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• Simple question about C++ constant syntax

  - by WilliamLou

  Here is some code copied from Thinking in C++ Vol1 Chapter 10. #include <iostream> using namespace std; int x = 100; class WithStatic { static int x; static int y; public: void print() const { cout << "WithStatic::x = " << x << endl; cout << "WithStatic::y = " << y << endl; } }; what's the meaning of const for the function print()? Thanks!

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• converting string to int in C++

  - by xbonez

  I am trying to convert a string I read in from a file to an int value so I can store it in an integer variable. This is what my code looks like: ifstream sin; sin.open("movie_output.txt"); string line; getline(sin,line); myMovie.setYear(atoi(line)); Over here, setYear is a mutator in the Movie class (myMovie is an object of Movie class) that looks like this: void Movie::setYear(unsigned int year) { year_ = year; } When I run the code, I get the following error: error C2664: 'atoi' : cannot convert parameter 1 from 'std::string' to 'const char *' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

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• debug error : max must have union class struct types

  - by hcemp

  this is my code: #include <iostream> using namespace std; class Sp { private : int a;int b; public: Sp(int x=0,int y=0):a(x),b(y){}; int max(int x,int y); }; int Sp::max(int a,int b) { return (a>b?a:b);}; int main() { int q,q1; cin>>q>>q1; Sp *mm=new Sp(q,q1); cout<< mm.max(q,q1); return 0; }

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• Implementing operator< in C++

  - by Vulcan Eager

  I have a class with a few numeric fields such as: class Class1 { int a; int b; int c; public: // constructor and so on... bool operator<(const Class1& other) const; }; I need to use objects of this class as a key in an std::map. I therefore implement operator<. What is the simplest implementation of operator< to use here?

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• pass by pointer is not working

  - by user323422

  #include"iostream" class CMessage { public:int a; CMessage(){} ~CMessage(){} }; void Testing(CMessage *f_pMessage)//l_pMessage is output parameter { f_pMessage = new CMessage(); f_pMessage->a = 1; } int main() { CMessage *l_pMessage =NULL; Testing(l_pMessage); std::cout<<l_pMessage->a;//getting l_pMessage = NULL; return 0; } when i called testing then inside testing f_pMessage is getting intialize but as ssoon as i after excuting testing function it should be store in l_Pmessage but it is showing NULL.confussed.....

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• printing using one '\n'

  - by Alex

  I am pretty sure all of you are familiar with the concept of the Big4, and I have several stuffs to do print in each of the constructor, assignment, destructor, and copy constructor. The restriction is this: I CAN'T use more than one newline (e.g., ƒn or std::endl) in any method I can have a method called print, so I am guessing print is where I will put that precious one and only '\n', my problem is that how can the method print which prints different things on each of the element I want to print in each of the Big4? Any idea? Maybe overloading the Big4?

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• How to initialise a STL vector/list with a class without invoking the copy constructor

  - by Warpspace

  I have a C++ program that uses a std::list containing instances of a class. If I call e.g. myList.push_back(MyClass(variable)); it goes through the process of creating a temporary variable, and then immediately copies it to the vector, and afterwards deletes the temporary variable. This is not nearly as efficient as I want, and sucks when you need a deep copy. I would love to have the constructor of my class new something and not have to implement a copy constructor just to allocate my memory for the second time and waste runtime. I'd also rather not have to immediately find the class instance from the vector/list and then manually allocate the memory (or do something horrible like allocate the memory in the copy constructor itself). Is there any way around this (I'm not using Visual Studio BTW)?

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• Are raw C++ pointers first class objects?

  - by Shailesh Kumar

  According to Wikipedia: An object is first-class when it: can be stored in variables and data structures can be passed as a parameter to a subroutine can be returned as the result of a subroutine can be constructed at runtime has intrinsic identity (independent of any given name) Somebody had once told me that raw pointers are not first class objects while smart pointers like std::auto_ptr are. But to me, a raw pointer (to an object or to a function) in C++ does seem to me to satisfy the conditions stated above to qualify as a first class object. Am I missing something?

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• How to declare a function that accepts a lambda?

  - by Andreas Bonini

  I read on the internet many tutorials that explained how to use lambdas with the standard library (such as std::find), and they all were very interesting, but I couldn't find any that explained how I can use a lambda for my own functions. For example: int main() { int test = 5; LambdaTest([&](int a) { test += a; }); return EXIT_SUCCESS; } How should I declare LambdaTest? What's the type of its first argument? And then, how can I call the anonymous function passing to it - for example - "10" as its argument?

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• How to call a method withgin a vector?

  - by Mike Dooley

  Hi! How do I call a method of an object which is stored within a vector? The following code fails... ClassA* class_derived_a = new ClassDerivedA; ClassA* class_another_a = new ClassAnotherDerivedA; vector<ClassA*> test_vector; test_vector.push_back(class_derived_a); test_vector.push_back(class_another_a); for (vector<ClassA*>::iterator it = test_vector.begin(); it != test_vector.end(); it++) it->printOutput(); The code retrieves the following error: test3.cpp:47: error: request for member ‘printOutput’ in ‘* it.__gnu_cxx::__normal_iterator<_Iterator, _Container::operator- with _Iterator = ClassA**, _Container = std::vector ’, which is of non-class type ‘ClassA*’ The problem seems to be it->printOutput(); but at the moment I don't know how to call the method properly, does anyone know? regards mikey

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• Multiple Producers Single Consumer Queue

  - by Talguy

  I am new to multithreading and have designed a program that receives data from two microcontroller measuring various temperatures (Ambient and Water) and draws the data to the screen. Right now the program is singly threaded and its performance SUCKS A BIG ONE. I get basic design approaches with multithreading but not well enough to create a thread to do a task but what I don't get is how to get threads to perform seperate task and place the data into a shared data pool. I figured that I need to make a queue that has one consumer and multiple producers (would like to use std::queue). I have seen some code on the gtkmm threading docs that show a single Con/Pro queue and they would lock the queue object produce data and signal the sleeping thread that it is finished then the producer would sleep. For what I need would I need to sleep a thread, would there be data conflicts if i didn't sleep any of the threads, and would sleeping a thread cause a data signifcant data delay (I need realtime data to be drawn 30 frames a sec) How would I go about coding such a queue using the gtkmm/glibmm library.

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• Multiplying complex with constant in C++

  - by Atilla Filiz

  The following code fails to compile #include <iostream> #include <cmath> #include <complex> using namespace std; int main(void) { const double b=3; complex <double> i(0, 1), comp; comp = b*i; comp=3*i; return 0; } with error: no match for ‘operator*’ in ‘3 * i’ What is wrong here, why cannot I multiply with immediate constants?

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• CryptGenRandom to generate asp.net session id

  - by DoDo

  Hi! does anyone have working example of CryptGenrRandom class to generate session id (need to use in my iis module). HCRYPTPROV hCryptProv; BYTE pbData[16]; if(CryptAcquireContext( &hCryptProv, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) { if(CryptGenRandom(hCryptProv, 8, pbData)) { std::string s(( const char *) pbData); printf(s.c_str()); } else { MyHandleError("Error during CryptGenRandom."); } } else { MyHandleError("Error during CryptAcquireContext!\n"); } i tried this code but, its not working quite well (i get it from msdn) and this example don't work for me ( http://www.codeproject.com/KB/security/plaintextsessionkey.aspx ) so if anyone know how to generate sessionid using this class plz let me know tnx anyway!

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• Difference between debug and release.

  - by atch

  Why when I'm debugging in debug mode everything in code below works as suppose to but when I switch to release I'm getting strange result? void say_hello(int argc, char* argv[])//In release mode argc has different values from 124353625 to 36369852 <include iostream> { std::cout << "In say_hello()\n"; } int main(int argc, char* argv[]) { say_hello(3,argv);//when instead of literal I enter "argc" everything is ok. return 0; } Thanks for help.

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• List of functions references

  - by Ockonal

  Hello, I'm using boost::function for making references to the functions. Can I make a list of references? For example: boost::function<bool (Entity &handle)> behaviorRef; And I need in a list of such pointers. For example: std::vector<behaviorRef> listPointers; Of course it's wrong code due to behaviorRef isn't a type. So the question is: how can I store a list of pointers for the function?

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• What's wrong with the following code ?

  - by dada

  #include <iostream> #include <string> using namespace std; int main(void) { string a; cin>>a; a.erase(a.end()-1); a.erase(a.begin()+1); string ge = "oae"; a.insert(a.begin()+1, ge); cout<<a<<endl; return 0; } It doesn't compile and i don't know why. Can you tell me what's wrong

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• Behavior with primitive data types' value out of range & C99's PRI* macros

  - by Yktula

  Say we have an 8-bit unsigned integer n (UINT8_MAX=255); what is the behavior of the compiler for n=256? Where can I find a table of default behavior when the value of a data type is out of range for different data types? Is there a pattern to how they behave when set out of range? #include <stdio.h> #include <inttypes.h> uint8_t n = UINT8_MAX; int main() { printf("%hhu ",n++); printf("%hhu",n); return 0; } Compiling with gcc -std=c99 -Wall *.c, this prints: 255 0 Also, is it acceptable to use C99's PRI* macros? How are they named?

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• C++ STL map.find() not finding my stuff

  - by Joe

  Hello, I have constructed a map and loaded it with data. If I iterate over all the elements I see they are all valid. However, the find method doesn't find my item. I'm sure it's something stupid I am doing. Here is snippet: // definitions // I am inserting a person class and using the firstname as the key typedef std::map<char*,Person *> mapType; mapType _myMap; mapType::iterator _mapIter; ... Person *pers = new Person(FirstName, LastName, Address, Phone); _myMap.insert(make_pair(pers->firstName, pers); ... ...later.... _mapIter = _myMap.find(firstName); // returns map.end _mapIter = _myMap.find("joe"); // returns map.end and I have no idea why :(

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• some verd problems in qt

  - by prabhakaran

  I am very new to qt, So whatever I facing is either errors or problems. Here goes some of them, 1)Just try to install it in VisualStudio, you will got enough for the day. 2)After you installed it as a separate qt(without embedding it inside visual studio).Open a c++ file in qt, = then you won't get any option to compile it. 3)Create a empty qt4 project like below #include<iostream> using namespace std; int main(int a,char * argv[]) { } Then build it, you will get a error like this C:\qt-greenhouse\Trolltech\Code_less_create_more\Trolltech\Code_less_create_more\Troll\4.6\qt\src\winmain/qtmain_win.cpp:131: undefined reference to `qMain(int, char**)' Can anybody clear any of these problems to me.

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• the problem only happens when i try create a release...

  - by ace

  I'm sorry if im not presenting this right, but i trully cannot understand what the problem is. i have a project to hand in, a code of 600 lines defined within a main, .cpp, and header file. if i compile the project with just a debugger and no release, it's fine. when i create it with the release, the following error occurs, for every function!!! 1st error: |36|multiple definition of `countLines(int&, std::vector const&)'| 2nd error: |36|first defined here| if someone will allow me and i can send them the entire code, that would be awesome - i have to have this done within 3 hours.

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• C++: Comparing list of doubles with some invalid values (QNAN)

  - by J.M.

  Hello, i need to compare two std::list < double , but some doubles may be invalid numbers (QNAN). If any invalid numbers are list entries the compare process won't work, because a comparison of the same invalid value will always result in 'false'. What is the easiest and most elegant way to solve the problem? My idea was to create copies of both lists, iterate through them and remove invalid values and then compare the remaining lists. The lists will typically have 20-50 values in them. Is there a more resource friendly way to solve it?

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• Container for database-like searches

  - by Milan Babuškov

  I'm looking for some STL, boost, or similar container to use the same way indexes are used in databases to search for record using a query like this: select * from table1 where field1 starting with 'X'; or select * from table1 where field1 like 'X%'; I thought about using std::map, but I cannot because I need to search for fields that "start with" some text, and not those that are "equal to". I could create a sorted vector or list and use binary search (breaking the set in 2 in each step by reading the element in the middle and seeing if it's more or less than 'X'), but I wonder if there is some ready-made container I could use without reinventing the wheel?

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• Why won't this compile and how can it be implemented so that it does?

  - by George Edison

  Here is some C++ code I'm playing around with: #include <iostream> #include <vector> #define IN , #define FOREACH(x,y) for(unsigned int i=0;i<y.size();i++) { x=y[i]; #define ENDFOREACH } using namespace std; int main() { vector<int> ints; ints.push_back(3); ints.push_back(4); ints.push_back(5); ints.push_back(6); FOREACH(int item IN ints) cout << item; ENDFOREACH return 0; } However, I get an error: macro "FOREACH" requires 2 arguments, but only 1 given The code compiles if I change the IN to a comma. How can I get the IN to take the place of a comma?

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• how to read integers from file and write each integer into different name integer?

  - by Aivaras

  This is how the program looks and i need to make all integers with different name. Like x,x1,x2 and so on... #include <cstdlib> #include <iostream> #include <fstream> using namespace std; int main() { ifstream iFile("input.txt"); // input.txt has integers, one per line while (true) { int x; iFile >> x; if( iFile.eof() ) break; cerr << x << endl; } system("Pause"); return 0; }

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• How to interrupt a waiting C++0x thread?

  - by doublep

  I'm considering to use C++0x threads in my application instead of Boost threads. However, I'm not sure how to reimplement what I have with standard C++0x threads since they don't seem to have an interrupt() method. My current setup is: a master thread that manages work; several worker threads that carry out master's commands. Workers call wait() on at least two different condition variables. Master has a "timed out" state: in this case it tells all workers to stop and give whatever result they got by then. With Boost threads master just uses interrupt_all() on a thread group, which causes workers to stop waiting. In case they are not waiting at the moment, master also sets a bool flag which workers check periodically. However, in C++0x std::thread I don't see any replacement for interrupt(). Do I miss something? If not, how can I implement the above scheme so that workers cannot just sleep forever?

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