I have a question on local class inserting to a map.
For testing I created a class in a function without newing so that the scope is in local.
Then I inserted the class as a value in a map.
I thought then the class would be destroyed since the scope of the class is local.
However it is showing the inserted value( which is 40 in below code).
Is it that the OS has lazy delete on memory? My code is as follow
#include <cstdlib>
#include <unordered_map>
#include <iostream>
using namespace std;
class MyClass{
public:
MyClass(int id){m_id = id;};
void PrintThis(){cout << " This is test "<< m_id << endl;};
int m_id;
};
class Test{
public:
unordered_map<int, MyClass> mapTest2;
void TestLocal(){
MyClass mc1(10);
MyClass mc2(20);
MyClass mc3(30);
mapTest2.insert(make_pair(1, mc1));
mapTest2.insert(make_pair(2, mc2));
mapTest2.insert(make_pair(3, mc3));
}
void TestLocal2(){
MyClass mc1(40);
MyClass mc2(50);
MyClass mc3(60);
mapTest2.insert(make_pair(4, mc1));
mapTest2.insert(make_pair(5, mc2));
mapTest2.insert(make_pair(6, mc3));
}
};
int main(){
Test* tt1 = new Test();
tt1->TestLocal();
tt1->TestLocal2();
auto search2 = tt1->mapTest2.find(4);
if(search2 != tt1->mapTest2.end()) {
MyClass mc = search2->second;
cout << mc.m_id << endl;
}else{
cout << "not Found2 " << endl;
}
}
Related
I am learning smart pointers in C++11.
I am facing an compilation error with operator overloading using smart pointers.
Can someone please suggest where I am doing wrong.
Below is the code
#include <iostream>
#include <memory>
using namespace std;
class myClass
{
private:
string name;
public:
myClass() { }
myClass(string n) : name(n) { }
shared_ptr<myClass> operator + (shared_ptr<myClass> &obj)
{
cout << "Operator Overloading" << endl;
shared_ptr<myClass> temp;
temp->name = this->name;
temp->name += obj->name;
return temp;
}
void print()
{
cout << name << endl;
}
};
int main()
{
shared_ptr<myClass> myPtr1 = make_shared<myClass>("Hello");
myPtr1->print();
shared_ptr<myClass> myPtr2 = make_shared<myClass>("Operator Overloading");
myPtr2->print();
shared_ptr<myClass> myPtr3;
myPtr3 = myPtr1 + myPtr2; //Calling Operator overloading
myPtr3->print();
return 0;
}
I'm writing a test program about c++ type erasure, the code is put on the end.
when I run program , the test case 2 output as follow:
A default cstr...0x7ffe0fe5158f
obj_:0x7ffe0fe5158f objaaa 0x7ffe0fe5158f
Print A 0x7ffe0fe5158f
my machine: Linux x86-64, gcc 4.8
In my opinion, "Object obj2(a2);" makes a class Model by lvalue reference, so it should call A's copy constructor,
but actually it did not work, it makes me confused.
someone can give a explanation, thank you in advance.
the program is list as follow:
#include <memory>
#include <iostream>
class Object {
public:
template <typename T>
Object(T&& obj) : object_(std::make_shared<Model<T>>(std::forward<T>(obj))) {
}
void PrintName() {
object_->PrintName();
}
private:
class Concept {
public:
virtual void PrintName() = 0;
};
template <typename T>
class Model : public Concept {
public:
Model(T&& obj) : obj_(std::forward<T>(obj)) {
std::cout << "obj_:" << std::addressof(obj_) <<" objaaa " << std::addressof(obj) << std::endl;
}
void PrintName() {
obj_.PrintName();
}
private:
T obj_;
};
private:
std::shared_ptr<Concept> object_;
};
class A {
public:
A(A& a) {
std::cout<< "A copy cstr...a" << this << std::endl;
}
A(A&& a) {
std::cout << "A move cstr...." <<this<< std::endl;
}
A() {
std::cout << "A default cstr..." <<this<< std::endl;
}
void PrintName() {
std::cout << "Print A " << this << std::endl;
}
};
int main(void)
{
// test case 1
Object obj{A()};
obj.PrintName();
// test case 2
A a2;
Object obj2(a2);
obj2.PrintName();
return 0;
}
In Object obj2(a2);, no copy is made. T in the constructor of Object is deduced to be A&, so it instantiates Model<A&>, which stores a reference to the original a2 object as its obj_ member.
Observe that in your debug output, a2's constructor, Model's constructor and PrintName all print the same address. You can further confirm that this address is in fact &a2.
Reading Scott Meyer's book "Effective Modern C++", Item 24 (and following), and Item 41, I wonder that this book opposes:
the individual constructors for lvalue and rvalue parameters
to
a template'd universal constructor solution
It says, that 1. has the disadvantage to duplicate code.
Whereas 2. has the disadvantage to potentially being used for unwanted types.
I wonder why the book does not mention a mixed model - as in the example code shown below.
It uses type-safe dedicated constructors for lvalue and rvalue but delegates to a single (private) generic implementation for "universal reference".
This avoids unwanted template types of a public "universal reference" constructor.
So is there is anything wrong with the approach below? Something I missed?
#include <iostream>
#include <string>
class MyClass
{
private:
enum class Dummy { Nop = 0 } ;
template <class T>
MyClass(Dummy, T&& data)
: _data(std::forward<T>(data))
{
std::cout << "MyClass universal reference template c'tor" << std::endl;
}
public:
// proxy c'tors delegating to universal reference c'tor
MyClass (std::string const & data)
: MyClass(Dummy::Nop, data)
{
std::cout << "MyClass lvalue c'tor" << std::endl;
}
MyClass (std::string && data)
: MyClass(Dummy::Nop, std::move(data))
{
std::cout << "MyClass rvalue c'tor" << std::endl;
}
private:
std::string _data;
};
int main(int, char**)
{
{
std::string str("demo");
MyClass myClass(str);
}
{
MyClass myClass("hello, world");
}
return 0;
}
And now let's put the book down and do it the right way:
Pros:
Optimal efficiency
Correct type limitations
DRY
Cons:
None
-
#include <iostream>
#include <string>
#include <type_traits>
class MyClass
{
public:
template <class T, std::enable_if_t<std::is_constructible<std::string, T>::value>* = nullptr>
MyClass(T&& data)
: _data(std::forward<T>(data))
{
std::cout << "MyClass universal reference template c'tor" << std::endl;
}
private:
std::string _data;
};
int main()
{
using namespace std::string_literals;
auto a = MyClass("hello"s);
auto b = MyClass("world");
const auto s = "Hello, World"s;
auto s2 = "Hello, World";
auto c = MyClass(s);
auto d = MyClass(s2);
// won't compile
// auto e = MyClass(10);
}
I'm trying to refactor a rather complicated piece of code and run into segfaults during loading the library. This is a minimal example of what I could single out to be the source of the segfault:
#include <iostream>
#include <string>
#include <map>
class Manager {
public:
class Action{
};
static bool registerAction(const Action* a, const char* name);
};
namespace Actions {
class ExampleAction : public Manager::Action {
};
namespace {
static bool available = Manager::registerAction(new ExampleAction(),"ExampleAction");
}
}
namespace {
typedef std::map<const std::string,const Manager::Action*> ActionList;
static ActionList sActions;
}
bool Manager::registerAction(const Action* a, const char* name){
std::cout << "attempting to register action " << a << " as " << name << std::endl;
sActions[name] = a;
std::cout << "done" << std::endl;
return true;
}
int main(){
std::cout << "hello!" << std::endl;
for(auto it:sActions){
std::cout << it.first << std::endl;
}
std::cout << "world!" << std::endl;
return 0;
}
It compiles fine with g++ 4.8.4 using the --std=c++11 flag, but upon execution, this happens:
attempting to register action 0x1ebe010 as ExampleAction
Segmentation fault (core dumped)
The line attempting to register comes first, which is of course expected, but the line assigning the value to the static map instance causes the crash, and I don't understand the reason. I'm probably being stupid here, but still - any suggestions on how to fix this?
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) {}