -- why is std::string::find different?

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• Get last/newly added element in std::set

  - by Dave17

  can you get the last or newly added element in std::set? for example say if the loop runs to collect the elements to fill in the std::set. if on the first run the set was, [0] "A" [1] "B" [2] "D" and, on second run, the set becomes [0] "A" [1] "B" [2] "C" [3] "D" How would you check if 'C' is the new element that was added?

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• Using an iterator without its container

  - by User1

  I am mixing some C and C++ libraries and have only a single pointer available to do some work in a callback function. All I need to do is iterate through a vector. Here's a simplified, untested example: bool call_back(void* data){ done=... if (!done) cout << *data++ << endl; return done; } Note that this function is in an extern "C" block in C++. call_back will be called until true is returned. I want it to cout the next element each time it's called. data is a pointer to something that I can pass from elsewhere in the code (an iterator in the above example, but can be anything). Something from data will likely be used to calculate done. I see two obvious options to give to data: Have data point to my vector. Have data point to an iterator of my vector. I can't use an iterator without having the .end() method available, right? I can't use a vector alone (unless maybe I start removing its data). I could make a struct with both vector and iterator, but is there a better way? What would you do?

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• how to specify a pointer to an overloaded function?

  - by davka

  I want to pass an overloaded function to the std::for_each() algorithm. e.g.: void f(char c); void f(int i); std::string s("example"); std::for_each(s.begin(), s.end(), f); I'd expect the compiler to resolve f() by the iterator type. Apparently, it (gcc 4.1.2) doesn't do it. So, how can I specify which f() I want? thanks a lot

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• Deleting a element from a vector of pointers in C++.

  - by Kranar

  I remember hearing that the following code is not C++ compliant and was hoping someone with much more C++ legalese than me would be able to confirm or deny it. std::vector<int*> intList; intList.push_back(new int(2)); intList.push_back(new int(10)); intList.push_back(new int(17)); for(std::vector<int*>::iterator i = intList.begin(); i != intList.end(); ++i) { delete *i; } intList.clear() The rationale was that it is illegal for a vector to contain pointers to invalid memory. Now obviously my example will compile and it will even work on all compilers I know of, but is it standard compliant C++ or am I supposed to do the following, which I was told is in fact the standard compliant approach: while(!intList.empty()) { int* element = intList.back(); intList.pop_back(); delete element; }

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• Iterator to last element in std::list

  - by Dave18

  #include <list> using std::list; int main() { list <int> n; n.push_back(1); n.push_back(2); n.push_back(3); list <int>::iterator iter = n.begin(); std::advance(iter, n.size() - 1); //iter is set to last element } is there any other way to have an iter to the last element in list?

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• Is there a sorted_vector class, which supports insert() etc.?

  - by Frank

  Often, it is more efficient to use a sorted std::vector instead of a std::set. Does anyone know a library class sorted_vector, which basically has a similar interface to std::set, but inserts elements into the sorted vector (so that there are no duplicates), uses binary search to find elements, etc.? I know it's not hard to write, but probably better not to waste time and use an existing implementation anyway.

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• What iterator to return for non-existing map How to signkey?

  - by Jack

  I want to have a function which searches for a key in a collection of maps and returns an iterator to the found key. But what should be returned in case the key cannot be found? I cannot return map::end since the collection of maps can be empty. Thanks.

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• Performance of vector::size() : is it as fast as reading a variable?

  - by zoli2k

  I have do an extensive calculation on a big vector of integers. The vector size is not changed during the calculation. The size of the vector is frequently accessed by the code. What is faster in general: using the vector::size() function or using helper constant vectorSize storing the size of the vector? I know that compilers usually able to inline the size() function when setting the proper compiler flags, however, making a function inline is something that a compiler may do but can not be forced.

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• Best way to handle storing (possibly NULL) char * in a std::string

  - by John

  class MyClass { public: void setVar(const char *str); private: std::string mStr; int maxLength; //we only store string up to this length }; What's the best approach to implement setVar when the external code is quite likely to pass in NULL for an empty string (and cannot be changed)? I currently do something a bit like: void MyClass::setVar(const char *str) { mStr.assign(str ? str : "",maxLength); } But it seems kind of messy. ideas?

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• what to use in place of std::map::emplace?

  - by kfmfe04

  For containers such as std::map< std::string, std::unique_ptr< Foo >>, it looks like emplace() has yet to be implemented in stdc++ as of gcc 4.7.2. Unfortunately, I can't store Foo directly by value as it is an abstract super-class. As a simple, but inefficient, place-holder, I've just been using std::map< std::string, Foo* > in conjunction with a std::vector< std::unique_ptr< Foo >> for garbage collection. Do you have a interim solution that is more efficient and more easily replaced once emplace() is available?

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• How to initialize std::vector from C-style array?

  - by dehmann

  What is the cheapest way to initialize a std::vector from a C-style array? Example: In the following class, I have a vector, but due to outside restrictions, the data will be passed in as C-style array: class Foo { std::vector<double> w_; public: void set_data(double* w, int len){ // how to cheaply initialize the std::vector? } Obviously, I can call w_.resize() and then loop over the elements, or call std::copy(). Are there any better methods?

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• difference between erase and remove/remove_if algorithms?

  - by Eric

  What is really the difference between the algorithms remove and remove_if and the member function erase? Does both of them result in a call to the removed objects destructor?

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• how-to initialize 'const std::vector<T>' like a c array

  - by vscharf

  Is there an elegant way to create and initialize a const std::vector<const T> like const T a[] = { ... } to a fixed (and small) number of values? I need to call a function frequently which expects a vector<T>, but these values will never change in my case. In principle I thought of something like namespace { const std::vector<const T> v(??); } since v won't be used outside of this compilation unit.

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• including a std::map within a struct? Is it ok?

  - by user553514

  class X_class{ public: struct extra {int extra1; int extra2; int extra3; }; enum a { n,m}; struct x_struct{ char b; char c; int d; int e; std::map <int, extra> myExtraMap; }; }; in my code I define : x_struct myStruct; why do I get compile errors compiling the above class? The error either says: 1) expected ; before < on the line --- where I defined the map (above) if I eliminate std:: or 2) error: invalid use of ::; error: expected ; before < token

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• C++ Standard Library Exception List??

  - by Brian

  Hi, Is there a reference about C++ Standard Library Exceptions? I just want to know that which functions may throw an exception or not. Thank you.

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• Can set_intersection be used with hash_set in C++?

  - by R S

  I am calculating intersection, union and differences of sets. I have a typedef of my set type: typedef set<node_type> node_set; When it is replaced with typedef hash_set<node_type> node_set; The results are different. It's a complicated program, and before I start debugging - am I doing it right? When I use functions like this: set_intersection(v_higher.begin(), v_higher.end(), neighbors[w].begin(), neighbors[w].end(), insert_iterator<node_set>(tmp1, tmp1.begin())); should they work seamlessly with both set and hash_set?

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