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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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  • Nested for_each with lambda not possible?

    - by Ela782
    The following code does not compile in VS2012, it gives error C2064: term does not evaluate to a function taking 1 arguments on the line of the second for_each (line 4 below). vector<string> v1; for_each(begin(v1), end(v1), [](string s1) { vector<string> v2; for_each(begin(v2), end(v2), [](string s2) { cout << "..."; }); }); I found some related stuff like http://connect.microsoft.com/VisualStudio/feedback/details/560907/capturing-variables-in-nested-lambdas which shows a bug (they are doing something different) but on the other hand that shows that what I print above should be possible. What's wrong with the above code?

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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 to negate a predicate function using operator ! in C++?

    - by Chan
    Hi, I want to erase all the elements that do not satisfy a criterion. For example: delete all the characters in a string that are not digit. My solution using boost::is_digit worked well. struct my_is_digit { bool operator()( char c ) const { return c >= '0' && c <= '9'; } }; int main() { string s( "1a2b3c4d" ); s.erase( remove_if( s.begin(), s.end(), !boost::is_digit() ), s.end() ); s.erase( remove_if( s.begin(), s.end(), !my_is_digit() ), s.end() ); cout << s << endl; return 0; } Then I tried my own version, the compiler complained :( error C2675: unary '!' : 'my_is_digit' does not define this operator or a conversion to a type acceptable to the predefined operator I could use not1() adapter, however I still think the operator ! is more meaningful in my current context. How could I implement such a ! like boost::is_digit() ? Any idea? Thanks, Chan Nguyen

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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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  • C++ std::vector memory/allocation

    - by aaa
    from a previous question about vector capacity, http://stackoverflow.com/questions/2663170/stdvector-capacity-after-copying, Mr. Bailey said: In current C++ you are guaranteed that no reallocation occurs after a call to reserve until an insertion would take the size beyond the value of the previous call to reserve. Before a call to reserve, or after a call to reserve when the size is between the value of the previous call to reserve and the capacity the implementation is allowed to reallocate early if it so chooses. So, if I understand correctly, in order to assure that no relocation happens until capacity is exceeded, I must do reserve twice? can you please clarify it? I am using vector as a memory stack like this: std::vector<double> memory; memory.reserve(size); memory.insert(memory.end(), matrix.data().begin(), matrix.data().end()); // smaller than size size_t offset = memory.size(); memory.resize(memory.capacity(), 0); I need to guarantee that relocation does not happen in the above. thank you. ps: I would also like to know if there is a better way to manage memory stack in similar manner other than vector

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  • A minimalistic smart array (container) class template

    - by legends2k
    I've written a (array) container class template (lets call it smart array) for using it in the BREW platform (which doesn't allow many C++ constructs like STD library, exceptions, etc. It has a very minimal C++ runtime support); while writing this my friend said that something like this already exists in Boost called MultiArray, I tried it but the ARM compiler (RVCT) cries with 100s of errors. I've not seen Boost.MultiArray's source, I've started learning templates only lately; template meta programming interests me a lot, although am not sure if this is strictly one that can be categorized thus. So I want all my fellow C++ aficionados to review it ~ point out flaws, potential bugs, suggestions, optimizations, etc.; something like "you've not written your own Big Three which might lead to...". Possibly any criticism that will help me improve this class and thereby my C++ skills. Edit: I've used std::vector since it's easily understood, later it will be replaced by a custom written vector class template made to work in the BREW platform. Also C++0x related syntax like static_assert will also be removed in the final code. smart_array.h #include <vector> #include <cassert> #include <cstdarg> using std::vector; template <typename T, size_t N> class smart_array { vector < smart_array<T, N - 1> > vec; public: explicit smart_array(vector <size_t> &dimensions) { assert(N == dimensions.size()); vector <size_t>::iterator it = ++dimensions.begin(); vector <size_t> dimensions_remaining(it, dimensions.end()); smart_array <T, N - 1> temp_smart_array(dimensions_remaining); vec.assign(dimensions[0], temp_smart_array); } explicit smart_array(size_t dimension_1 = 1, ...) { static_assert(N > 0, "Error: smart_array expects 1 or more dimension(s)"); assert(dimension_1 > 1); va_list dim_list; vector <size_t> dimensions_remaining(N - 1); va_start(dim_list, dimension_1); for(size_t i = 0; i < N - 1; ++i) { size_t dimension_n = va_arg(dim_list, size_t); assert(dimension_n > 0); dimensions_remaining[i] = dimension_n; } va_end(dim_list); smart_array <T, N - 1> temp_smart_array(dimensions_remaining); vec.assign(dimension_1, temp_smart_array); } smart_array<T, N - 1>& operator[](size_t index) { assert(index < vec.size() && index >= 0); return vec[index]; } size_t length() const { return vec.size(); } }; template<typename T> class smart_array<T, 1> { vector <T> vec; public: explicit smart_array(vector <size_t> &dimension) : vec(dimension[0]) { assert(dimension[0] > 0); } explicit smart_array(size_t dimension_1 = 1) : vec(dimension_1) { assert(dimension_1 > 0); } T& operator[](size_t index) { assert(index < vec.size() && index >= 0); return vec[index]; } size_t length() { return vec.size(); } }; Sample Usage: #include "smart_array.h" #include <iostream> using std::cout; using std::endl; int main() { // testing 1 dimension smart_array <int, 1> x(3); x[0] = 0, x[1] = 1, x[2] = 2; cout << "x.length(): " << x.length() << endl; // testing 2 dimensions smart_array <float, 2> y(2, 3); y[0][0] = y[0][1] = y[0][2] = 0; y[1][0] = y[1][1] = y[1][2] = 1; cout << "y.length(): " << y.length() << endl; cout << "y[0].length(): " << y[0].length() << endl; // testing 3 dimensions smart_array <char, 3> z(2, 4, 5); cout << "z.length(): " << z.length() << endl; cout << "z[0].length(): " << z[0].length() << endl; cout << "z[0][0].length(): " << z[0][0].length() << endl; z[0][0][4] = 'c'; cout << z[0][0][4] << endl; // testing 4 dimensions smart_array <bool, 4> r(2, 3, 4, 5); cout << "z.length(): " << r.length() << endl; cout << "z[0].length(): " << r[0].length() << endl; cout << "z[0][0].length(): " << r[0][0].length() << endl; cout << "z[0][0][0].length(): " << r[0][0][0].length() << endl; // testing copy constructor smart_array <float, 2> copy_y(y); cout << "copy_y.length(): " << copy_y.length() << endl; cout << "copy_x[0].length(): " << copy_y[0].length() << endl; cout << copy_y[0][0] << "\t" << copy_y[1][0] << "\t" << copy_y[0][1] << "\t" << copy_y[1][1] << "\t" << copy_y[0][2] << "\t" << copy_y[1][2] << endl; return 0; }

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  • std::basic_string full specialization (g++ conflict)

    - by SoapBox
    I am trying to define a full specialization of std::basic_string< char, char_traits<char>, allocator<char> > which is typedef'd (in g++) by the <string> header. The problem is, if I include <string> first, g++ sees the typedef as an instantiation of basic_string and gives me errors. If I do my specialization first then I have no issues. I should be able to define my specialization after <string> is included. What do I have to do to be able to do that? My Code: #include <bits/localefwd.h> //#include <string> // <- uncommenting this line causes compilation to fail namespace std { template<> class basic_string< char, char_traits<char>, allocator<char> > { public: int blah() { return 42; } size_t size() { return 0; } const char *c_str() { return ""; } void reserve(int) {} void clear() {} }; } #include <string> #include <iostream> int main() { std::cout << std::string().blah() << std::endl; } The above code works fine. But, if I uncomment the first #include <string> line, I get the following compiler errors: blah.cpp:7: error: specialization of ‘std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ after instantiation blah.cpp:7: error: redefinition of ‘class std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ /usr/include/c++/4.4/bits/stringfwd.h:52: error: previous definition of ‘class std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ blah.cpp: In function ‘int main()’: blah.cpp:22: error: ‘class std::string’ has no member named ‘blah’ Line 52 of /usr/include/c++/4.4/bits/stringfwd.h: template<typename _CharT, typename _Traits = char_traits<_CharT>, typename _Alloc = allocator<_CharT> > class basic_string; As far as I know this is just a forward delcaration of the template, NOT an instantiation as g++ claims. Line 56 of /usr/include/c++/4.4/bits/stringfwd.h: typedef basic_string<char> string; As far as I know this is just a typedef, NOT an instantiation either. So why are these lines conflicting with my code? What can I do to fix this other than ensuring that my code is always included before <string>?

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  • resort on a std::vector vs std::insert

    - by Abruzzo Forte e Gentile
    I have a sorted std::vector of relative small size ( from 5 to 20 elements ). I used std::vector since the data is continuous so I have speed because of cache. On a specific point I need to remove an element from this vector. I have now a doubt: which is the fastest way to remove this value between the 2 options below? setting that element to 0 and call sort to reorder: this has complexity but elements are on the same cache line. call erase that will copy ( or memcpy who knows?? ) all elements after it of 1 place ( I need to investigate the behind scense of erase ). Do you know which one is faster? I think that the same approach could be thought about inserting a new element without hitting the max capacity of the vector. Regards AFG

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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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  • Javascript object encapsulation that tracks changes

    - by Raynos
    Is it possible to create an object container where changes can be tracked Said object is a complex nested object of data. (compliant with JSON). The wrapper allows you to get the object, and save changes, without specifically stating what the changes are Does there exist a design pattern for this kind of encapsulation Deep cloning is not an option since I'm trying to write a wrapper like this to avoid doing just that. The solution of serialization should only be considered if there are no other solutions. An example of use would be var foo = state.get(); // change state state.update(); // or state.save(); client.tell(state.recentChange()); A jsfiddle snippet might help : http://jsfiddle.net/Raynos/kzKEp/ It seems like implementing an internal hash to keep track of changes is the best option. [Edit] To clarify this is actaully done on node.js on the server. The only thing that changes is that the solution can be specific to the V8 implementation.

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  • How to call operator<< on "this" in a descendant of std::stringstream?

    - by romkyns
    class mystream : public std::stringstream { public: void write_something() { this << "something"; } }; This results in the following two compile errors on VC++10: error C2297: '<<' : illegal, right operand has type 'const char [10]' error C2296: '<<' : illegal, left operand has type 'mystream *const ' Judging from the second one, this is because what this points at can't be changed, but the << operator does (or at least is declared as if it does). Correct? Is there some other way I can still use the << and >> operators on this?

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  • Confused about std::runtime_error vs. std::logic_error

    - by David Gladfelter
    I recently saw that the boost program_options library throws a logic_error if the command-line input was un-parsable. That challenged my assumptions about logic_error vs. runtime_error. I assumed that logic errors (logic_error and its derived classes) were problems that resulted from internal failures to adhere to program invariants, often in the form of illegal arguments to internal API's. In that sense they are largely equivalent to ASSERT's, but meant to be used in released code (unlike ASSERT's which are not usually compiled into released code.) They are useful in situations where it is infeasible to integrate separate software components in debug/test builds or the consequences of a failure are such that it is important to give runtime feedback about the invalid invariant condition to the user. Similarly, I thought that runtime_errors resulted exclusively from runtime conditions outside of the control of the programmer: I/O errors, invalid user input, etc. However, program_options is obviously heavily (primarily?) used as a means of parsing end-user input, so under my mental model it certainly should throw a runtime_error in the case of bad input. Where am I going wrong? Do you agree with the boost model of exception typing?

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  • Does operator precedence in C++ differ for pointers and iterators?

    - by oraz
    The code below demonstrates this difference: #include <iostream> #include <string> int main() { char s[] = "ABCD"; std::string str(s); char *p = s; while(*p) { *p++ = tolower(*p); // <-- incr after assignment } std::cout << s << std::endl; std::string::iterator it = str.begin(), end = str.end(); while(it != end) { *it++ = tolower(*it); // <-- incr before assignment ? } std::cout << str << std::endl; return 0; } the code above outputs: abcd bcd if we separate assignment operation and increment operator: while(it != end) { *it = tolower(*it); // <-- incr before assignment ? it++; } the output will be as expected. What's wrong with the original code? $ g++ --version g++ (GCC) 3.4.4 (cygming special, gdc 0.12, using dmd 0.125) Copyright (C) 2004 Free Software Foundation, Inc.

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  • Is the "==" operator required to be defined to use std::find

    - by user144182
    Let's say I have: class myClass std::list<myClass> myList where myClass does not define the == operator and only consists of public fields. In both VS2010 and VS2005 the following does not compile: myClass myClassVal = myList.front(); std::find( myList.begin(), myList.end(), myClassVal ) complaining about lack of == operator. I naively assumed it would do a value comparison of the myClass object's public members, but I am almost positive this is not correct. I assume if I define a == operator or perhaps use a functor instead, it will solve the problem. Alternatively, if my list was holding pointers instead of values, the comparison would work. Is this right or should I be doing something else?

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  • C++ Generic List Assignment

    - by S73417H
    I've clearly been stuck in Java land for too long... Is it possible to do the C++ equivalent of the following Java code: // Method List<Bar> getBars() { return new LinkedList<Bar>(); } // Assignment statement. List<Foo> stuff = getBars(); Where Foo is a sub-class of Bar. So in C++.... std::list<Bar> & getBars() { std::list<Bar> bars; return bars; } std::list<Foo> stuff = getBars(); Hope that makes sense....

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  • Copy method optimization in compilers

    - by Dženan
    Hi All! I have the following code: void Stack::operator =(Stack &rhs) { //do the actual copying } Stack::Stack(Stack &rhs) //copy-constructor { top=NULL; //initialize this as an empty stack (which it is) *this=rhs; //invoke assignment operator } Stack& Stack::CopyStack() { return *this; //this statement will invoke copy contructor } It is being used like this: unsigned Stack::count() { unsigned c=0; Stack copy=CopyStack(); while (!copy.empty()) { copy.pop(); c++; } return c; } Removing reference symbol from declaration of CopyStack (returning a copy instead of reference) makes no difference in visual studio 2008 (with respect to number of times copying is invoked). I guess it gets optimized away - normally it should first make a copy for the return value, then call assignment operator once more to assign it to variable sc. What is your experience with this sort of optimization in different compilers? Regards, Dženan

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  • Using pair in c++

    - by user1543957
    Can someone please tell why i am unable to compile the following program #include<iostream> #include<string> #include<cmath> #include<iostream> #include<cfloat> #define MOD 10000009 using namespace std; double distance(pair<int,int> p1,pair<int,int> p2) { double dist; dist = sqrt( (p1.first-p2.first)*(p1.first-p2.first) + (p1.second-p2.second)*(p1.second-p2.second) ); return(dist); } int main() { int N,i,j; cin >> N; pair<int,int> pi[N]; for(i=0;i<N;i++) { cin >> pi[i].first >> pi[i].second; } for(i=0;i<N;i++) { cout << pi[i].first << " "<< pi[i].second << endl; } distance(pi[0],pi[1]); // This line is giving error return 0; }

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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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  • accessing nth element (value) of a vector after sorting

    - by memC
    dear experts, This question is an extension of this question I asked. I have a std::vector vec_B.which stores instances of class Foo. The order of elements in this vector changes in the code. Now, I want to access the value of the current "last element" or current 'nth' element of the vector. If I use the code below to get the last element using getLastFoo() method, it doesn't return the correct value. For example, to begin with the last element of the vector has Foo.getNumber() = 9. After sorting it in descending order of num, for the last element, Foo.getNumber() = 0. But with the code below, it still returns 9.. that means it is still pointing to the original element that was the last element. What change should I make to the code below so that "lastFoo" points to the correct last element? class Foo { public: Foo(int i); ~Foo(){}; int getNum(); private: int num; }; Foo:Foo(int i){ num = i; } int Foo::getNum(){ return num; } class B { public: Foo* getLastFoo(); B(); ~B(){}; private: vector<Foo> vec_B; }; B::B(){ int i; for (i = 0; i< 10; i++){ vec_B.push_back(Foo(i)); } // Do some random changes to the vector vec_B so that elements are reordered. For // example rearrange elements in decreasing order of 'num' //... } Foo* B::getLastFoo(){ &vec_B.back(); }; int main(){ B b; Foo* lastFoo; lastFoo = b.getLastFoo() cout<<lastFoo->getNumber(); return 0; }

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  • need help with C++ using maps to keep track of words in a INPUT file

    - by eNetik
    Let say i have a text file with today is today but tomorrow is today tomorrow then using maps how can i keep track of the words that are repeated? and on which line it repeats? so far i have each string in the file read in as a temp and it is stored in the following way: map<string,int> storage; int count = 1 // for the first line of the file if(infile.is_open()){ while( !infile.eof() ){ getline(in, line); istringstream my_string(line); while(my_string.good()){ string temp; my_string >> temp; storage[temp] = count } count++;// so that every string read in the next line will be recorded as that line. } } map<string,int>::iterator m; for(int m = storage.begin(); m!= storage.end(); m++){ out<<m->first<<": "<<"line "<<m->second<<endl; } right now the output is just but: line 1 is: line 2 today: line 2 tomorrow: line 2 But instead.. it should print out(no repeating strings): today : line 1 occurred 2 times, line 2 occurred 1 time. is: line 1 occurred 1 time, line 2 occurred 1 time. but: line 1 occurred 1 time. tomorrow: line 2 occurred 2 times. Note: the order of the string does not matter. Any help would be appreciated. Thanks.

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