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• Executing certain code for every method call in C++

  - by Luís Guilherme

  I have a C++ class I want to inspect. So, I would like to all methods print their parameters and the return, just before getting out. The latter looks somewhat easy. If I do return() for everything, a macro #define return(a) cout << (a) << endl; return (a) would do it (might be wrong) if I padronize all returns to parenthesized (or whatever this may be called). If I want to take this out, just comment out the define. However, printing inputs seems more difficult. Is there a way I can do it, using C++ structures or with a workaroud hack?

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• Is there a standard Cyclic Iterator in C++

  - by Hippicoder

  Based on the following question: Check if one string is a rotation of other string I was thinking of making a cyclic iterator type that takes a range, and would be able to solve the above problem like so: std::string s1 = "abc" ; std::string s2 = "bca" ; std::size_t n = 2; // number of cycles cyclic_iterator it(s2.begin(),s2.end(),n); cyclic_iterator end; if (std::search(it, end, s1.begin(),s1.end()) != end) { std::cout << "s1 is a rotation of s2" << std::endl; } My question, Is there already something like this available? I've check Boost and STL doesn't have one. I've got a simple hand-written one but would rather use an already made/tested implementation.

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• Visual c++ opencv 2.1 contains()

  - by kaushalyjain

  how to check whether a given point is contained in a rectangle using the contains() function in Rect_ construct... Please give me the exact function and its parameters.Like when i type this Point b(2,2); Rect a(10,10,50,50); cout<< Rect_::contains(b); There is a compile error saying 1c:\users\kaushal\documents\visual studio 2008\projects\test1\test1.cpp(23) : error C2352: 'cv::Rect_<Tp::contains' : illegal call of non-static member function 1c:\opencv2.1\include\opencv\cxcore.hpp(385) : see declaration of 'cv::Rect<_Tp::contains'

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• recursive program

  - by wilson88

  I am trying to make a recursive program that calculates interest per year.It prompts the user for the startup amount (1000), the interest rate (10%)and number of years(1).(in brackets are samples) Manually I realised that the interest comes from the formula YT(1 + R)----- interest for the first year which is 1100. 2nd year YT(1 + R/2 + R2/2) //R squared 2nd year YT(1 + R/3 + R2/3 + 3R3/) // R cubed How do I write a recursive program that will calculate the interest? Below is the function which I tried double calculateInterest(double startUp, double rate, double duration) { double cpdInterest = (duration*startUp)*(1 + rate); if (duration == 0) { return cpdInterest; } else if (duration < 0) { cout << "Please enter a valid year"; } else { calculateInterest(cpdInterest,rate,duration); } return cpdInterest; }

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• static array in c++ forgets its size

  - by Karel Bílek

  In this small example, c++ forgets size of an array, passed to a constructor. I guess it is something simple, but I cannot see it. In classes.h, there is this code: #ifndef CLASSES_INC #define CLASSES_INC #include <iostream> class static_class { public: static_class(int array[]) { std::cout<<sizeof(array)/sizeof(int)<<"\n"; } }; class my_class{ public: static static_class s; static int array[4]; }; #endif In classes.cpp, there is this code: #include "classes.h" int my_class::array[4]={1, 2, 3, 4}; static_class my_class::s = static_class(my_class::array); In main.cpp, there is only simple #include "classes.h" int main () { return 0; } Now, the desired output (from the constructor of static_class) is 4. But what I get is 1. Why is that?

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• C operating system createprozess

  - by Tyzak

  hello, i'm new at C Programming (i learned c++) i want to create a process with windows.h at first i just want to start my main programm that creates a process ( -- starts an other programm) that's my code, but it doesn't really work, i removed every unnessasery line of code but "void sleep(700)" (or "sleep (700)" for testing if the windows methods work, but i get an error, that "sleep" cant be found. #include <iostream> #include <windows.h> #include <string> using namespace std; void main() { //bool ret; //startupinfo stupinfo; //prozess_information pro2info; //Getstartupinfo (&stupinfo); //createprozess(null, "C:\\bsss10\\betriebssystemePRA1.exe", null, null, false, create_new_console, null, // null, &stupinfo, &pro2info); sleep (700); cout<< "hello"; } thanks in advance

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• how to read character from console in c++?

  - by tsubasa

  I'm struggling with reading characters from console in c++. Here is what I tried to do: char x; char y; char z; cout<<"Please enter your string: "; string s; getline(cin,s); istringstream is(s); is>> x >> y >> z; The problem is if the user enter something like this "1 20 100": x will get 1 y will get 2 z will get 0 What I want to get is x = 1; y = 20; z = 100; Anybody has suggestions?

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• c++ warning: address of local variable

  - by chnet

  int * ref () { int tmp = 100; return &tmp; } int main () { int * a = ref(); cout << *a << endl; } I know the function ref () is allocated stack space. It will get destroyed as soon as the function exits. So the complier will give warning information. But my question is why the returning result is still correct.

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• In the following implementation of static_strlen, why are the & and parentheses around str necessary

  - by Ben

  If I change the type to const char str[Len], I get the following error: error: no matching function for call to ‘static_strlen(const char [5])’ Am I correct that static_strlen expects an array of const char references? My understanding is that arrays are passed as pointers anyway, so what need is there for the elements to be references? Or is that interpretation completely off-the-mark? #include <iostream> template <size_t Len> size_t static_strlen(const char (&str)[Len]) { return Len - 1; } int main() { std::cout << static_strlen("oyez") << std::endl; return 0; }

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• Visual Studio 2005 - VC++ compiler C1001 on Windows 7

  - by Fritz H

  When I try to build a simple "Hello World" C++ app on Windows 7 Beta, using Visual Studio 2005 (VC++2005) I get a rather generic error C1001 error (Internal compiler error) The compiler seems to just crash, and Windows pops up its (un)helpful This program has stopped working dialog. The file it complains about is mcp1.cpp. Has anyone come across this before? Cheers, Fritz EDIT: The code is: #include <iostream> int main(int argc, char** argv) { std::cout << "Hello!"; return 0; } EDIT 2: I have installed SP1 as well as SP1 for Vista. VS popped up a warning saying it needs SP1 for Vista, but installing it makes no difference. No ideas about what I can possibly do to fix this?

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• Threading and pcap issues.

  - by cftmon

  I have a GUI program that allows a user a scan a network, the issue is that when the pcap_loop function is called, my GUI program becomes unresponsive.(the pcap_loop blocks the current thread). When i try to use pthreads, i got a SIGSEGV fault at the pcap_loop function.Why?It's as if the thread can't see the procPacket function itself. void procPacket(u_char *arg, const struct pcap_pkthdr *pkthdr, const u_char *packet) { //show packets here } void* pcapLooper(void* param) { pcap_t* handler = (pcap_t*) param; pcap_loop(handler, 900 ,procPacket, NULL ); } //some function that runs when a button is pressed //handler has been opened through pcap_open_live pthread_t scanner; int t = pthread_create(&scanner,NULL,&pcapLooper, &handler ); if(t) { std::cout << "failed" << std::endl; } pthread_join(scanner,NULL); //do other stuff.

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• I asked this yesterday, after the input given I'm still having trouble implementing..

  - by Josh

  I'm not sure how to fix this or what I did wrong, but whenever I enter in a value it just closes out the run prompt. So, seems I do have a problem somewhere in my coding. Whenever I run the program and input a variable, it always returns the same answer.."The content at location 76 is 0." On that note, someone told me that "I don't know, but I suspect that Program A incorrectly has a fixed address being branched to on instructions 10 and 11." - mctylr but I'm not sure how to fix that.. I'm trying to figure out how to incorporate this idea from R Samuel Klatchko.. I'm still not sure what I'm missing but I can't get it to work.. const int OP_LOAD = 3; const int OP_STORE = 4; const int OP_ADD = 5; ... const int OP_LOCATION_MULTIPLIER = 100; mem[0] = OP_LOAD * OP_LOCATION_MULTIPLIER + ...; mem[1] = OP_ADD * OP_LOCATION_MULTIPLIER + ...; operand = memory[ j ] % OP_LOCATION_MULTIPLIER; operation = memory[ j ] / OP_LOCATION_MULTIPLIER; I'm new to programming, I'm not the best, so I'm going for simplicity. Also this is an SML program. Anyway, this IS a homework assignment and I'm wanting a good grade on this. So I was looking for input and making sure this program will do what I'm hoping they are looking for. Anyway, here are the instructions: Write SML (Simpletron Machine language) programs to accomplish each of the following task: A) Use a sentinel-controlled loop to read positive number s and compute and print their sum. Terminate input when a neg number is entered. B) Use a counter-controlled loop to read seven numbers, some positive and some negative, and compute + print the avg. C) Read a series of numbers, and determine and print the largest number. The first number read indicates how many numbers should be processed. Without further a due, here is my program. All together. int main() { const int READ = 10; const int WRITE = 11; const int LOAD = 20; const int STORE = 21; const int ADD = 30; const int SUBTRACT = 31; const int DIVIDE = 32; const int MULTIPLY = 33; const int BRANCH = 40; const int BRANCHNEG = 41; const int BRANCHZERO = 41; const int HALT = 43; int mem[100] = {0}; //Making it 100, since simpletron contains a 100 word mem. int operation; //taking the rest of these variables straight out of the book seeing as how they were italisized. int operand; int accum = 0; // the special register is starting at 0 int j; // This is for part a, it will take in positive variables in a sent-controlled loop and compute + print their sum. Variables from example in text. memory [0] = 1010; memory [01] = 2009; memory [02] = 3008; memory [03] = 2109; memory [04] = 1109; memory [05] = 4300; memory [06] = 1009; j = 0; //Makes the variable j start at 0. while ( true ) { operand = memory[ j ]%100; // Finds the op codes from the limit on the memory (100) operation = memory[ j ]/100; //using a switch loop to set up the loops for the cases switch ( operation ){ case 10: //reads a variable into a word from loc. Enter in -1 to exit cout <<"\n Input a positive variable: "; cin >> memory[ operand ]; break; case 11: // takes a word from location cout << "\n\nThe content at location " << operand << "is " << memory[operand]; break; case 20:// loads accum = memory[ operand ]; break; case 21: //stores memory[ operand ] = accum; break; case 30: //adds accum += mem[operand]; break; case 31: // subtracts accum-= memory[ operand ]; break; case 32: //divides accum /=(memory[ operand ]); break; case 33: // multiplies accum*= memory [ operand ]; break; case 40: // Branches to location j = -1; break; case 41: //branches if acc. is < 0 if (accum < 0) j = 5; break; case 42: //branches if acc = 0 if (accum == 0) j = 5; break; case 43: // Program ends exit(0); break; } j++; } return 0; }

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• Reading a text file in c++

  - by Yavuz Karacabey

  string numbers; string fileName = "text.txt"; ifstream inputFile; inputFile.open(fileName.c_str(),ios_base::in); inputFile >> numbers; inputFile.close(); cout << numbers; And my text.txt file is: 1 2 3 4 5 basically a set of integers separated by tabs. The problem is the program only reads the first integer in the text.txt file and ignores the rest for some reason. If I remove the tabs between the integers it works fine, but with tabs between them, it won't work. What causes this? As far as I know it should ignore any white space characters or am I mistaken? If so is there a better way to get each of these numbers from the text file?

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• c++ and c# speed compared

  - by Mack

  I was worried about C#'s speed when it deals with heavy calculations, when you need to use raw CPU power. I always thought that C++ is much faster than C# when it comes to calculations. So I did some quick tests. The first test computes prime numbers < an integer n, the second test computes some pandigital numbers. The idea for second test comes from here: Pandigital Numbers C# prime computation: using System; using System.Diagnostics; class Program { static int primes(int n) { uint i, j; int countprimes = 0; for (i = 1; i <= n; i++) { bool isprime = true; for (j = 2; j <= Math.Sqrt(i); j++) if ((i % j) == 0) { isprime = false; break; } if (isprime) countprimes++; } return countprimes; } static void Main(string[] args) { int n = int.Parse(Console.ReadLine()); Stopwatch sw = new Stopwatch(); sw.Start(); int res = primes(n); sw.Stop(); Console.WriteLine("I found {0} prime numbers between 0 and {1} in {2} msecs.", res, n, sw.ElapsedMilliseconds); Console.ReadKey(); } } C++ variant: #include <iostream> #include <ctime> int primes(unsigned long n) { unsigned long i, j; int countprimes = 0; for(i = 1; i <= n; i++) { int isprime = 1; for(j = 2; j < (i^(1/2)); j++) if(!(i%j)) { isprime = 0; break; } countprimes+= isprime; } return countprimes; } int main() { int n, res; cin>>n; unsigned int start = clock(); res = primes(n); int tprime = clock() - start; cout<<"\nI found "<<res<<" prime numbers between 1 and "<<n<<" in "<<tprime<<" msecs."; return 0; } When I ran the test trying to find primes < than 100,000, C# variant finished in 0.409 seconds and C++ variant in 5.553 seconds. When I ran them for 1,000,000 C# finished in 6.039 seconds and C++ in about 337 seconds. Pandigital test in C#: using System; using System.Diagnostics; class Program { static bool IsPandigital(int n) { int digits = 0; int count = 0; int tmp; for (; n > 0; n /= 10, ++count) { if ((tmp = digits) == (digits |= 1 << (n - ((n / 10) * 10) - 1))) return false; } return digits == (1 << count) - 1; } static void Main() { int pans = 0; Stopwatch sw = new Stopwatch(); sw.Start(); for (int i = 1; i <= 123456789; i++) { if (IsPandigital(i)) { pans++; } } sw.Stop(); Console.WriteLine("{0}pcs, {1}ms", pans, sw.ElapsedMilliseconds); Console.ReadKey(); } } Pandigital test in C++: #include <iostream> #include <ctime> using namespace std; int IsPandigital(int n) { int digits = 0; int count = 0; int tmp; for (; n > 0; n /= 10, ++count) { if ((tmp = digits) == (digits |= 1 << (n - ((n / 10) * 10) - 1))) return 0; } return digits == (1 << count) - 1; } int main() { int pans = 0; unsigned int start = clock(); for (int i = 1; i <= 123456789; i++) { if (IsPandigital(i)) { pans++; } } int ptime = clock() - start; cout<<"\nPans:"<<pans<<" time:"<<ptime; return 0; } C# variant runs in 29.906 seconds and C++ in about 36.298 seconds. I didn't touch any compiler switches and bot C# and C++ programs were compiled with debug options. Before I attempted to run the test I was worried that C# will lag well behind C++, but now it seems that there is a pretty big speed difference in C# favor. Can anybody explain this? C# is jitted and C++ is compiled native so it's normal that a C++ will be faster than a C# variant. Thanks for the answers!

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• Explain ML type inference to a C++ programmer

  - by Tsubasa Gomamoto

  How does ML perform the type inference in the following function definition: let add a b = a + b Is it like C++ templates where no type-checking is performed until the point of template instantiation after which if the type supports the necessary operations, the function works or else a compilation error is thrown ? i.e. for example, the following function template template <typename NumType> NumType add(NumType a, NumType b) { return a + b; } will work for add<int>(23, 11); but won't work for add<ostream>(cout, fout); Is what I am guessing is correct or ML type inference works differently? PS: Sorry for my poor English; it's not my native language.

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• Reading in 4 bytes at a time

  - by alphomega

  I have a big file full of integers that I'm loading in. I've just started using C++, and I'm trying out the filestream stuff. From everything I've read, it appears I can only read in bytes, So I've had to set up a char array, and then cast it as a int pointer. Is there a way I can read in 4 bytes at a time, and eliminate the need for the char array? const int HRSIZE = 129951336; //The size of the table char bhr[HRSIZE]; //The table int *dwhr; int main() { ifstream fstr; /* load the handranks.dat file */ std::cout << "Loading table.dat...\n"; fstr.open("table.dat"); fstr.read(bhr, HRSIZE); fstr.close(); dwhr = (int *) bhr; }

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• Why can't I access a const vector with iterator?

  - by tsubasa

  My example is as below. I found out the problem is with "const" in function void test's parameter. I don't know why the compiler does not allow. Could anybody tell me? Thanks. vector<int> p; void test(const vector<int> &blah) { vector<int>::iterator it; for (it=blah.begin(); it!=blah.end(); it++) { cout<<*it<<" "; } } int main() { p.push_back(1); p.push_back(2); p.push_back(3); test(p); return 0; }

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• Compilation Error on Recursive Variadic Template Function

  - by Maxpm

  I've prepared a simple variadic template test in Code::Blocks, but I'm getting an error: No matching function for call to 'OutputSizes()' Here's my source code: #include <iostream> #include <typeinfo> using namespace std; template <typename FirstDatatype, typename... DatatypeList> void OutputSizes() { std::cout << typeid(FirstDatatype).name() << ": " << sizeof(FirstDatatype) << std::endl; OutputSizes<DatatypeList...>(); } int main() { OutputSizes<char, int, long int>(); return 0; } I'm using GNU GCC with -std=C++0x. Using std=gnu++0x makes no difference.

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• Compile error with initializer_list when trying to use it to initialize member value of class

  - by ilektron

  I am trying to make a class initializable from an initialization_list in a class constructor's constructor's initialization list. It works for a std::map, but not for my custom class. I don't see any difference other than templates are used in std::map. #include <iostream> #include <initializer_list> #include <string> #include <sstream> #include <map> using std::string; class text_thing { private: string m_text; public: text_thing() { } text_thing(text_thing& other); text_thing(std::initializer_list< std::pair<const string, const string> >& il); text_thing& operator=(std::initializer_list< std::pair<const string, const string> >& il); operator string() { return m_text; } }; class static_base { private: std::map<string, string> m_test_map; text_thing m_thing; static_base(); public: static static_base& getInstance() { static static_base instance; return instance; } string getText() { return (string)m_thing; } }; typedef std::pair<const string, const string> spair; text_thing::text_thing(text_thing& other) { m_text = other.m_text; } text_thing::text_thing(std::initializer_list< std::pair<const string, const string> >& il) { std::stringstream text_gen; for (auto& apair : il) { text_gen << "{" << apair.first << ", " << apair.second << "}" << std::endl; } } text_thing& text_thing::operator=(std::initializer_list< std::pair<const string, const string> >& il) { std::stringstream text_gen; for (auto& apair : il) { text_gen << "{" << apair.first << ", " << apair.second << "}" << std::endl; } return *this; } static_base::static_base() : m_test_map{{"test", "1"}, {"test2", "2"}}, // Compiler fine with this m_thing{{"test", "1"}, {"test2", "2"}} // Compiler doesn't like this { } int main() { std::cout << "Starting the program" << std::endl; std::cout << "The text thing: " << std::endl << static_base::getInstance().getText(); } I get this compiler output g++ -O0 -g3 -Wall -c -fmessage-length=0 -std=c++11 -MMD -MP -MF"static_base.d" -MT"static_base.d" -o "static_base.o" "../static_base.cpp" Finished building: ../static_base.cpp Building file: ../test.cpp Invoking: GCC C++ Compiler g++ -O0 -g3 -Wall -c -fmessage-length=0 -std=c++11 -MMD -MP -MF"test.d" -MT"test.d" -o "test.o" "../test.cpp" ../test.cpp: In constructor ‘static_base::static_base()’: ../test.cpp:94:40: error: no matching function for call to ‘text_thing::text_thing(<brace-enclosed initializer list>)’ m_thing{{"test", "1"}, {"test2", "2"}} ^ ../test.cpp:94:40: note: candidates are: ../test.cpp:72:1: note: text_thing::text_thing(std::initializer_list<std::pair<const std::basic_string<char>, const std::basic_string<char> > >&) text_thing::text_thing(std::initializer_list< std::pair<const string, const string> >& il) ^ ../test.cpp:72:1: note: candidate expects 1 argument, 2 provided ../test.cpp:67:1: note: text_thing::text_thing(text_thing&) text_thing::text_thing(text_thing& other) ^ ../test.cpp:67:1: note: candidate expects 1 argument, 2 provided ../test.cpp:23:2: note: text_thing::text_thing() text_thing() ^ ../test.cpp:23:2: note: candidate expects 0 arguments, 2 provided make: *** [test.o] Error 1 Output of gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.1-2ubuntu1~13.04' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.8.1 (Ubuntu 4.8.1-2ubuntu1~13.04) It compiles fine with the std::map constructed this way, and if I modify the static_base to return the strings from the maps, all is fine and dandy. Please help me understand what is going on here.

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• Pass temporary object to function that takes pointer

  - by Happy Mittal

  I tried following code : #include<iostream> #include<string> using namespace std; string f1(string s) { return s="f1 called"; } void f2(string *s) { cout<<*s<<endl; } int main() { string str; f2(&f1(str)); } But this code doesn't compile. What I think is : f1 returns by value so it creates temporary, of which I am taking address and passing to f2. Now Please explain me where I am thinking wrong?

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• Why are gettimeofday() intervals occasionally negative?

  - by Andres Jaan Tack

  I have an experimental library whose performance I'm trying to measure. To do this, I've written the following: struct timeval begin; gettimeofday(&begin, NULL); { // Experiment! } struct timeval end; gettimeofday(&end, NULL); // Print the time it took! std::cout << "Time: " << 100000 * (end.tv_sec - begin.tv_sec) + (end.tv_usec - begin.tv_usec) << std::endl; Occasionally, my results include negative timings, some of which are nonsensical. For instance: Time: 226762 Time: 220222 Time: 210883 Time: -688976 What's going on?

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• How I can print the wchar_t values to console?

  - by zed91

  Example: #include <iostream> using namespace std; int main() { wchar_t en[] = L"Hello"; wchar_t ru[] = L"??????"; //Russian language cout << ru << endl << en; return 0; } This code only prints HEX-values like adress. How to print the wchar_t string?

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• Overloading operator>> for case insensitive string

  - by TheSOFan

  Given the definition of ci_string from cpp.reference.com, how would we go about implementing operator? My attempts at it involved std::read, but it doesn't seem to work (that is, gcount() properly counts the number of characters entered, but there is no output) #include <iostream> #include <cctype> #include <string> // ci_string definition goes here std::istream& operator>>(std::istream& in, ci_string& str) { return in.read(&*str.begin(), 4); } int main() { ci_string test_str; std::cin >> test_str; std::cout << test_str; return 0; }

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• Why do I get a "warning: no newline at end of file" ?

  - by user198729

  The file is a helloworld.cpp: #include <iostream> using namespace std; int main() { if(true) cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!! return 0; } But when I build it,get a warning: g++ -Wall -O2 -c -o hw.o hw.cpp hw.cpp:8:2: warning: no newline at end of file g++ -o myprog hw.o If I add a newline at the end,the warning will go. Why is that newline at end of file recommended in a cpp source file?

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• More elegant way to make a C++ member function change different member variables based on template p

  - by Eric Moyer

  Today, I wrote some code that needed to add elements to different container variables depending on the type of a template parameter. I solved it by writing a friend helper class specialized on its own template parameter which had a member variable of the original class. It saved me a few hundred lines of repeating myself without adding much complexity. However, it seemed kludgey. I would like to know if there is a better, more elegant way. The code below is a greatly simplified example illustrating the problem and my solution. It compiles in g++. #include <vector> #include <algorithm> #include <iostream> namespace myNS{ template<class Elt> struct Container{ std::vector<Elt> contents; template<class Iter> void set(Iter begin, Iter end){ contents.erase(contents.begin(), contents.end()); std::copy(begin, end, back_inserter(contents)); } }; struct User; namespace WkNS{ template<class Elt> struct Worker{ User& u; Worker(User& u):u(u){} template<class Iter> void set(Iter begin, Iter end); }; }; struct F{ int x; explicit F(int x):x(x){} }; struct G{ double x; explicit G(double x):x(x){} }; struct User{ Container<F> a; Container<G> b; template<class Elt> void doIt(Elt x, Elt y){ std::vector<Elt> v; v.push_back(x); v.push_back(y); Worker<Elt>(*this).set(v.begin(), v.end()); } }; namespace WkNS{ template<class Elt> template<class Iter> void Worker<Elt>::set(Iter begin, Iter end){ std::cout << "Set a." << std::endl; u.a.set(begin, end); } template<> template<class Iter> void Worker<G>::set(Iter begin, Iter end){ std::cout << "Set b." << std::endl; u.b.set(begin, end); } }; }; int main(){ using myNS::F; using myNS::G; myNS::User u; u.doIt(F(1),F(2)); u.doIt(G(3),G(4)); } User is the class I was writing. Worker is my helper class. I have it in its own namespace because I don't want it causing trouble outside myNS. Container is a container class whose definition I don't want to modify, but is used by User in its instance variables. doIt<F> should modify a. doIt<G> should modify b. F and G are open to limited modification if that would produce a more elegant solution. (As an example of one such modification, in the real application F's constructor takes a dummy parameter to make it look like G's constructor and save me from repeating myself.) In the real code, Worker is a friend of User and member variables are private. To make the example simpler to write, I made everything public. However, a solution that requires things to be public really doesn't answer my question. Given all these caveats, is there a better way to write User::doIt?

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