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• Basic question on c++ header file inclusion ?

  - by siva

  What are the differences between below 3 programs ?. Is <iostream> a header file or C++ standard library ? 1 #include<iostream> using namespace std; int main() { return 0; } 2 #include<iostream> int main() { return 0; } 3 #include<iostream.h> int main() { return 0; } Thanks in advance.

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• Why doesn't the C++ default destructor destroy my objects?

  - by Oszkar

  The C++ specification says the default destructor deletes all non-static members. Nevertheless, I can't manage to achieve that. I have this: class N { public: ~N() { std::cout << "Destroying object of type N"; } }; class M { public: M() { n = new N; } // ~M() { //this should happen by default // delete n; // } private: N* n; }; Then this should print the given message, but it doesn't: M* m = new M(); delete m; //this should invoke the default destructor

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• How To Parse String File Txt Into Array With C++

  - by Ibnu Syuhada

  I am trying to write a C++ program, but I am not familiar with C++. I have a .txt file, which contains values as follows: 0 0.0146484 0.0292969 0.0439453 0.0585938 0.0732422 0.0878906 What I have done in my C++ code is as follows: #include <iostream> #include <fstream> using namespace std; int main() { string line; ifstream myReadFile; myReadFile.open("Qi.txt"); if(myReadFile.is_open()) { while(myReadFile.good()) { getline(myReadFile,line); cout << line << endl; } myReadFile.close(); } return 0; } I would like to make the output of the program an array, i.e. line[0] = 0 line[1] = 0.0146484 line[2] = 0.0292969 line[3] = 0.0439453 line[4] = 0.0585938 line[5] = 0.0732422 line[6] = 0.0878906

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• Another thread safe queue implementation

  - by jensph

  I have a class, Queue, that I tried to make thread safe. It has these three member variables: std::queue<T> m_queue; pthread_mutex_t m_mutex; pthread_cond_t m_condition; and a push and pop implemented as: template<class T> void Queue<T>::push(T value) { pthread_mutex_lock( &m_mutex ); m_queue.push(value); if( !m_queue.empty() ) { pthread_cond_signal( &m_condition ); } pthread_mutex_unlock( &m_mutex ); } template<class T> bool Queue<T>::pop(T& value, bool block) { bool rtn = false; pthread_mutex_lock( &m_mutex ); if( block ) { while( m_queue.empty() ) { pthread_cond_wait( &m_condition, &m_mutex ); } } if( !m_queue.empty() ) { value = m_queue.front(); m_queue.pop(); rtn = true; } pthread_mutex_unlock( &m_mutex ); return rtn; } Unfortunately there are occasional issues that may be the fault of this code. That is, there are two threads and sometimes thread 1 never comes out of push() and at other times thread 2 never comes out of pop() (the block parameter is true) though the queue isn't empty. I understand there are other implementations available, but I'd like to try to fix this code, if needed. Anyone see any issues? The constructor has the appropriate initializations: Queue() { pthread_mutex_init( &mMutex, NULL ); pthread_cond_init( &mCondition, NULL ); } and the destructor, the corresponding 'destroy' calls.

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• Modify input stream data on the fly

  - by Frizi

  I would like to implement a std::stream modifier/parser, that is doing data manipulation on the fly. Is it possible to create it in form of stream manipulator? For example, i want to strip all the line comments (from any // to the end of line) out of the stdin and pass it to stdout. string str; istream strippingCin = cin >> stripcomments; while(strippingCin.good()) { strippingCin >> str; cout << str; } There may be also a large file input instead of cin, so i don't want to load full stream data into memory at once. Is it possible without writing my own stream class? Maybe is there another route i should take instead?

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• delegating into private parts

  - by FredOverflow

  Sometimes, C++'s notion of privacy just baffles me :-) class Foo { struct Bar; Bar* p; public: Bar* operator->() const { return p; } }; struct Foo::Bar { void baz() { std::cout << "inside baz\n"; } }; int main() { Foo::Bar b; // error: 'struct Foo::Bar' is private within this context Foo f; f->baz(); // fine } Since Foo::Bar is private, I cannot declare b in main. Yet I can call methods from Foo::Bar just fine. Why the hell is this allowed? Was that an accident or by design?

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• C++ Char without Limit

  - by Lienau

  I'm pretty well versed in C#, but I decided it would be a good idea to learn C++ as well. The only thing I can't figure out is chars. I know you can use the string lib but I also want to figure out chars. I know you can set a char with a limit like this: #include <iostream> using namespace std; int main() { char c[128] = "limited to 128"; cout << c << endl; system("pause"); return 0; } But how would I make a char without a limit? I've seen chars with * but I though that was for pointers. Any help is greatly appreciated.

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• Evaluation of (de)reference operators

  - by Micha

  I have an (uncommented...) source file which I'm trying to understand. static const Map *gCurMap; static std::vector<Map> mapVec; then auto e = mapVec.end(); auto i = mapVec.begin(); while(i!=e) { // ... const Map *map = gCurMap = &(*(i++)); // ... } I don't understand what &(*(i++)) does. It does not compile when just using i++, but to me it looks the same, because I'm "incrementing" i, then I'm requesting the value at the given address and then I'm requesting the address of this value?!

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• Basic C++ code for multiplication of 2 matrix or vectors (C++ beginner)

  - by Ice

  I am a new C++ user and I am also doing a major in Maths so thought I would try implement a simple calculator. I got some code off the internet and now I just need help to multiply elements of 2 matrices or vectors. Matrixf multiply(Matrixf const& left, Matrixf const& right) { // error check if (left.ncols() != right.nrows()) { throw std::runtime_error("Unable to multiply: matrix dimensions not agree."); } /* I have all the other part of the code for matrix*/ /** Now I am not sure how to implement multiplication of vector or matrix.**/ Matrixf ret(1, 1); return ret; }

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• C++ - Efficient container for large amounts of searchable data?

  - by Francisco P.

  Hello, everybody! I am implementing a text-based version of Scrabble for a College project. My dictionary is quite large, weighing in at around 400.000 words (std::string). Searching for a valid word will suck, big time, in terms of efficiency if I go for a vector<string> ( O(n) ). Are there any good alternatives? Keep in mind, I'm enrolled in freshman year. Nothing TOO complex! Thanks for your time! Francisco

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• Another problem with decltype

  - by There is nothing we can do

  template<class IntT, IntT low = IntT(), IntT high = IntT()> struct X { static_assert(std::is_same<decltype(low),decltype(high)>::value,"Different types not allowed");//this should give error if types are different decltype(low) a; decltype(high) b; X():a(decltype(a)()),b(decltype(b)())//WHY THIS DOES NOT COMPILE? { cout << typeid(a).name() << '\n'; cout << typeid(b).name() << '\n'; } }; int _tmain(int argc, _TCHAR* argv[]) { X<char,1,'a'> x;//this according to static_assert shouldn't compile but it does return 0; } Using VS2010. Please see 3 comments in code above.

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• Normal C++ code in Qt doesnt build and run

  - by Nick

  Hello. I am using Qt under linux, if it matters. I ran successfully under Geany (a simple c++ compiler) the following: //my first program in C++ Hello World! include using namespace std; int main () {cout << "Hello World!"; return 0;} I opened Qt source file and copied the exact same code and i can't build or run. Thank you for your responses to this simple problem.

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• C++ a map to a 2 dimensional vector

  - by user1701545

  I want to create a C++ map where key is, say, int and value is a 2-D vector of double: map myMap; suppose I filled it and now I would like to update the second vector mapped by each key (for example divide each element by 2). How would I access that vector iteratively? The "itr-second[0]" syntax in the statement below is obviously wrong. What would be the right syntax for that action? for(std::map<in, vector<vector<double> > > itr = myMap.begin(); itr != myMap.end();++itr) { for(int i = 0;i < itr->second[0].size();++i) { itr->second[0][i] /= 2; } } thanks, rubi

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• Shuffle array variables in a pre-specified order, without using extra memory of "size of input array"

  - by Eternal Learner

  Input : A[4] = {0,4,-1,1000} - Actual Array P[4] = {1,0,3,2} - Order to be reshuffled Output: A[4] = {4,0,1000,-1} Condition : Don't use an additional array as memory. Can use an extra variable or two. Problem : I have the below program in C++, but this fails for certain inputs of array P. #include<iostream> using namespace std; void swap(int *a_r,int *r) { int temp = *r; *r = *a_r; *a_r = temp; } int main() { int A[4] = {0,4,-1,1000}; int P[4] = {3,0,1,2}; int value = A[0] , dest = P[0]; for(int i=0; i<4;i++) { swap(&A[dest],&value); dest = P[dest]; } for(int i=0;i<4;i++) cout<<A[i]<<" "; }

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• Compilation failing - no #include - boost

  - by jwoolard

  Hi, I'm trying to compile a third-party library, but g++ is complaining about the following line: typedef boost::shared_ptr<MessageConsumer> MessageConsumerPtr; The strange thing is, there is no #include directive in the file - and it is clearly supposed to be this way; there are about 60 files with the same (or very similar) issues. Clearly if there was an #include directive referencing the relevant boost header this would compile cleanly. My question is: how can I get g++ to somehow automagically find the relevant symbol (in all instances of this issue, it is a namespace that can't be found - usually std:: or boost::) by either automatically processing the relevant header (or some other mechanism). Thanks. Edit My current g++ call looks like: g++ -fPIC -O3 -DUSING_PCH -D_REENTRANT -I/usr/include/boost -I./ -c MessageInterpreter.cpp -o MessageInterpreter.o

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• Using * Width & Precision Specifiers With boost::format

  - by John Dibling

  I am trying to use width and precision specifiers with boost::format, like this: #include <boost\format.hpp> #include <string> int main() { int n = 5; std::string s = (boost::format("%*.*s") % (n*2) % (n*2) % "Hello").str(); return 0; } But this doesn't work because boost::format doesn't support the * specifier. Boost throws an exception when parsing the string. Is there a way to accomplish the same goal, preferably using a drop-in replacement?

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• Switch/Case statements in C++

  - by vgoklani

  Regarding the switch/case statement in the C++ code below: "Case 1" is obviously false, so how/why does it enter the do-while loop? #include <iostream> using namespace std; int main() { int test = 4; switch(test) { case 1: do { case 2: test++; case 3: test++; case 4: cout << "How did I get inside the do-while loop?" << endl; break; case 5: test++; } while(test > 0); cout << test << endl; } }

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• specyfic syntax question

  - by bua

  Hi there, Is it possible to create template to the initialization like: template <typename C> typename C::value_type fooFunction(C& c) {...}; std::vector<string> vec_instance; fooFunction(cont<0>(vec_instance)); fooFunction(cont<1>(vec_instance)); In general i'm interested is it possible to specify template using integer (ie. 0) instead of true type name. And how to achieve above?

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• Can nohup change the result of a C++ code?

  - by Biga

  I am having this very weird behaviour with a C++ code: It gives me different results when running with and without 'nohup' (reproducible in cygwin and linux). I mean, if I get the same executable and run it like './run' or run it like 'nohup ./run out.log', I get different results! I use std::cout to output to screen, all lines ending with endl; I use ifstream for the input file; I use ofstream for output, all lines ending with endl. I am using g++ 4. Any idea what is going on?

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• trying to sort a simple string in c++

  - by every_answer_gets_a_point

  #include "stdio.h" #include "conio.h" #include <iostream> using namespace std; int main (void) { char my_char[] = "happy birthday"; int i; bool j=false; char my_char_temp[1]; do { for (i=0;i<sizeof(my_char)-2;i++) { j=false; if (my_char[i+1] < my_char[i]) { my_char_temp[0]=my_char[i+1]; my_char[i+1] = my_char[i]; my_char[i] = my_char_temp[0]; j=true; } } }while (j); cout << my_char; } what am i doing wrong? im just trying to sort the letters within the char the output i get is completely wrong

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• How does make_pair know the types of its args?

  - by bobobobo

  The definition for make_pair in the MSVC++ "utility" header is: template<class _Ty1, class _Ty2> inline pair<_Ty1, _Ty2> make_pair(_Ty1 _Val1, _Ty2 _Val2) { // return pair composed from arguments return (pair<_Ty1, _Ty2>(_Val1, _Val2)); } I use make_pair all the time though without putting the argument types in angle brackets: map<string,int> theMap ; theMap.insert( make_pair( "string", 5 ) ) ; Shouldn't I have to tell make_pair that the first argument is std::string and not char* ? How does it know?

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• C++ runtime, display exception message

  - by aaa

  hello. I am using gcc on linux to compile C++ code. There are some exceptions which should not be handled and should close program. However, I would like to be able to display exception string: For example: throw std::runtime_error(" message"); does not display message, only type of error. I would like to display messages as well. Is there way to do it? it is a library, I really do not want to put catch statements and let library user decide. However, right now library user is fortran, which does not allow to handle exceptions. in principle, I can put handlers in wrapper code, but rather not to if there is a way around Thanks

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• Why does the Win32-API have so many custom types?

  - by sub

  I'm new to the Win32 API and the many new types begin to confuse me. Some functions take 1-2 ints and 3 UINTS as arguments. Why can't they just use ints? What are UINTS? Then, there are those other types: DWORD LPCWSTR LPBOOL Again, I think the "primitive" C types would be enough - why introduce 100 new types? This one was a pain: WCHAR* I had to iterate through it and push_back every character to an std::string as there wasn't another way to convert it to one. Horrible. Why WCHAR? Why reinvent the wheel? They could have just used char* instead, or?

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• Why does this C++ char array seem to be able to hold more than its size?

  - by Phenom

  #include <iostream> using namespace std; typedef struct { char streetName[5]; } RECORD; int main() { RECORD r; cin >> r.streetName; cout << r.streetName << endl; } When I run this program, if I enter in more than 5 characters, the output will show the whole string I entered. It does not truncate at 5 characters. Why is that? How can I get this to work correctly?

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• Make object by it's name

  - by Ockonal

  Hello, is it possible to return exemplar of object using passed type name (string) in c++? I have some base abstract class Base and a few derivates. Example code: class Base { /* ... */ }; class Der1 : public Base { /* ... */ }; class Der2 : public Base { /* ... */ }; And I need function like: Base *objectByType(const std::string &name); Number of derivates classes are changeable and I don't want to make something like switching of name and returning by hands new object type. Is it possible in c++ to do that automatically anyway? p.s. usage should looks like: dynamic_cast<Der1>(objectByType("Der1")); I need pure c++ code (crossplatform). Using boost is permissible.

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