When I try to implement friend function inside the class as shown below, I get an error, and I don't know why... It only happens when a non-argumental friend function is declared inside the class. The error is
display() is not defined in this scope
Code:
#include<iostream>
using namespace std;
class test{
private:
int x = 5;
public:
friend void display(){
test obj;
cout << obj.x << endl;
}
};
int main(){
display();
return 0;
}
Output should be simply: 5
But I get this error:
display is not defined()
I'm not sure why you need to do this, but if you have to, define the display() method outside the class.
class Test
{
private:
int x = 5;
public:
friend void display();
};
void display()
{
Test obj;
cout << obj.x << endl;
}
int main()
{
display();
return 0;
}
Though in general, I try to avoid friend classes/functions. I don't know exactly what you're trying to accomplish, but I'd rethink your approach.
Related
I'm trying to overload << to print the protected members of a class as a string, but when I try to use it in another class doing std::cout << player2; I get "0x7f60b0100" as output.
"player2" is an Actor*, so I'm not sure what's happening.
class Actor {
private:
string type;
protected:
int health;
int damage;
vector<MoveType> moves;
public:
Actor(string type, int health): type{ type }, health{ health }{damage=0;}
virtual void Hit(int damage){health = health-damage;}
virtual void Heal(int amount){health=+amount;}
const vector<MoveType>& GetMoves() const {return moves;}
bool IsDead() { return health <= 0; }
friend ostream& operator<<(ostream& out, const Actor& actor){
return (out << "DAMAGE DONE: " << actor.damage << "HEALTH: "<< actor.health);
}
};
As you've said it's a pointer to an Actor instance, so that's what you get printed, the value of this pointer.
You need to derefernce the pointer:
std::cout << *player2;
Historically, I've been using trait classes to hold information and apply that into a "generic" function that runs the same "algorithm." Only differed by the trait class. For example: https://onlinegdb.com/ryUo7WRmN
enum selector { SELECTOR1, SELECTOR2, SELECTOR3, };
// declaration
template < selector T> struct example_trait;
template<> struct example_trait<SELECTOR1> {
static constexpr size_t member_var = 3;
static size_t do_something() { return 0; }
};
template<> struct example_trait<SELECTOR2> {
static constexpr size_t member_var = 5;
static size_t do_something() { return 0; }
};
// pretend this is doing something useful but common
template < selector T, typename TT = example_trait<T> >
void function() {
std::cout << TT::member_var << std::endl;
std::cout << TT::do_something() << std::endl;
}
int main()
{
function<SELECTOR1>();
function<SELECTOR2>();
return 0;
}
I'm not sure how to create "generic" algorithms this when dealing with polymorphic classes.
For example: https://onlinegdb.com/S1hFLGC7V
Below I have created an inherited class hierarchy. In this example I have a base catch-all example that defaults all the parameters to something (0 in this case). And then each derived class sets overrides specific methods.
#include <iostream>
#include <memory>
#include <type_traits>
#include <assert.h>
using namespace std;
struct Base {
virtual int get_thing_one() {
return 0;
}
virtual int get_thing_two() {
return 0;
}
virtual int get_thing_three() {
return 0;
}
virtual int get_thing_four() {
return 0;
}
};
struct A : public Base {
virtual int get_thing_one() override {
return 1;
}
virtual int get_thing_three() override {
return 3;
}
};
struct B : public Base {
virtual int get_thing_one() override {
return 2;
}
virtual int get_thing_four() override{
return 4;
}
};
Here I created a simple factory, not elegant but for illustrative purposes
// example simple factory
std::shared_ptr<Base> get_class(const int input) {
switch(input)
{
case 0:
return std::shared_ptr<Base>(std::make_shared<A>());
break;
case 1:
return std::shared_ptr<Base>(std::make_shared<B>());
break;
default:
assert(false);
break;
}
}
So this is the class of interest. It is a class does "something" with the data from the classes above. The methods below are a simple addition example but imagine a more complicated algorithm that is very similar for every method.
// class that uses the shared_ptr
class setter {
private:
std::shared_ptr<Base> l_ptr;
public:
setter(const std::shared_ptr<Base>& input):l_ptr(input)
{}
int get_thing_a()
{
return l_ptr->get_thing_one() + l_ptr->get_thing_two();
}
int get_thing_b()
{
return l_ptr->get_thing_three() + l_ptr->get_thing_four();
}
};
int main()
{
constexpr int select = 0;
std::shared_ptr<Base> example = get_class(select);
setter l_setter(example);
std::cout << l_setter.get_thing_a() << std::endl;
std::cout << l_setter.get_thing_b() << std::endl;
return 0;
}
How can I make the "boilerplate" inside the setter class more generic? I can't use traits as I did in the example above because I can't tie static functions with an object. So is there a way to make the boilerplate example more common?
Somewhere along the lines of having a selector, say
enum thing_select { THINGA, THINGB, };
template < thing_select T >
struct thing_traits;
template <>
struct thing_traits<THINGA>
{
static int first_function() --> somehow tied to shared_ptr<Base> 'thing_one' method
static int second_function() --> somehow tied to shared_ptr<Base> 'thing_two' method
}
template <>
struct thing_traits<THINGB>
{
static int first_function() --> somehow tied to shared_ptr<Base> 'thing_three' method
static int second_function() --> somehow tied to shared_ptr<Base> 'thing_four' method
}
// generic function I'd like to create
template < thing_select T, typename TT = thing_traits<T> >
int perform_action(...)
{
return TT::first_function(..) + TT::second_function(..);
}
I ideally would like to modify the class above to something along the lines of
// Inside setter class further above
int get_thing_a()
{
return perform_action<THINGA>(...);
}
int get_thing_b()
{
return perform_action<THINGB>(...);
}
The answer is, maybe I can't, and I need to pass int the shared_ptr as a parameter and call the specific methods I need instead of trying to tie a shared_ptr method to a static function (in hindsight, that doesn't sound like a good idea...but I wanted to bounce my idea)
Whoever makes the actual call will need a reference of the object, one way or the other. Therefore, assuming you want perform_action to perform the actual call, you will have to pass the parameter.
Now, if you really want to store which function of Base to call as a static in thing_traits without passing a parameter, you can leverage pointer to member functions:
template <>
struct thing_traits<THINGA>
{
static constexpr int (Base::*first_function)() = &Base::get_thing_one;
...
}
template < thing_select T, typename TT = thing_traits<T>>
int perform_action(Base & b)
{
return (b.*TT::first_function)() + ...;
}
You can also play instead with returning a function object that does the call for you (and the inner function takes the parameter).
It all depends on who you need to make the call and what information/dependencies you assume you have available in each class/template.
Here is what I am supposed to do.
Here is what I did.(on codeblocks IDE with minGW compiler)
#include <iostream>
using namespace std;
class person
{
private:
string name;
int age;
double height;
double weight;
public:
void set_private_members(string n, int a, double h, double w)
{
name = n;
age = a;
weight = h;
height = w;
}
void print_private_members()
{
cout <<"The name of person is:- "<<name<<"\n The age of person is:- " <<age<<"\n The weight of person is:- "<<weight;
cout <<"\n The height of person is:- "<<height<<endl;
}
};
void modify_person(person);
int main()
{
person p;
p.set_private_members("Ayush",19,165.7,47.2);
cout <<"We are in main function right now \n";
p.print_private_members();
modify_person(p);
cout <<"We are now back in main function \n The values of object passed to modify_person function are as \n";
p.print_private_members();
return 0;
}
void modify_person(person z)
{
cout <<"We are in modify_person function \n Modifying person details... \n";
z.set_private_members("Priyanshi",15,159.1,50.6);
cout <<"The details are now as:- \n";
z.print_private_members();
}
Here is the output to this code
however the expected output is to have priyanshi details in the class object when print_private_members function is called in main() for the last time.
Who is wrong? Me or they? I think if I call modify_person by reference then the expected should come.
I was trying to work on the below code but the program crashes:
#include <iostream>
#include <string>
#include <map>
using namespace std;
typedef void (*callBackMethod)(string);
class CTest
{
private:
map<string, callBackMethod> mapMethod;
void testMethod(string msg)
{
cout << msg << endl;
}
public:
CTest()
{
addFunction("AA", (callBackMethod) &CTest::testMethod);
}
void addFunction(string funName, callBackMethod methodName)
{
mapMethod[funName] = methodName;
}
callBackMethod getMethod(string funName)
{
auto fun = mapMethod.find(funName);
if(fun == mapMethod.end()) { return nullptr; }
return fun->second;
}
void runFunction(string funName)
{
getMethod(funName)("test");
}
};
int main()
{
CTest test;
test.runFunction("AA");
return 0;
}
I have a requirement where I need to pass private methods to a map. The program compiles with warning:
converting from 'void (CTest::*)(std::__cxx11::string) {aka void (CTest::*)(std::__cxx11::basic_string<char>)}' to 'callBackMethod {aka void (*)(std::__cxx11::basic_string<char>)}'
and when I execute this, it crashes.
When I move the callback method outside of the class it works. My requirement is to make the program flow this was (hide the methods from external call which needs to be added to a map).
Looking forward to your comments.
If you need to point to both CTest member functions and free functions, then you can use std::function<void(std::string)>.
#include <iostream>
#include <string>
#include <map>
#include <functional>
using namespace std;
using callBackFunction = std::function<void(string)>;
void testFunction(string msg)
{
cout << "[" << __PRETTY_FUNCTION__ << "] " << msg << endl;
}
class CTest
{
private:
map<string, callBackFunction> mapMethod;
void testMethod(string msg)
{
cout << "[" << __PRETTY_FUNCTION__ << "] " << msg << endl;
}
public:
CTest()
{
addFreeFunction("AA", testFunction);
addMemberFunction("BB", &CTest::testMethod);
}
void addMemberFunction(string funName, void(CTest::*methodName)(string))
{
using std::placeholders::_1;
mapMethod[funName] = std::bind(methodName, this, _1);
}
void addFreeFunction(string funName, void(*methodName)(string))
{
mapMethod[funName] = methodName;
}
callBackFunction getMethod(string funName)
{
auto fun = mapMethod.find(funName);
if(fun == mapMethod.end()) { return nullptr; }
return fun->second;
}
void runFunction(string funName)
{
getMethod(funName)("test");
}
};
int main()
{
CTest test;
test.runFunction("AA");
test.runFunction("BB");
return 0;
}
Notice that CTest must insert elements into the map in a different way depending on what type of function you are passing, since for member functions you must provide the object for which it is to be invoked, this in this example. This is achived by using std::bind.
Since you want to use member variables you need to specify the signature differently in your typedef:
In C++ Builder the following can be done:
typedef void(__closure *callBackMethod)(string);
If you do that, I do suggest that you keep a smart pointer to the object that the member belongs to so that you can check if the object is still valid before calling the function otherwise it will crash the application.
The __closure keyword is a C++ Builder extension to work around the requirement to use fully qualified member names source
To handle both global and member functions we have the following:
typedef void(__closure *callBackMethodMember)(string);
typedef void (*callBackMethodGlobal)(string);
/* And then on 2 overloaded functions */
void addFunction(string funName, callBackMethodMember methodName) {}
void addFunction(string funName, callBackMethodGlobal methodName) {}
I have to implement A valve Open function (for specified duration).
I am using boost::asio::deadline_timer
My class member function to open valve is:
bool Valves::valveOpen(ValveType type)
{
switch (type)
{
case eVentValve:
tblMap_.digitalInput[eVentValveK1].setBit();
if (tblMap_.digitalOutput[eOutK1VentValve].getBit())
{
isVentOpen_ = true;
}
return isVentOpen_;
case eVacuumPumpValve:
....
....
}
Class member function to close the valve is:
bool Valves::valveClose(ValveType type)
{
switch (type)
{
case eVentValve:
tblMap_.digitalInput[eVentValveK1].clearBit();
if (!tblMap_.digitalOutput[eOutK1VentValve].getBit())
{
isVentOpen_ = false;
}
return !isVentOpen_;
case eVacuumPumpValve:
....
....
}
I am trying to achieve the timer action as below
bool Valves::valveTimedOpen(ValveType type, int sec)
{
boost::asio::io_service io;
switch (type)
{
case eVentValve:
{
std::bind(&Valves::valveOpen, this, type); //Here
boost::asio::deadline_timer t(io, boost::posix_time::seconds(sec));
t.async_wait(std::bind(&Valves::valveClose, this, type));
boost::thread th(boost::bind(&boost::asio::io_service::run, &io));
return true;
}
case eVacuumPumpValve:
.....
.....
}
The code hits the line Here i.e.
std::bind(&Valves::valveOpen, this, type); but it does not go to bool Valves::valveOpen(ValveType type) function.
Can someone let me know the issue with this code?
Variables io and t go out of scope as soon as valveTimedOpen exits. You need to rethink the way you interact with the boost asio components e.g. the io_service could be a member of your class, and the timer could be dynamically allocated and needs to be deleted in the completion handler.
Also, keep in mind that if you plan on re-using an io_service object, you also need to reset it before calling run again.
auto fn = std::bind(&Test::Open, shared_from_this(), std::placeholders::_1);
fn(type);
Calls the Open() correctly.
io_service and boost::deadline_timer I have to make class member as suggested by #Ralf
Working Code:
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/thread.hpp>
#include <boost/asio.hpp>
class Test : public std::enable_shared_from_this <Test>
{
public:
Test() :io(), timer(io){}
void Open(int num);
void Close(int num);
void TimedOpen(int num, int dur);
void Run();
private:
boost::asio::io_service io;
boost::asio::deadline_timer timer;
};
void Test::Open(int num)
{
std::cout << "Open for Number : " << num << std::endl;
}
void Test::Close(int num)
{
std::cout << "Close for Number : " << num << std::endl;
}
void Test::TimedOpen(int num, int dur)
{
io.reset();
auto fn = std::bind(&Test::Open, shared_from_this(), std::placeholders::_1);
fn(num);
timer.expires_from_now( boost::posix_time::seconds(dur));
timer.async_wait(std::bind(&Test::Close, shared_from_this(), num));
Run();
std::cout << "Function Exiting" << std::endl;
}
void Test::Run()
{
boost::thread th(boost::bind(&boost::asio::io_service::run, &io));
}
int main()
{
auto t = std::make_shared<Test>();
t->TimedOpen(5, 5);
char line[128];
while (std::cin.getline(line, 128))
{
if (strcmp(line, "\n")) break;
}
return 0;
}