So I was Playing around with c++11 Varidiacs, and I wanted to create a thing called CallClass, basically a class that warps a function, for later call,when all variables are set(truly I have No Idea If It can Be Useful):
#include <tuple>
template <typename OBJ,typename F,typename... VARGS>
class CallClass
{
public:
CallClass(OBJ& object,F callFunction)
:_object(&object),_func(callFunction)
{ }
CallClass(const CallClass& other)
:_func_args(other._func_args)
,_object(other._object)
,_func(other._func)
{ }
template <size_t INDEX>
auto get(){ return std::get<INDEX>(_func_args); }
template <size_t INDEX,typename T>
void set(const T& val){ std::get<INDEX>(_func_args) = val; }
template <size_t INDEX,typename T>
void set(T&& val){ std::get<INDEX>(_func_args) = val; }
auto Call()
{
//throws segmentation Fault Here
return InnerCall<0>(_func_args);
}
virtual ~CallClass() {}
protected:
private:
std::tuple<VARGS...> _func_args;
OBJ* _object;
F _func;
template <size_t INDEX,typename... ARGS>
auto InnerCall(std::tuple<VARGS...>& tup,ARGS... args)
{
auto arg = std::get<INDEX>(tup);
return InnerCall<INDEX + 1>(tup,args...,arg);
}
template <size_t INDEX,VARGS...>
auto InnerCall(std::tuple<VARGS...>& tup,VARGS... args)
{
return (_object->*_func)(args...);
}
};
Now when I try to compile(compiling using IDE:code::blocks, configured to use MINGW On windows ), it prints Compiler:Segmentation Fault, anybody any Ideas?
Usage:
class obj{
public:
obj(int a)
:_a(a)
{ }
virtual ~obj() {}
int add(int b,int c){
return _a + b + c;
}
private:
int _a;
};
int main(){
obj ob(6);
CallClass<obj,decltype(obj::add),int,int> callAdd(ob,obj::add);
callAdd.set<0,int>(5);
callAdd.set<1,int>(7);
cout << "result is " << callAdd.Call() << endl;
return 0;
}
After a Bit of a search i stumbled upon a similar issue, in a way.
apparently the way I'm unpacking the tuple is an issue, so i decided to use a different approach as shown in: enter link description here
had to add a few changes to suit my needs:
changes:
namespace detail
{
template <typename OBJ,typename F, typename Tuple, bool Done, int Total, int... N>
struct call_impl
{
static auto call(OBJ& obj,F f, Tuple && t)
{
return call_impl<OBJ,F, Tuple, Total == 1 + sizeof...(N), Total, N..., sizeof...(N)>::call(obj,f, std::forward<Tuple>(t));
}
};
template <typename OBJ,typename F, typename Tuple, int Total, int... N>
struct call_impl<OBJ,F, Tuple, true, Total, N...>
{
static auto call(OBJ& obj,F f, Tuple && t)
{
return (obj.*f)(std::get<N>(std::forward<Tuple>(t))...);
}
};
}
// user invokes this
template <typename OBJ,typename F, typename Tuple>
auto call(OBJ& obj,F f, Tuple && t)
{
typedef typename std::decay<Tuple>::type ttype;
return detail::call_impl<OBJ,F, Tuple, 0 == std::tuple_size<ttype>::value, std::tuple_size<ttype>::value>::call(obj,f, std::forward<Tuple>(t));
}
and changed Call():
auto Call()
{
std::tuple<VARGS...> func_args = _func_args;
return call(*_object,_func, std::move(func_args));
}
I will probably make a few more changes, like passing the tuple as a reference, and making the structs a part of my class.
Related
I'm looking into a solution of building containers which track stored size of their elements in addition to basic functions.
So far I didn't saw a solution which doesn't create a huge amount of boilerplate code of each invalidating member of container. This also assumes that stored elements cannot change size after being stored.
Unless standard containers have some feature that allows to inject such behaviour. The following example should be working one, albeit abridged for brevity. The declarations used are:
typedef uint8_t Byte;
typedef Byte PacketId;
template <class T>
struct CollectionTraits {
typedef T collection_type;
typedef typename collection_type::value_type value_type;
typedef typename collection_type::size_type size_type;
typedef typename collection_type::iterator iterator;
typedef typename collection_type::reference reference;
typedef typename collection_type::const_iterator const_iterator;
const_iterator begin() const { return _collection.begin(); }
const_iterator end() const { return _collection.end(); }
iterator begin() { return _collection.begin(); }
iterator end() { return _collection.end(); }
size_type size() const { return _collection.size(); }
protected:
T _collection;
};
struct Packet : CollectionTraits<std::vector<Byte>>
{
PacketId id;
};
The container itself:
struct PacketList : CollectionTraits<std::deque<Packet>>
{
public:
typedef Packet::size_type data_size;
void clear() { _collection.clear(); _total_size = 0; }
data_size total_size() const { return _total_size; }
void push_back(const Packet& v) {
_collection.push_back(v);
_add(v);
}
void push_back(const Packet&& v) {
_collection.push_back(std::move(v));
_add(v);
}
void push_front(const Packet& v) {
_collection.push_front(v);
_add(v);
}
void push_front(const Packet&& v) {
_collection.push_front(std::move(v));
_add(v);
}
void pop_back() {
_remove(_collection.back());
_collection.pop_back();
}
void erase(const_iterator first, const_iterator last) {
for(auto it = first; it != last; ++it) _remove(*it);
_collection.erase(first, last);
}
PacketList() : _total_size(0) {}
PacketList(const PacketList& other) : _total_size(other._total_size) {}
private:
void _add(const Packet& v) { _total_size += v.size(); }
void _remove(const Packet& v) { _total_size -= v.size(); }
data_size _total_size;
};
The interface in result should similar to a standard container. Is there a way to avoid this amount of repeated code? Is there some standard solution for this problem?
I have a method that takes an index-able object as a template parameter, something like:
template <typename OBJ>
int foo(int n, OBJ o)
{
int x = 0;
for (int i = 0; i < n; ++i) {
x += o[i];
}
return x;
}
Is there a way I can pass a lambda function in for the o parameter? In other words, having the lambda be call-able via the [] operator rather than the () operator?
template<class F>
struct square_bracket_invoke_t {
F f;
template<class T>
auto operator[](T&& t)const
-> typename std::result_of< F const&(T&&) >::type
{ return f(std::forward<T>(t)); }
};
template<class F>
square_bracket_invoke_t< typename std::decay<F>::type >
make_square_bracket_invoke( F&& f ) {
return {std::forward<F>(f)};
}
Live example.
Code is C++11 and has basically zero overhead.
int main() {
std::cout << foo( 6, make_square_bracket_invoke([](int x){ return x; } ) ) << "\n";
}
result is 0+1+2+3+4+5 aka 15.
Is this a good idea? Maybe. But why stop there?
For max amusement:
const auto idx_is = make_square_bracket_invoke([](auto&&f){return make_square_bracket_invoke(decltype(f)(f));});
int main() {
std::cout << foo( 6, idx_is[[](int x){ return x; }] ) << "\n";
}
You can do that by:
Creating a class template, a functor, that has the operator[] defined.
Implementing the operator[] in terms of the operator() of a std::function.
Storing the lambda in a wrapped std::function as a member variable of the class template.
Here's a demonstrative program.
#include <iostream>
#include <functional>
template <typename OBJ>
int foo(int n, OBJ o)
{
int x = 0;
for (int i = 0; i < n; ++i) {
x += o[i];
}
return x;
}
template <typename> struct Functor;
template <typename R> struct Functor<R(int)>
{
using ftype = std::function<R(int)>;
Functor(ftype f) : f_(f) {}
R operator[](int i) const { return f_(i); }
ftype f_;
};
int main()
{
Functor<int(int)> f = {[](int i) -> int {return i*i;}};
std::cout << foo(10, f) << std::endl;
}
and its output
285
Live demo
PS
Functor is not the appropriate name here. It does not overload the function call operator. I suspect there is a more appropriate name.
Well, if it helps, here's a way to forward a wrapper class's operator[] to your lambda's operator().
template<class F>
struct SubscriptWrapper_t {
F f_;
template<class T> auto operator[](T const& t_) const -> decltype(f_(t_)) {
return f_(t_);
}
};
template<class F>
SubscriptWrapper_t<typename std::decay<F>::type> SubscriptWrapper(F&& f_) {
return{std::forward<F>(f_)};
}
I use wrappers like this a lot. They're convenient, and they don't seem to have any computational overhead, at least when compiled by GCC. You can make one for at or even make one for find.
EDIT: Updated for C++11 (and updated to be able to return a reference)
A sketch of a wrapper type that would do this.
template<typename UnaryFunction>
class index_wrapper
{
public:
index_wrapper(UnaryFunction func) : func(std::move(func)) {}
template<typename T>
std::invoke_result_t<UnaryFunction, T> operator[](T&& t)
{ return func(std::forward<T>(t)); }
private:
UnaryFunction func;
};
With usage
#include <iostream>
template <typename OBJ>
int foo(int n, OBJ o)
{
int x = 0;
for (int i = 0; i < n; ++i) {
x += o[i];
}
return x;
}
int main()
{
index_wrapper f([](int i) -> int { return i*i; });
std::cout << foo(10, f) << std::endl;
}
You might want to restrict it to a single parameter type, so that you can provide member type aliases similar to std::vector::reference et.al.
How to pass lambda as template parameter.
For example this code
template<void (*callback)()>
void function() {
callback();
}
int main() {
function<[]() -> void { std::cout << "Hello world\n"; }>();
}
fails with error "invalid template argument for 'function', expected compile-time constant expression".
What I'm doing wrong.
Edit
I want to implement something like this
template<typename T,
T (*deserializer)(buffer *data),
void (*serializer)(T item, buffer *data)>
class Type {
public:
T item;
Type(T item) : item(item) {
}
Type(buffer *data) {
deserialize(data);
}
void serialize(buffer *data) {
serializer(item, data);
}
void deserialize(buffer *data) {
deserializer(data);
}
};
typedef Type<int, [](buffer* data) -> int { return -1; }, [](int item, buffer* data) -> void {}> IntType
typedef Type<long, [](buffer* data) -> long { return -1; }, [](long item, buffer* data) -> void {}> LongType
Lambdas in C++14, including their conversion to function pointers, are not constexpr.
In C++17, this is going to change. There are no stable compilers with that feature implemented that I'm aware of (if you find one, can you mention it in the comments below?).
At that point
constexpr auto tmp = []() -> void { std::cout << "Hello world\n"; };
function<+tmp>();
will definitely work. I am uncertain if
function<+[]() -> void { std::cout << "Hello world\n"; }>()
would work; there are some rules about lambdas in unevaluated contexts and inside template argument lists that may be separate from the constexpr lambda problem and may apply here.
We can hack it in C++14.
Create a template class that stores a static copy of a lambda and exposes a static function with the same signature (f_ptr) that calls that static copy of a lambda.
Instantiate it once globally with your lambda.
Pass a pointer to the f_ptr to your template.
So:
template<class L> struct stateless; // todo
template<class L> stateless<L> make_stateless(L l){return std::move(l);}
auto foo = make_stateless( []() -> void { std::cout << "Hello world\n"; } );
function< &foo::f_ptr >();
this is almost certainly not what you want.
The kind of template in the example does not take a type as a parameter, but rather a value. This value needs to be determinable at runtime, in order to instantiate the template, and the value of a lambda is not a compile time constant, so this method just wont do. The common way of sending functors to a function is:
template<typename Func>
void foo(Func&& f)
{
f();
}
And since you want a class template (please put such information in the question, not the comments), here is an example using classes:
#include <utility>
template<typename Func>
class MyClass
{
public:
MyClass(Func&& f) : f(f) {}
void Run() { f(); }
private:
Func f;
};
template<typename Func>
MyClass<Func> MakeMyClass(Func&& f)
{
return { std::forward<Func>(f) };
}
int main()
{
auto x = MakeMyClass( [](){} );
x.Run();
}
I want to pass two functions with different signature into a same slot of class template parameter (one each time).
Ignore the strict syntax, this is what I want :-
void hf1 (int a) { std::cout<< "hf1" <<std::endl; }
void hf2 (int a, int i){ std::cout<< "hf2 "<<i<<std::endl; }
template<hfX> class Collection{
int i_=56;
public: test(){
if( "hfX has 1 param" ){hfX(0);} //call hf1
else {hfX(0,i_);} //call hf2
}
};
int main(){
Collection<&hf1> test1; test1.test(); // print "hf1"
Collection<&hf2> test2; test2.test(); // print "hf2 56"
}
Here is the code that works OK, but its usage is not so convenient :-
template<typename ... AA> using hfX = void(*)(AA ... );
void hf1 (int a) { std::cout<< "hf1" <<std::endl; }
void hf2 (int a, int i) { std::cout<< "hf2 "<<i <<std::endl; }
template <typename Tf, Tf F>
class Collection;
template <typename ... I, hfX<I...> F>
class Collection<hfX<I...>, F>{
public:
int i_=56;
template <std::size_t N = sizeof...(I)>
typename std::enable_if<N == 1U, void>::type test (){
F(0);
}
template <std::size_t N = sizeof...(I)>
typename std::enable_if<N == 2U, void>::type test (){
F(0,i_);
}
};
The usage:-
int main () {
Collection<hfX<int>, hf1> test1; //<--- #A dirty signature
Collection<hfX<int,int>, hf2> test2; //<--- #B dirty signature
test1.test(); // print "hf1"
test2.test(); // print "hf2 56"
}
Live version : https://ideone.com/f20BEk
Question
It would be nice if I can call it without explicit redundant signature.
Collection<hf1> test1; //or &hf
Collection<hf2> test2;
How to improve code (especially around hfX and Collection) to make its usage easier?
I don't know how to make what do you want with functions.
But if you can, instead of functions, accept to use static method in classes or structs (and pass that classes/structs as template argument)...
#include <iostream>
struct sf1
{ static void hf (int a) { std::cout << "hf1" << std::endl; } };
struct sf2
{ static void hf (int a, int i) { std::cout << "hf2 " << i << std::endl; } };
template <typename S>
class Collection
{
private:
int i_ = 56;
public:
template <typename T = S>
decltype(T::hf(0)) test() { S::hf(0); /*call sf1::hf */ }
template <typename T = S>
decltype(T::hf(0, 0)) test() { S::hf(0,i_); /*call sf2::hf */ }
};
int main ()
{
Collection<sf1> test1; test1.test(); // print "hf1"
Collection<sf2> test2; test2.test(); // print "hf2 56"
}
I have the following code:
#include <iostream>
#include <functional>
class test
{
public:
typedef std::function<bool(int)> Handler;
void handler(Handler h){h(5);}
};
class test2
{
public:
template< typename Ret2, typename Ret, typename Class, typename Param>
inline Ret2 MemFn(Ret (Class::*f)(Param), int arg_num)
{
if (arg_num == 1)
return std::bind(f, this, std::placeholders::_1);
}
bool f(int x){ std::cout << x << std::endl; return true;}
};
int main()
{
test t;
test2 t2;
t.handler(t2.MemFn<test::Handler>(&test2::f, 1));
return 0;
}
It works as expected.
I would like to be able to call this:
t.handler(t2.MemFn<test::Handler>(&test2::f));
instead of
t.handler(t2.MemFn<test::Handler>(&test2::f, 1));
Basically I need MemFn to determine in runtime what Handler expects as the number of arguments.
Is that even possible?
You may create some type_traits to have your info, something like:
template <typename T> struct function_trait;
template <typename Ret, typename ... Args>
struct function_trait<std::function<Ret(Args...)>>
{
static constexpr std::size_t args_count = sizeof...(Args);
};
And so your method may look like:
template<typename Ret2, typename Ret, typename Class, typename Param>
inline Ret2 MemFn(Ret (Class::*f)(Param))
{
if (function_trait<Ret2>::args_count == 1)
return std::bind(f, this, std::placeholders::_1);
throw std::runtime_error("invalid number of arguments");
}