Related
I came across this strange code snippet which compiles fine:
class Car
{
public:
int speed;
};
int main()
{
int Car::*pSpeed = &Car::speed;
return 0;
}
Why does C++ have this pointer to a non-static data member of a class? What is the use of this strange pointer in real code?
It's a "pointer to member" - the following code illustrates its use:
#include <iostream>
using namespace std;
class Car
{
public:
int speed;
};
int main()
{
int Car::*pSpeed = &Car::speed;
Car c1;
c1.speed = 1; // direct access
cout << "speed is " << c1.speed << endl;
c1.*pSpeed = 2; // access via pointer to member
cout << "speed is " << c1.speed << endl;
return 0;
}
As to why you would want to do that, well it gives you another level of indirection that can solve some tricky problems. But to be honest, I've never had to use them in my own code.
Edit: I can't think off-hand of a convincing use for pointers to member data. Pointer to member functions can be used in pluggable architectures, but once again producing an example in a small space defeats me. The following is my best (untested) try - an Apply function that would do some pre &post processing before applying a user-selected member function to an object:
void Apply( SomeClass * c, void (SomeClass::*func)() ) {
// do hefty pre-call processing
(c->*func)(); // call user specified function
// do hefty post-call processing
}
The parentheses around c->*func are necessary because the ->* operator has lower precedence than the function call operator.
This is the simplest example I can think of that conveys the rare cases where this feature is pertinent:
#include <iostream>
class bowl {
public:
int apples;
int oranges;
};
int count_fruit(bowl * begin, bowl * end, int bowl::*fruit)
{
int count = 0;
for (bowl * iterator = begin; iterator != end; ++ iterator)
count += iterator->*fruit;
return count;
}
int main()
{
bowl bowls[2] = {
{ 1, 2 },
{ 3, 5 }
};
std::cout << "I have " << count_fruit(bowls, bowls + 2, & bowl::apples) << " apples\n";
std::cout << "I have " << count_fruit(bowls, bowls + 2, & bowl::oranges) << " oranges\n";
return 0;
}
The thing to note here is the pointer passed in to count_fruit. This saves you having to write separate count_apples and count_oranges functions.
Another application are intrusive lists. The element type can tell the list what its next/prev pointers are. So the list does not use hard-coded names but can still use existing pointers:
// say this is some existing structure. And we want to use
// a list. We can tell it that the next pointer
// is apple::next.
struct apple {
int data;
apple * next;
};
// simple example of a minimal intrusive list. Could specify the
// member pointer as template argument too, if we wanted:
// template<typename E, E *E::*next_ptr>
template<typename E>
struct List {
List(E *E::*next_ptr):head(0), next_ptr(next_ptr) { }
void add(E &e) {
// access its next pointer by the member pointer
e.*next_ptr = head;
head = &e;
}
E * head;
E *E::*next_ptr;
};
int main() {
List<apple> lst(&apple::next);
apple a;
lst.add(a);
}
Here's a real-world example I am working on right now, from signal processing / control systems:
Suppose you have some structure that represents the data you are collecting:
struct Sample {
time_t time;
double value1;
double value2;
double value3;
};
Now suppose that you stuff them into a vector:
std::vector<Sample> samples;
... fill the vector ...
Now suppose that you want to calculate some function (say the mean) of one of the variables over a range of samples, and you want to factor this mean calculation into a function. The pointer-to-member makes it easy:
double Mean(std::vector<Sample>::const_iterator begin,
std::vector<Sample>::const_iterator end,
double Sample::* var)
{
float mean = 0;
int samples = 0;
for(; begin != end; begin++) {
const Sample& s = *begin;
mean += s.*var;
samples++;
}
mean /= samples;
return mean;
}
...
double mean = Mean(samples.begin(), samples.end(), &Sample::value2);
Note Edited 2016/08/05 for a more concise template-function approach
And, of course, you can template it to compute a mean for any forward-iterator and any value type that supports addition with itself and division by size_t:
template<typename Titer, typename S>
S mean(Titer begin, const Titer& end, S std::iterator_traits<Titer>::value_type::* var) {
using T = typename std::iterator_traits<Titer>::value_type;
S sum = 0;
size_t samples = 0;
for( ; begin != end ; ++begin ) {
const T& s = *begin;
sum += s.*var;
samples++;
}
return sum / samples;
}
struct Sample {
double x;
}
std::vector<Sample> samples { {1.0}, {2.0}, {3.0} };
double m = mean(samples.begin(), samples.end(), &Sample::x);
EDIT - The above code has performance implications
You should note, as I soon discovered, that the code above has some serious performance implications. The summary is that if you're calculating a summary statistic on a time series, or calculating an FFT etc, then you should store the values for each variable contiguously in memory. Otherwise, iterating over the series will cause a cache miss for every value retrieved.
Consider the performance of this code:
struct Sample {
float w, x, y, z;
};
std::vector<Sample> series = ...;
float sum = 0;
int samples = 0;
for(auto it = series.begin(); it != series.end(); it++) {
sum += *it.x;
samples++;
}
float mean = sum / samples;
On many architectures, one instance of Sample will fill a cache line. So on each iteration of the loop, one sample will be pulled from memory into the cache. 4 bytes from the cache line will be used and the rest thrown away, and the next iteration will result in another cache miss, memory access and so on.
Much better to do this:
struct Samples {
std::vector<float> w, x, y, z;
};
Samples series = ...;
float sum = 0;
float samples = 0;
for(auto it = series.x.begin(); it != series.x.end(); it++) {
sum += *it;
samples++;
}
float mean = sum / samples;
Now when the first x value is loaded from memory, the next three will also be loaded into the cache (supposing suitable alignment), meaning you don't need any values loaded for the next three iterations.
The above algorithm can be improved somewhat further through the use of SIMD instructions on eg SSE2 architectures. However, these work much better if the values are all contiguous in memory and you can use a single instruction to load four samples together (more in later SSE versions).
YMMV - design your data structures to suit your algorithm.
You can later access this member, on any instance:
int main()
{
int Car::*pSpeed = &Car::speed;
Car myCar;
Car yourCar;
int mySpeed = myCar.*pSpeed;
int yourSpeed = yourCar.*pSpeed;
assert(mySpeed > yourSpeed); // ;-)
return 0;
}
Note that you do need an instance to call it on, so it does not work like a delegate.
It is used rarely, I've needed it maybe once or twice in all my years.
Normally using an interface (i.e. a pure base class in C++) is the better design choice.
IBM has some more documentation on how to use this. Briefly, you're using the pointer as an offset into the class. You can't use these pointers apart from the class they refer to, so:
int Car::*pSpeed = &Car::speed;
Car mycar;
mycar.*pSpeed = 65;
It seems a little obscure, but one possible application is if you're trying to write code for deserializing generic data into many different object types, and your code needs to handle object types that it knows absolutely nothing about (for example, your code is in a library, and the objects into which you deserialize were created by a user of your library). The member pointers give you a generic, semi-legible way of referring to the individual data member offsets, without having to resort to typeless void * tricks the way you might for C structs.
It makes it possible to bind member variables and functions in the uniform manner. The following is example with your Car class. More common usage would be binding std::pair::first and ::second when using in STL algorithms and Boost on a map.
#include <list>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
class Car {
public:
Car(int s): speed(s) {}
void drive() {
std::cout << "Driving at " << speed << " km/h" << std::endl;
}
int speed;
};
int main() {
using namespace std;
using namespace boost::lambda;
list<Car> l;
l.push_back(Car(10));
l.push_back(Car(140));
l.push_back(Car(130));
l.push_back(Car(60));
// Speeding cars
list<Car> s;
// Binding a value to a member variable.
// Find all cars with speed over 60 km/h.
remove_copy_if(l.begin(), l.end(),
back_inserter(s),
bind(&Car::speed, _1) <= 60);
// Binding a value to a member function.
// Call a function on each car.
for_each(s.begin(), s.end(), bind(&Car::drive, _1));
return 0;
}
You can use an array of pointer to (homogeneous) member data to enable a dual, named-member (i.e. x.data) and array-subscript (i.e. x[idx]) interface.
#include <cassert>
#include <cstddef>
struct vector3 {
float x;
float y;
float z;
float& operator[](std::size_t idx) {
static float vector3::*component[3] = {
&vector3::x, &vector3::y, &vector3::z
};
return this->*component[idx];
}
};
int main()
{
vector3 v = { 0.0f, 1.0f, 2.0f };
assert(&v[0] == &v.x);
assert(&v[1] == &v.y);
assert(&v[2] == &v.z);
for (std::size_t i = 0; i < 3; ++i) {
v[i] += 1.0f;
}
assert(v.x == 1.0f);
assert(v.y == 2.0f);
assert(v.z == 3.0f);
return 0;
}
One way I've used it is if I have two implementations of how to do something in a class and I want to choose one at run-time without having to continually go through an if statement i.e.
class Algorithm
{
public:
Algorithm() : m_impFn( &Algorithm::implementationA ) {}
void frequentlyCalled()
{
// Avoid if ( using A ) else if ( using B ) type of thing
(this->*m_impFn)();
}
private:
void implementationA() { /*...*/ }
void implementationB() { /*...*/ }
typedef void ( Algorithm::*IMP_FN ) ();
IMP_FN m_impFn;
};
Obviously this is only practically useful if you feel the code is being hammered enough that the if statement is slowing things done eg. deep in the guts of some intensive algorithm somewhere. I still think it's more elegant than the if statement even in situations where it has no practical use but that's just my opnion.
Pointers to classes are not real pointers; a class is a logical construct and has no physical existence in memory, however, when you construct a pointer to a member of a class it gives an offset into an object of the member's class where the member can be found; This gives an important conclusion: Since static members are not associated with any object so a pointer to a member CANNOT point to a static member(data or functions) whatsoever
Consider the following:
class x {
public:
int val;
x(int i) { val = i;}
int get_val() { return val; }
int d_val(int i) {return i+i; }
};
int main() {
int (x::* data) = &x::val; //pointer to data member
int (x::* func)(int) = &x::d_val; //pointer to function member
x ob1(1), ob2(2);
cout <<ob1.*data;
cout <<ob2.*data;
cout <<(ob1.*func)(ob1.*data);
cout <<(ob2.*func)(ob2.*data);
return 0;
}
Source: The Complete Reference C++ - Herbert Schildt 4th Edition
Here is an example where pointer to data members could be useful:
#include <iostream>
#include <list>
#include <string>
template <typename Container, typename T, typename DataPtr>
typename Container::value_type searchByDataMember (const Container& container, const T& t, DataPtr ptr) {
for (const typename Container::value_type& x : container) {
if (x->*ptr == t)
return x;
}
return typename Container::value_type{};
}
struct Object {
int ID, value;
std::string name;
Object (int i, int v, const std::string& n) : ID(i), value(v), name(n) {}
};
std::list<Object*> objects { new Object(5,6,"Sam"), new Object(11,7,"Mark"), new Object(9,12,"Rob"),
new Object(2,11,"Tom"), new Object(15,16,"John") };
int main() {
const Object* object = searchByDataMember (objects, 11, &Object::value);
std::cout << object->name << '\n'; // Tom
}
Suppose you have a structure. Inside of that structure are
* some sort of name
* two variables of the same type but with different meaning
struct foo {
std::string a;
std::string b;
};
Okay, now let's say you have a bunch of foos in a container:
// key: some sort of name, value: a foo instance
std::map<std::string, foo> container;
Okay, now suppose you load the data from separate sources, but the data is presented in the same fashion (eg, you need the same parsing method).
You could do something like this:
void readDataFromText(std::istream & input, std::map<std::string, foo> & container, std::string foo::*storage) {
std::string line, name, value;
// while lines are successfully retrieved
while (std::getline(input, line)) {
std::stringstream linestr(line);
if ( line.empty() ) {
continue;
}
// retrieve name and value
linestr >> name >> value;
// store value into correct storage, whichever one is correct
container[name].*storage = value;
}
}
std::map<std::string, foo> readValues() {
std::map<std::string, foo> foos;
std::ifstream a("input-a");
readDataFromText(a, foos, &foo::a);
std::ifstream b("input-b");
readDataFromText(b, foos, &foo::b);
return foos;
}
At this point, calling readValues() will return a container with a unison of "input-a" and "input-b"; all keys will be present, and foos with have either a or b or both.
Just to add some use cases for #anon's & #Oktalist's answer, here's a great reading material about pointer-to-member-function and pointer-to-member-data.
https://www.dre.vanderbilt.edu/~schmidt/PDF/C++-ptmf4.pdf
with pointer to member, we can write generic code like this
template<typename T, typename U>
struct alpha{
T U::*p_some_member;
};
struct beta{
int foo;
};
int main()
{
beta b{};
alpha<int, beta> a{&beta::foo};
b.*(a.p_some_member) = 4;
return 0;
}
I love the * and & operators:
struct X
{
int a {0};
int *ptr {NULL};
int &fa() { return a; }
int *&fptr() { return ptr; }
};
int main(void)
{
X x;
int X::*p1 = &X::a; // pointer-to-member 'int X::a'. Type of p1 = 'int X::*'
x.*p1 = 10;
int *X::*p2 = &X::ptr; // pointer-to-member-pointer 'int *X::ptr'. Type of p2 = 'int *X::*'
x.*p2 = nullptr;
X *xx;
xx->*p2 = nullptr;
int& (X::*p3)() = X::fa; // pointer-to-member-function 'X::fa'. Type of p3 = 'int &(X::*)()'
(x.*p3)() = 20;
(xx->*p3)() = 30;
int *&(X::*p4)() = X::fptr; // pointer-to-member-function 'X::fptr'. Type of p4 = 'int *&(X::*)()'
(x.*p4)() = nullptr;
(xx->*p4)() = nullptr;
}
Indeed all is true as long as the members are public, or static
I think you'd only want to do this if the member data was pretty large (e.g., an object of another pretty hefty class), and you have some external routine which only works on references to objects of that class. You don't want to copy the member object, so this lets you pass it around.
A realworld example of a pointer-to-member could be a more narrow aliasing constructor for std::shared_ptr:
template <typename T>
template <typename U>
shared_ptr<T>::shared_ptr(const shared_ptr<U>, T U::*member);
What that constructor would be good for
assume you have a struct foo:
struct foo {
int ival;
float fval;
};
If you have given a shared_ptr to a foo, you could then retrieve shared_ptr's to its members ival or fval using that constructor:
auto foo_shared = std::make_shared<foo>();
auto ival_shared = std::shared_ptr<int>(foo_shared, &foo::ival);
This would be useful if want to pass the pointer foo_shared->ival to some function which expects a shared_ptr
https://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr
Pointer to members are C++'s type safe equivalent for C's offsetof(), which is defined in stddef.h: Both return the information, where a certain field is located within a class or struct. While offsetof() may be used with certain simple enough classes also in C++, it fails miserably for the general case, especially with virtual base classes. So pointer to members were added to the standard. They also provide easier syntax to reference an actual field:
struct C { int a; int b; } c;
int C::* intptr = &C::a; // or &C::b, depending on the field wanted
c.*intptr += 1;
is much easier than:
struct C { int a; int b; } c;
int intoffset = offsetof(struct C, a);
* (int *) (((char *) (void *) &c) + intoffset) += 1;
As to why one wants to use offsetof() (or pointer to members), there are good answers elsewhere on stackoverflow. One example is here: How does the C offsetof macro work?
How to initialize variable(s) dynamically i.e. being not as constant, in class declaration which will be invoked/used without need of object instantiation such as a constructor for member function call immediately gets returned in a function ?
There must, side by side of that variable(s) in that class, be a constant member.
enum Value :int { NO,LOW,MED,HIGH };
template<Value V> class valuation {
public:
valuation(const ints) : pos(s) {};
valuation(int a, int b, int c) : j(a), k(b), l(c) {};
value() {
if (!V)
return (j+k+l) * pos;
}
private:
const int pos;
int j,k,l;
};
int a_function(int s) {
return valuation<NO>(s).value(); // no need object at all
}
int main() {
// somehow in run-time j, k, l has been given 1,2,3, or other valid values
int v = a_function(7) // e.g.
}
Thanks in advance
I think you want to declare your value method -- or at least a version of it -- as a static member. This will allow you to call the method without instantiating an object from your class.
I have a function that takes a vector-like input. To simplify things, let's use this print_in_order function:
#include <iostream>
#include <vector>
template <typename vectorlike>
void print_in_order(std::vector<int> const & order,
vectorlike const & printme) {
for (int i : order)
std::cout << printme[i] << std::endl;
}
int main() {
std::vector<int> printme = {100, 200, 300};
std::vector<int> order = {2,0,1};
print_in_order(order, printme);
}
Now I have a vector<Elem> and want to print a single integer member, Elem.a, for each Elem in the vector. I could do this by creating a new vector<int> (copying a for all Elems) and pass this to the print function - however, I feel like there must be a way to pass a "virtual" vector that, when operator[] is used on it, returns this only the member a. Note that I don't want to change the print_in_order function to access the member, it should remain general.
Is this possible, maybe with a lambda expression?
Full code below.
#include <iostream>
#include <vector>
struct Elem {
int a,b;
Elem(int a, int b) : a(a),b(b) {}
};
template <typename vectorlike>
void print_in_order(std::vector<int> const & order,
vectorlike const & printme) {
for (int i : order)
std::cout << printme[i] << std::endl;
}
int main() {
std::vector<Elem> printme = {Elem(1,100), Elem(2,200), Elem(3,300)};
std::vector<int> order = {2,0,1};
// how to do this?
virtual_vector X(printme) // behaves like a std::vector<Elem.a>
print_in_order(order, X);
}
It's not really possible to directly do what you want. Instead you might want to take a hint from the standard algorithm library, for example std::for_each where you take an extra argument that is a function-like object that you call for each element. Then you could easily pass a lambda function that prints only the wanted element.
Perhaps something like
template<typename vectorlike, typename functionlike>
void print_in_order(std::vector<int> const & order,
vectorlike const & printme,
functionlike func) {
for (int i : order)
func(printme[i]);
}
Then call it like
print_in_order(order, printme, [](Elem const& elem) {
std::cout << elem.a;
});
Since C++ have function overloading you can still keep the old print_in_order function for plain vectors.
Using member pointers you can implement a proxy type that will allow you view a container of objects by substituting each object by one of it's members (see pointer to data member) or by one of it's getters (see pointer to member function). The first solution addresses only data members, the second accounts for both.
The container will necessarily need to know which container to use and which member to map, which will be provided at construction. The type of a pointer to member depends on the type of that member so it will have to be considered as an additional template argument.
template<class Container, class MemberPtr>
class virtual_vector
{
public:
virtual_vector(const Container & p_container, MemberPtr p_member_ptr) :
m_container(&p_container),
m_member(p_member_ptr)
{}
private:
const Container * m_container;
MemberPtr m_member;
};
Next, implement the operator[] operator, since you mentioned that it's how you wanted to access your elements. The syntax for dereferencing a member pointer can be surprising at first.
template<class Container, class MemberPtr>
class virtual_vector
{
public:
virtual_vector(const Container & p_container, MemberPtr p_member_ptr) :
m_container(&p_container),
m_member(p_member_ptr)
{}
// Dispatch to the right get method
auto operator[](const size_t p_index) const
{
return (*m_container)[p_index].*m_member;
}
private:
const Container * m_container;
MemberPtr m_member;
};
To use this implementation, you would write something like this :
int main() {
std::vector<Elem> printme = { Elem(1,100), Elem(2,200), Elem(3,300) };
std::vector<int> order = { 2,0,1 };
virtual_vector<decltype(printme), decltype(&Elem::a)> X(printme, &Elem::a);
print_in_order(order, X);
}
This is a bit cumbersome since there is no template argument deduction happening. So lets add a free function to deduce the template arguments.
template<class Container, class MemberPtr>
virtual_vector<Container, MemberPtr>
make_virtual_vector(const Container & p_container, MemberPtr p_member_ptr)
{
return{ p_container, p_member_ptr };
}
The usage becomes :
int main() {
std::vector<Elem> printme = { Elem(1,100), Elem(2,200), Elem(3,300) };
std::vector<int> order = { 2,0,1 };
auto X = make_virtual_vector(printme, &Elem::a);
print_in_order(order, X);
}
If you want to support member functions, it's a little bit more complicated. First, the syntax to dereference a data member pointer is slightly different from calling a function member pointer. You have to implement two versions of the operator[] and enable the correct one based on the member pointer type. Luckily the standard provides std::enable_if and std::is_member_function_pointer (both in the <type_trait> header) which allow us to do just that. The member function pointer requires you to specify the arguments to pass to the function (non in this case) and an extra set of parentheses around the expression that would evaluate to the function to call (everything before the list of arguments).
template<class Container, class MemberPtr>
class virtual_vector
{
public:
virtual_vector(const Container & p_container, MemberPtr p_member_ptr) :
m_container(&p_container),
m_member(p_member_ptr)
{}
// For mapping to a method
template<class T = MemberPtr>
auto operator[](std::enable_if_t<std::is_member_function_pointer<T>::value == true, const size_t> p_index) const
{
return ((*m_container)[p_index].*m_member)();
}
// For mapping to a member
template<class T = MemberPtr>
auto operator[](std::enable_if_t<std::is_member_function_pointer<T>::value == false, const size_t> p_index) const
{
return (*m_container)[p_index].*m_member;
}
private:
const Container * m_container;
MemberPtr m_member;
};
To test this, I've added a getter to the Elem class, for illustrative purposes.
struct Elem {
int a, b;
int foo() const { return a; }
Elem(int a, int b) : a(a), b(b) {}
};
And here is how it would be used :
int main() {
std::vector<Elem> printme = { Elem(1,100), Elem(2,200), Elem(3,300) };
std::vector<int> order = { 2,0,1 };
{ // print member
auto X = make_virtual_vector(printme, &Elem::a);
print_in_order(order, X);
}
{ // print method
auto X = make_virtual_vector(printme, &Elem::foo);
print_in_order(order, X);
}
}
You've got a choice of two data structures
struct Employee
{
std::string name;
double salary;
long payrollid;
};
std::vector<Employee> employees;
Or alternatively
struct Employees
{
std::vector<std::string> names;
std::vector<double> salaries;
std::vector<long> payrollids;
};
C++ is designed with the first option as the default. Other languages such as Javascript tend to encourage the second option.
If you want to find mean salary, option 2 is more convenient. If you want to sort the employees by salary, option 1 is easier to work with.
However you can use lamdas to partially interconvert between the two. The lambda is a trivial little function which takes an Employee and returns a salary for him - so effectively providing a flat vector of doubles we can take the mean of - or takes an index and an Employees and returns an employee, doing a little bit of trivial data reformatting.
template<class F>
struct index_fake_t{
F f;
decltype(auto) operator[](std::size_t i)const{
return f(i);
}
};
template<class F>
index_fake_t<F> index_fake( F f ){
return{std::move(f)};
}
template<class F>
auto reindexer(F f){
return [f=std::move(f)](auto&& v)mutable{
return index_fake([f=std::move(f),&v](auto i)->decltype(auto){
return v[f(i)];
});
};
}
template<class F>
auto indexer_mapper(F f){
return [f=std::move(f)](auto&& v)mutable{
return index_fake([f=std::move(f),&v](auto i)->decltype(auto){
return f(v[i]);
});
};
}
Now, print in order can be rewritten as:
template <typename vectorlike>
void print(vectorlike const & printme) {
for (auto&& x:printme)
std::cout << x << std::endl;
}
template <typename vectorlike>
void print_in_order(std::vector<int> const& reorder, vectorlike const & printme) {
print(reindexer([&](auto i){return reorder[i];})(printme));
}
and printing .a as:
print_in_order( reorder, indexer_mapper([](auto&&x){return x.a;})(printme) );
there may be some typos.
I have the following setup
class obj {
public:
int a();
int b();
int c();
int d();
};
std::vector<obj> obj_vector;
//Fill obj_vector
and I would like to access the methods (this is for an interface that expects vectors of ints, individually).
void func(vector<int>a ,vector<int> b,vector<int> c,vector<int> d);
I thought initially that extending the vector was the best approach, but have been warned against it.
I've implemented
get_a_vec(vector<obj>);
etc but this is a bit clumsy. Are there any nice alternatives?
Thanks in advance!
Write a function which retrieves the desired member from a single object.
Then pass that function to std::transform.
If you have a int get_a_member(obj&) function, for example, simply call
std::vector<int> avec;
std::transform(obj_vector.begin(), obj_vector.end(), get_a_member);
If you want a more complex or reusable function, make it a functor instead. Then you can pass parameters to it in its constructor (for example telling it which member to return).
You could write a generic function to extract any class member from a sequence of objects:
template <typename InIt, typename OutIt, typename Class, typename Member>
void extract(InIt start, InIt end, OutIt out, Member Class::*member) {
for (; start != end; ++start, ++out) {
*out = (*start).*member;
}
}
// Example
int main()
{
std::vector<obj> objs {{1,2,3,4},{5,6,7,8}};
std::vector<int> ints;
extract(objs.begin(), objs.end(), back_inserter(ints), &obj::a);
std::copy(ints.begin(), ints.end(),
std::ostream_iterator<int>(std::cout, "\n"));
}
What I understood is that you want a function that extracts the values of a from your objs in a given vector and puts them in another vector, if this is true, then I suggest using a friend function.
Declaring a friend function in the class obj allows this function to access private data members of that class.
class obj {
friend vector<int> get_a_vec(const vector<obj>&);
friend vector<int> get_b_vec(const vector<obj>&);
friend vector<int> get_c_vec(const vector<obj>&);
friend vector<int> get_d_vec(const vector<obj>&);
int a;
int b;
int c;
int d;
};
vector<int> get_a_vec(const vector<obj>& vec)
{
vector<int> result(vec.size());
for (unsigned i = 0;i<vec.size();i++)
result->at(i) = vec[i].a;
return result;
}
I need to implement an std::map with <std::string, fn_ptr> pairs. The function pointers are pointers to methods of the same class that owns the map. The idea is to have direct access to the methods instead of implementing a switch or an equivalent.
( I am using std::string as keys for the map )
I'm quite new to C++, so could anyone post some pseudo-code or link that talks about implementing a map with function pointers? ( pointers to methods owned by the same class that owns the map )
If you think there's a better approach to my problem, suggestions are also welcome.
This is about the simplest I can come up with. Note no error checking, and the map could probably usefully be made static.
#include <map>
#include <iostream>
#include <string>
using namespace std;
struct A {
typedef int (A::*MFP)(int);
std::map <string, MFP> fmap;
int f( int x ) { return x + 1; }
int g( int x ) { return x + 2; }
A() {
fmap.insert( std::make_pair( "f", &A::f ));
fmap.insert( std::make_pair( "g", &A::g ));
}
int Call( const string & s, int x ) {
MFP fp = fmap[s];
return (this->*fp)(x);
}
};
int main() {
A a;
cout << a.Call( "f", 0 ) << endl;
cout << a.Call( "g", 0 ) << endl;
}
A template implementation could look like:
class Factory {
public:
enum which {
foo, bar, baz
};
template<which w>
A* newA(...);
...
};
template<Factory::which w>
A* Factory::newA(...) {
/* default implementation */
throw invalid_argument();
}
template<>
A* Factory::newA<Factory::foo>(...) {
/* specialization for a 'foo' style A */
...
}
....
This requires that the value used to determine which newA is called be known at compile time. You could potentially use a const char * as the template parameter, but it's not guaranteed to work on all compilers.
Yet another option is to create helper factories, one for each factory creation method, and store those in the map. This isn't a huge advantage over storing method pointers, but does let you define a default creation method and simplifies fetching things from the map (there's no need to check that the key exists, because you'll get a default factory). On the downside, an entry for each unknown key would be added to the map.
Also, if you use an enum rather than a string for the key type, you shouldn't need to worry about checking whether a key exists in the map. While it's possible for someone to pass an invalid enum key to newA, they'd have to explicitly cast the argument, which means they're not going to do it by accident. I'm having a hard time imagining a case where someone would purposefully cause a crash in newA; the potential scenarios involve security, but an application programmer could crash the app without using your class.
Since C++14, we can use a generic lambda to get rid easily of pointers to member methods.
It follows a minimal, working example of a forward function made up with a generic lambda function:
#include<utility>
#include<map>
#include<string>
#include<iostream>
struct SomeClass { };
struct SomeOtherClass { };
struct Test {
void test(SomeClass) { std::cout << "SomeClass" << std::endl; }
void test(SomeOtherClass) { std::cout << "SomeOtherClass" << std::endl; }
};
int main() {
Test test;
auto l = [&test](auto c){ test.test(c); };
std::map<std::string, decltype(l)> m;
m.emplace("foo", l);
m.emplace("bar", l);
m.at("foo")(SomeClass{});
m.at("bar")(SomeOtherClass{});
}
Another option is to use delegates as oppose to function pointers. This delegate implementation is pretty fast, supports polymorphisms, and plays well with stl containers.
You could have something like:
class MyClass {
public:
// defines
typedef fastdelegate::FastDelegate2<int, int, int> MyDelegate;
typedef std::map<std::string, MyDelegate> MyMap;
// populate your map of delegates
MyClass() {
_myMap["plus"] = fastdelegate::MakeDelegate(this, &Plus);
_myMap["minus"] = fastdelegate::MakeDelegate(this, &Minus);
}
bool Do(const std::string& operation, int a, int b, int& res){
MyMap::const_iterator it = _myMap.find(operation);
if (it != _myMap.end()){
res = it.second(a,b);
return true;
}
return false;
}
private:
int Plus (int a, int b) { return a+b; }
int Minus(int a, int b) { return a-b; }
MyMap _myMap;
};