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• how to get stl map to construct/destruct inserted object only once.

  - by Alberto Toglia

  I have found a very prejudicial fact about stl maps. For some reason I cant get objects being inserted in the map to get constructed/destructed only once. Example: struct MyObject{ MyObject(){ cout << "constructor" << endl; } ~MyObject(){ cout << "destructor" << endl; } }; int main() { std::map<int, MyObject> myObjectsMap; myObjectsMap[0] = MyObject(); return 0; } returns: constructor destructor destructor constructor destructor If I do: typedef std::pair<int, MyObject> MyObjectPair; myObjectsMap.insert( MyObjectPair(0,MyObject())); returns: constructor destructor destructor destructor I'm inserting Objects responsible for their own memory allocation, so when destructed they'll clean themselves up, being destructed several times is causing me some trouble.

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• C++ Access variable value using string representing variable's name

  - by Paul Ridgway

  Hello everyone, If the title was not clear, I will try to clarify what I am asking: Imagine I have a variable called counter, I know I can see its current value by doing something like: std::cout << counter << std::endl; However, assume I have lots of variables and I don't know which I'm going to want to look at until runtime. Does anyone know a way I can fetch the value of a variable by using its name, for example: std::cout << valueOf("counter") << std::endl; I feel being able to do this might make debugging large complex projects easier. Thanks in advance for your time. PS: Please do not respond with 'Google it', I have, though maybe not with the best query to get the answer I'm looking for...

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• Question on Virtual Methods

  - by bobber205

  IF both methods are declared as virtual, shouldn't both instances of Method1() that are called be the derived class's Method1()? I am seeing BASE then DERIVED called each time. I am doing some review for an interview and I want to make sure I have this straight. xD class BaseClass { public: virtual void Method1() { cout << "Method 1 BASE" << endl; } }; class DerClass: public BaseClass { public: virtual void Method1() { cout << "Method 1 DERVIED" << endl; } }; DerClass myClass; ((BaseClass)myClass).Method1(); myClass.Method1(); Method 1 BASE Method 1 DERVIED

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• [C++] Question on Virtual Methods

  - by bobber205

  IF both methods are declared as virtual, shouldn't both instances of Method1() that are called be the derived class's Method1()? I am seeing BASE then DERIVED called each time. I am doing some review for an interview and I want to make sure I have this straight. xD class BaseClass { public: virtual void Method1() { cout << "Method 1 BASE" << endl; } }; class DerClass: public BaseClass { public: virtual void Method1() { cout << "Method 1 DERVIED" << endl; } }; DerClass myClass; ((BaseClass)myClass).Method1(); myClass.Method1(); Method 1 BASE Method 1 DERVIED

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• C++ pointer to functions, Beginner Question...

  - by BobAlmond

  Hi all, I want to ask about pointer in C++ I have some simple code: int add(int a, int b){ return a+b; } int runner(int x,int y, int (*functocall)(int, int)){ return (*functocall)(x,y); } now, suppose I call those functions using this way : cout<<runner(2,5,&add); or maybe cout<<runner(2,5,add); is there any difference? because when I tried, the result is the same and with no error. Thanks a lot

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• How to define a custom iterator in C++

  - by Robert Martin

  I've seen a number of posts on SO about how to define custom iterators, but nothing that seems to exactly answers my question, which is... How do I create an iterator that hides a nested for loop? For instance, I have a class Foo, inside of the Foo is a Bar, and inside of the Bar is a string. I could write for (const Foo& foo : foo_set) for (const Bar& bar : foo.bar_set) if (bar.my_string != "baz") cout << bar.my_string << endl; but instead I want to be able to do something like: for (const string& good : foo_set) cout << good << endl; How do I do something like this?

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• C++ beginner question regarding chars

  - by Samwhoo

  I'm just messing around with some C++ at the moment trying to make a simple tic-tac-toe game and I'm running into a bit of a problem. This is my code: #include <iostream> using namespace std; class Square { public: char getState() const; void setState(char); Square(); ~Square(); private: char * pState; }; class Board { public: Board(); ~Board(); void printBoard() const; Square getSquare(short x, short y) const; private: Square board[3][3]; }; int main() { Board board; board.getSquare(1,2).setState('1'); board.printBoard(); return 0; } Square::Square() { pState = new char; *pState = ' '; } Square::~Square() { delete pState; } char Square::getState() const { return *pState; } void Square::setState(char set) { *pState = set; } Board::~Board() { } Board::Board() { } void Board::printBoard() const { for (int x = 0; x < 3; x++) { cout << "|"; for (int y = 0; y < 3; y++) { cout << board[x][y].getState(); } cout << "|" << endl; } } Square Board::getSquare(short x, short y) const { return board[x][y]; } Forgive me if there are blatantly obvious problems with it or it's stupidly written, this is my first program in C++ :p However, the problem is that when I try and set the square 1,2 to the char '1', it doesn't print out as a 1, it prints out as some strange character I didn't recognise. Can anyone tell me why? :) Thanks in advance.

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• C++ Arrays of Structure access

  - by learningtolive

  Hi, I'm studying C++ from Schildt's book and don't quite understand what does he mean under third structure; Can somebody explain this - To access a specific structure within an array of structures, you must index the structure name. For example, to display the on_hand member of the third structure, you would write cout cout << invtry[2].on_hand; Some code: struct type{ char item[40]; double cost; double retail; int on_hand; int lead_time; }invtry[SIZE];

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• can we write this in C++ switch ?

  - by jellly

  #include <iostream> using namespace std; int main(){ char i; cin >>i; switch (i){ case ('e'||'i'||'o'||'u'||'a'): cout<<"Vowel"; break; case ('+'||'-'||'/'||'*'||'%'): cout<<"Op"; break; } return 0; } if not than how can we use comparison or logical operators in switch ? & why cant we declare and initialize variable in single case without using scope ?

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• C++ STL: Trouble with string iterators

  - by Rosarch

  I'm making a simple command line Hangman game. void Hangman::printStatus() { cout << "Lives remaining: " << livesRemaining << endl; cout << getFormattedAnswer() << endl; } string Hangman::getFormattedAnswer() { return getFormattedAnswerFrom(correctAnswer.begin(), correctAnswer.end()); } string Hangman::getFormattedAnswerFrom(string::const_iterator begin, string::const_iterator end) { return begin == end? "" : displayChar(*begin) + getFormattedAnswerFrom(++begin, end); } char Hangman::displayChar(const char c) { return c; } (Eventually, I'll change this so displayChar() displays a - or a character if the user has guessed it, but for simplicity now I'm just returning everything.) When I build and run this from VS 2010, I get a popup box: Debug Assertion Failed! xstring Line: 78 Expression: string iterator not dereferenceable What am I doing wrong?

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• Receiving integers, but also want to test for char.

  - by Wayne Haworth

  Say I am looking to receive a series of numeric values and read them into an int, but I also want to test if the user hit key 'x'. I am sure I am missing something obvious, and have tried a few things but seem to be stuck. This is what I have so far... cout << endl << "Enter key (or 'x' to exit): "; cin key; if (key == 'x') { cout << "exiting";} // continue on...

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• Array not returned correctly

  - by hp1

  I am trying to return a simple array, but I am not getting the correct result. I am getting the following arr1[0] = 1 arr1[1] = 32767 result while the result should have been arr1[0] = 1 arr1[1] = 15 Please suggest. int *sum(int a, int b){ int arr[2]; int *a1; int result = a+b; arr[0]= 1; arr[1]= result; a1 = arr; return a1; } int main(){ int *arr1 = sum(5,10); cout<<"arr1[0] = "<<arr1[0]<<endl; cout<<"arr1[1] = "<<arr1[1]<<endl; return 0; }

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• C++ open() fails for no apparant reason

  - by jondoe

  The following code: char filename[64]; ifstream input; cout << "Please enter the filename: " << endl; cin >> filename; input.open(filename); if (!input.is_open()) { cout << "Opening file " << filename << " failed." << endl; exit(1); } fails, it enters the if() and exits. What could possibly be the cause for this? I'm using Microsoft Visual C++. When I hardcoded the filename as a constant it instead ended up garbled: http://pici.se/pictures/CNQEnwhgo.png Suggestions?

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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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• glReadPixels() returning non-accurate value

  - by max

  I'm trying to implement the flood fill algorithm. But glReadPixels() is returning float RGB values of a pixel which are slightly different from the actual value set by me, causing the algorithm to fail. Why is this happening? Outputting returned RGB values to check. #include<iostream> #include<GL/glut.h> using namespace std; float boundaryColor[3]={0,0,0}, interiorColor[3]={0,0,0.5}, fillColor[3]={1,0,0}; float readPixel[3]; void init(void) { glClearColor(0,0,0.5,0); glMatrixMode(GL_PROJECTION); gluOrtho2D(0,500,0,500); } void setPixel(int x,int y) { glColor3fv(fillColor); glBegin(GL_POINTS); glVertex2f(x,y); glEnd(); } void getPixel(int x, int y, float *color) { glReadPixels(x,y,1,1,GL_RGB,GL_FLOAT,color); } void floodFill(int x,int y) { getPixel(x,y,readPixel); //outputting values here to check cout<<readPixel[0]<<endl; cout<<readPixel[1]<<endl; cout<<readPixel[2]<<endl; if( readPixel[0]==interiorColor[0] && readPixel[1]==interiorColor[1] && readPixel[2]==interiorColor[2] ) { setPixel(x,y); floodFill(x+1,y); floodFill(x,y+1); floodFill(x-1,y); floodFill(x,y-1); } } void display() { glClear(GL_COLOR_BUFFER_BIT); glColor3fv(boundaryColor); glLineWidth(3); glBegin(GL_LINE_STRIP); glVertex2i(150,150); glVertex2i(150,350); glVertex2i(350,350); glVertex2i(350,150); glVertex2i(150,150); glEnd(); floodFill(200,200); glFlush(); } int main(int argc,char** argv) { glutInit(&argc,argv); glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); glutInitWindowPosition(100,100); glutInitWindowSize(500,500); glutCreateWindow("Flood fill"); init(); glutDisplayFunc(display); glutMainLoop(); }

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• Why is the destructor of the class called twice ?

  - by dicaprio

  Apologies if the question sounds silly, I was following experts in SO and trying some examples myself, and this is one of them. I did try the search option but didn't find an answer for this kind. class A { public: A(){cout<<"A Contruction"<<endl;} ~A(){cout<<"A destruction"<<endl;} }; int main() { vector<A> t; t.push_back(A()); // After this line, when the scope of the object is lost. } Why is the destructor of the class called twice ?

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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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• How to update a vector in method

  - by gurpinars

  I'm new to C++ and trying to understand vectors. My goal is to update a vector in method: #include <vector> #include <iostream> using namespace std; void test(vector<int>& array){ for(int i=0;i<10;i++){ array.push_back(i); } } int main(){ // some integer value vector<int> array(10); test(array); for(int i=0;i<array.size();++i) cout<<array.at(i)<<endl; cout<<"array size:"<<array.size()<<endl; return 0; } output: 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 array size:20 I haven't figure out why 10 zeros add vector at first?

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• char pointer array in c#

  - by james

  consider the following c++ code #include "stdafx.h" #include<iostream> using namespace std; void ping(int,char* d[]); void ping(int a,char *b[]) { int size; size=sizeof(b)/sizeof(int); // total size of array/size of array data type //cout<<size; for(int i=0;i<=size;i++) cout<<"ping "<<a<<b[i]<<endl; } int _tmain(int argc, _TCHAR* argv[]) { void (*funcptr)(int,char* d[]); char* c[]={"a","b"}; funcptr= ping; funcptr(10,c); return 0; } how can i implement the same in c#.. m new to c#. how can i have char pointer array in c#?

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• how to take input for uint32_t variable

  - by sksingh73

  i have declared a variable as uint32_t num; when i take input for this variable i.e cinnum; cout< when i run this program, i gives num value as 12345678, it works fine & give cout. but when i give input as 1234abcd. it creates problem & displays only 1234. please suggest solution to resolve this problem.

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• Why is this C++ code working ?

  - by gregseth

  Why doesn't the program segfault on the p->info() call? struct A { int info() { retrun (this) ? 1 : -1; } }; int main() { A* p = NULL; if (p->info() == 1) { cout << "I'm alive!" << endl; } else { cout << "I'm NULL..." << endl; } return 0; }

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• How to know if the argument that is passed to the function is a class, union or enum in c++?

  - by Narek

  I want to define an operator<< for all enums, to cout the value and print that it is an enum like this: code: enum AnyEnum{A,B,C}; AnyEnum enm = A; cout << enm <<endl; output: This is an enum which has a value equal to 0 I know a way of doing this with Boost library by using is_enum struct. But I don’t understand how it works. So that's why, in general, I am interested how to identify if the veriable is a class type, union type or an enum (in compile time).

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• Need help with map (c++, STL)

  - by Mike Dooley

  Hi folks! Actually I'm new to C++. I tried something out (actually the map container) but it doesn't work the way I assumed it will... Before posting my code, I will explain it shortly. I created 3 classes: ClassA ClassDerivedA ClassAnotherDerivedA The two last ones are derived from "ClassA". Further I created a map: map<string,ClassA> test_map; I put some objects (from Type ClassDerivedA and ClassAnotherDerivedA) into the map. Keep in mind: the mapped value is from type "ClassA". This will only work because of Polymorphism. Finally I created an iterator which runs over my map and compares the user input with my keys in the map. If they match, it will call a specific method called "printOutput". And there is the Problem: Although i declared "printOutput" as "virtual" the only method called is the one from my base class, but why? and here is the code: #include <iostream> #include <map> using namespace std; class ClassA { public: virtual void printOutput() { cout << "ClassA" << endl; } }; class ClassDerivedA : public ClassA { public: void printOutput() { cout << "ClassDerivedA" << endl; } }; class ClassAnotherDerivedA: public ClassA { public: void printOutput() { cout << "ClassAnotherDerivedA" << endl; } }; int main() { ClassDerivedA class_derived_a; ClassAnotherDerivedA class_another_a; map<string,ClassA> test_map; test_map.insert(pair<string,ClassA>("deriveda", class_derived_a)); test_map.insert(pair<string,ClassA>("anothera", class_another_a)); string s; while( cin >> s ) { if( s != "quit" ) { map<string,ClassA>::iterator it = test_map.find(s); if(it != test_map.end()) it->second.printOutput(); } else break; } } Blockquote

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• array of objects of a class

  - by anurag18294

  #include class test{ int a; char b; public: test() { cout<<"\n\nDefault constructor being called"; } test(int i,char j) { a=i; b=j; cout<<"\n\nConstructor with arguments called"; } }; int main() { test tarray[5]; test newobj(31,'z'); }; In the above code snippet can we intialize values to tarray[5].

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• C++: Print only one char

  - by Martijn Courteaux

  Hi, When I read one char* from std::cin and I want to write it to std::cout, it prints until it finds a \0 in memory. So what did was: char c; cin >> c; char* pChar = &c; pChar++; *pChar = '\0'; println(&c); // My own method: void println(char * str) { cout << str << endl; } But I don't think this is a safe action. Is there a safer way to do this?

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