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• "Right" way to deallocate an std::vector object

  - by Jacob

  The first solution is: std::vector<int> *vec = new std::vector<int>; assert(vec != NULL); // ... delete vec; An alternative is: std::vector<int> v; //... vec.clear(); vec.swap(std::vector<int>(vec)); The second solution's a bit of a trick --- what's the "right" way to do it?

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• overloading "<<" with a struct (no class) cout style

  - by monkeyking

  I have a struct that I'd like to output using either 'std::cout' or some other output stream. Is this possible without using classes? Thanks #include <iostream> #include <fstream> template <typename T> struct point{ T x; T y; }; template <typename T> std::ostream& dump(std::ostream &o,point<T> p) const{ o<<"x: " << p.x <<"\ty: " << p.y <<std::endl; } template<typename T> std::ostream& operator << (std::ostream &o,const point<T> &a){ return dump(o,a); } int main(){ point<double> p; p.x=0.1; p.y=0.3; dump(std::cout,p); std::cout << p ;//how? return 0; } I tried different syntax' but I cant seem to make it work.

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• std::set<T>::erase(key). What if key isn't there?

  - by Armen Tsirunyan

  if I erase an element from an std::set and pass the key, not the iterator, and the key isn't in the set right now, will an exception be thrown? The thing is every second sentence in the MSDN documentation says: "this does blah blah, but it doesn't conform to the standard". So I need to know the standard behaviour. I just couldn't find it in the standard. Redirecting to the relevant clause will do as well. Thanks.

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• C++ Segmentation fault in binary_function

  - by noryb009

  I'm using Visual Studio 2010 Beta 2 (also tried with NetBeans), and I'm having a segmentation fault in the following code: // One of the @link s20_3_3_comparisons comparison functors@endlink. template <class _Tp> struct less : public binary_function<_Tp, _Tp, bool> { bool operator()(const _Tp& __x, const _Tp& __y) const { return __x < __y; } //this is the problem line }; I don't know what in my program calls it, but I am trying to find out. (I think it's a map) Does anyone know what to do, or has encountered this before?

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• Using an initializer_list on a map of vectors

  - by Hooked

  I've been trying to initialize a map of <ints, vector<ints> > using the new 0X standard, but I cannot seem to get the syntax correct. I'd like to make a map with a single entry with key:value = 1:<3,4 #include <initializer_list> #include <map> #include <vector> using namespace std; map<int, vector<int> > A = {1,{3,4}}; .... It dies with the following error using gcc 4.4.3: error: no matching function for call to std::map<int,std::vector<int,std::allocator<int> >,std::less<int>,std::allocator<std::pair<const int,std::vector<int,std::allocator<int> > > > >::map(<brace-enclosed initializer list>) Edit Following the suggestion by Cogwheel and adding the extra brace it now compiles with a warning that can be gotten rid of using the -fno-deduce-init-list flag. Is there any danger in doing so?

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• std::map and behavior of already inserted data

  - by Konstantin

  Does std::map move around already inserted values when inserting new data ?

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• C++'s unordered_map / hash_map / Google's dense_hash - how to input binary data (buf+len) and insert

  - by shlomif

  Hi all, I have two questions about Google's dense_hash_map, which can be used instead of the more standard unordered_map or hash_map: How do I use an arbitrary binary data memory segment as a key: I want a buffer+length pair, which may still contain some NUL (\0) characters. I can see how I use a NUL-terminated char * string , but that's not what I want. How do I implement an operation where I look if a key exists, and if not - insert it and if it does return the pointer to the existing key and let me know what actually happened. I'd appreciate it if anyone can shed any light on this subject. Regards, -- Shlomi Fish

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• How do I create a set with std::pair thats sorted based on the ::second pair member using bind

  - by Stone Free

  I know I could use the following: template <typename Pair> struct ComparePairThroughSecond : public std::unary_function<Pair, bool> { bool operator ()(const Pair& p1, const Pair& p2) const { return p1.second < p2.second; } }; std::set<std::pair<int, long>, ComparePairThroughSecond> somevar; but wondered if it could be done with boost::bind

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• Function template accepting nothing less than a bidirectional iterator or a pointer

  - by san

  I need a function template that accepts two iterators that could be pointers. If the two arguments are random_access iterators I want the return type to be an object of std::iterator<random_access_iterator_tag, ...> type else a std::iterator<bidirectional_iterator_tag, ...> type. I also want the code to refuse compilation if the arguments are neither a bidirectional iterator, nor a pointer. I cannot have dependency on third party libraries e.g. Boost Could you help me with the signature of this function so that it accepts bidirectional iterators as well as pointers, but not say input_iterator, output_iterator, forward_iterators. One partial solution I can think of is the following template<class T> T foo( T iter1, T iter2) { const T tmp1 = reverse_iterator<T>(iter1); const T tmp2 = reverse_iterator<T>(iter2); // do something } The idea is that if it is not bidirectional the compiler will not let me construct a reverse_iterator from it.

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• std::map keys in C++

  - by Soumava

  I have a requirement to create two different maps in C++. The Key is of type CHAR * and the Value is a pointer to a struct. I am filling 2 maps with these pairs, in separate iterations. After creating both maps I need find all such instances in which the value of the string referenced by the CHAR * are same. For this i am using the following code : typedef struct _STRUCTTYPE { .. } STRUCTTYPE, *PSTRUCTTYPE; typedef pair {CHAR *,PSTRUCTTYPE} kvpair; .. CHAR *xyz; PSTRUCTTYPE abc; after filling the information; Map.insert (kvpair(xyz,abc)); the above is repeated x times for the first map, and y times for the second map. after both are filled out; std::map {CHAR *, PSTRUCTTYPE} :: iterator Iter,findIter; for (Iter=iteratedMap-begin();Iter!=iteratedMap-end();mapIterator++) { char *key = Iter-first; printf("%s\n",key); findIter=otherMap-find(key); //printf("%u",findIter-second); if (findIter!=otherMap-end()) { printf("Match!\n"); } } The above code does not show any match, although the list of keys in both maps show obvious matches. My understanding is that the equals operator for CHAR * just equates the memory address of the pointers. My question is, what should i do to alter the equals operator for this type of key or could I use a different datatype for the string? *note : {} has been used instead of angle brackets as the content inside angle brackets was not showing up in the post.

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• Does std::multiset guarantee insertion order?

  - by Naveen

  I have a std::multiset which stores elements of class A. I have provided my own implementation of operator< for this class. My question is if I insert two equivalent objects into this multiset is their order guaranteed? For example, first I insert a object a1 into the set and then I insert an equivalent object a2 into this set. Can I expect the a1 to come before a2 when I iterate through the set? If no, is there any way to achieve this using multiset?

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• Making uppercase of std::string

  - by Daniel K.

  Which implementation do you think is better? std::string ToUpper( const std::string& source ) { std::string result; result.reserve( source.length() ); std::transform( source.begin(), source.end(), result.begin(), std::ptr_fun<int, int>( std::toupper ) ); return result; } and... std::string ToUpper( const std::string& source ) { std::string result( source.length(), '\0' ); std::transform( source.begin(), source.end(), result.begin(), std::ptr_fun<int, int>( std::toupper ) ); return result; } Difference is that the first one uses reserve method after the default constructor, but the second one uses the constructor accepting the number of characters.

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• Why can't we have an immutable version of operator[] for map

  - by Yan Cheng CHEOK

  The following code works fine : std::map<int, int>& m = std::map<int, int>(); int i = m[0]; But not the following code : // error C2678: binary '[' : no operator... const std::map<int, int>& m = std::map<int, int>(); int i = m[0]; Most of the time, I prefer to make most of my stuff to become immutable, due to reason : http://www.javapractices.com/topic/TopicAction.do?Id=29 I look at map source code. It has mapped_type& operator[](const key_type& _Keyval) Is there any reason, why std::map unable to provide const mapped_type& operator[](const key_type& _Keyval) const

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• Virtual methods as Comp function to sort

  - by wilsongoncalo.mp

  Hello everyone! I'm new to C++ and i'm trying to use std::sort function to sort a vector of Solutions. The code is something like this (solution list is a *vector): void SolutionSet::sort(Comparator &comparator) { std::sort(solutionsList_->begin(), solutionsList_->end(), &comparator::compare); } The comparator param is a Comparator´s child class instance , and the compare method is virtual at Comparator class and implemented by all Comparator's child classes. And i want to use that function as a comparator function at std:sort(). Is this possible? If it is, can someone tell me how? Because with the previous code, it doesn't work. If i've not made myself clear, please just ask! Thank you guys!

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• What's the bug in the following code ?

  - by Johannes

  #include <iostream> #include <algorithm> #include <vector> #include <boost/array.hpp> #include <boost/bind.hpp> int main() { boost::array<int, 4> a = {45, 11, 67, 23}; std::vector<int> v(a.begin(), a.end()); std::vector<int> v2; std::transform(v.begin(), v.end(), v2.begin(), boost::bind(std::multiplies<int>(), _1, 2)); std::copy(v2.begin(), v2.end(), std::ostream_iterator<int>(std::cout, " ")); } When run, this gives a creepy segmentation fault. Please tell me where I'm going wrong.

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• push(ing)_back objects pointers within a loop

  - by Jose Manuel Albornoz

  Consider the following: I have a class CDevices containing, amongst others, a string member class CDevice { public: CDevice(void); ~CDevice(void); // device name std::string Device_Name; etc... } and somewhere else in my code I define another class that contains a vector of pointers to CDevices class CDevice; class CServers { public: CServers(void); ~CServers(void); // Devices vector vector<CDevice*> Devices; etc... } The problem appears in the following lines in my main.c pDevice = new CDevice; pDevice->Device_Name = "de"; Devices.push_back(pDevice); pDevice->Device_Name = " revolotiunibus"; Devices.push_back(pDevice); pDevice->Device_Name = " orbium"; Devices.push_back(pDevice); pDevice->Device_Name = " coelestium"; Devices.push_back(pDevice); for(int i = 0; i < (int)Devices.size(); ++i) cout << "\nLoad name = " << Devices.at(i)->Device_Name << endl; The output I get is " coelestium" repeated four times: each time I push_back a new element into the vector all of the already existing elements take the value of the one which has just been added. I have also tried using iterators to recover each element in the vector with the same results. Could someone please tell me what's wrong here? Thankx

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• Is it safe to take the address of std::wstring's internal pointer?

  - by LCC

  I have an interface which is used like the following: if (SUCCEEDED(pInterface->GetSize(&size)) { wchar_t tmp = new wchar_t[size]; if (SUCCEEDED(pInterface->GetValue(tmp, size))) { std::wstring str = tmp; // do some work which doesn't throw } delete[] tmp; } Is it safe and portable to do this instead? if (SUCCEEDED(pInterface->GetSize(&size)) { std::wstring str; str.resize(size); if (SUCCEEDED(pInterface->GetValue(&str[0], size))) { // do some work } } Now, obviously this works (doesn't crash/corrupt memory) or I wouldn't have asked, but I'm mostly wanting to know if there's a compelling reason not to do this.

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• How can I copy one map into another using std::copy?

  - by Frank

  I would like to copy the content of one std::map into another. Can I use std::copy for that? Obviously, the following code won't work: int main() { typedef std::map<int,double> Map; Map m1; m1[3] = 0.3; m1[5] = 0.5; Map m2; m2[1] = 0.1; std::copy(m1.begin(), m1.end(), m2.begin()); return 0; } Is there any way to make it work with std::copy? Thanks!

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• C++ map to track when the end of map is reached

  - by eNetik

  Currently I have a map that prints out the following map<string, map<int,int> > mapper; map<int,int>::iterator inner; map<string, map<int,int> >::iterator outer; for(outer = mapper.begin(); outer != mapper.end(); outer++){ cout<<outer->first<<": "; for(inner = outer->second.begin(); inner != outer->second.end(); inner++){ cout<<inner->first<<","<<inner->second<<","; } } As of now this prints out the following stringone: 1,2,3,4,6,7,8, stringtwo: 3,5,6,7, stringthree: 2,3,4,5, What i want it to print out is stringone: 1,2,3,4,6,7,8 stringtwo: 3,5,6,7 stringthree: 2,3,4,5 how can i check for the end of the map inside my inner map? Any help would be appreciated Thank you

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• How to convert c++ std::list element to multimap iterator

  - by user63898

  Hello all, I have std::list<multimap<std::string,std::string>::iterator> > Now i have new element: multimap<std::string,std::string>::value_type aNewMmapValue("foo1","test") I want to avoid the need to set temp multimap and do insert to the new element just to get its iterator back so i could to push it back to the: std::list<multimap<std::string,std::string>::iterator> > can i somehow avoid this creation of the temp multimap. Thanks

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• Two dimensional strings in C++

  - by dada

  I want to write something like 2d strings in C++. I tried with : vector< vector<string> > table; int m,n,i,j; string s; cin>>n>>m; for(i=0;i<n;i++) { for(j=0;j<m;j++) { cin>>s; table[i][j] = s; } } cout << "\n\n\n\n"; for(i=0;i<n;i++) { for(j=0;j<m;j++) { cout<<table[i][j]<<" "; } cout<<"\n"; } no compile errors, but when i enter input like: 10 20 .....#.............. .....#.............. .....#.............. .....#.............. ######.............. .......###.......... .......#.#.......... .......###...####### .............#.....# .............####### It gives me segmentation fault. Why ? What's wrong ? And how it should be done so it would work correctly ? Thank you.

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• C++ Vector at/[] operator speed

  - by sub

  In order to give functions the option to modify the vector I can't do curr = myvec.at( i ); doThis( curr ); doThat( curr ); doStuffWith( curr ); But I have to do: doThis( myvec.at( i ) ); doThat( myvec.at( i ) ); doStuffWith( myvec.at( i ) ); (as the answers of my other question pointed out) I'm going to make a hell lot of calls to myvec.at() then. How fast is it, compared to the first example using a variable to store the result? Is there a different option for me? Can I somehow use pointers? When it's getting serious there will be thousands of calls to myvec.at() per second. So every little performance-eater is important.

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• C++ associative array with arbitrary types for values

  - by Gerald Kaszuba

  What is the best way to have an associative array with arbitrary value types for each key in C++? Currently my plan is to create a "value" class with member variables of the types I will be expecting. For example: class Value { int iValue; Value(int v) { iValue = v; } std::string sValue; Value(std::string v) { sValue = v; } SomeClass *cValue; Value(SomeClass *v) { cValue = c; } }; std::map<std::string, Value> table; A downside with this is you have to know the type when accessing the "Value". i.e.: table["something"] = Value(5); SomeClass *s = table["something"].cValue; // broken pointer Also the more types that are put in Value, the more bloated the array will be. Any better suggestions?

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• Find missing number in sequence in set or list

  - by Dave18

  If a std::set or std::list contains a sequence of natural numbers (1, 2, 3..). would there be a function in standard library to find the missing number?

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• How to iterate a list of vector

  - by michael

  Hi, If I have a vector of object vector, how can I check if A* myA is inside that vector?

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