I want to perfect-forward a call to a function, but I always want to return a const version of the returned type. I mean, when returning a reference, I want a const& instead. How can I achieve that?
I don't care about non-reference types. After all, returning a const T (without a reference) is not much different than T.
I care only about a const at the top level. If T is a pointer type, or some other more complex type, I do not want to add constness somewhere deeper.
If I just forward the call normally:
template<class Func, class... Args>
decltype(auto) Example(Func f, Args&&... args)
{
return fun(std::forward<Args>(args)...);
}
and f happens to return, say, int& then I just get just that: int&. But I would like to get a const int& instead.
decltype(auto) const, or decltype(auto const) seem to be illegal.
Here is a possible solution. References are transformed to const&. Any other type remains unchanged. However, I would like to hear a second opinion, if there are some corner cases I didn't consider.
#include <utility>
template<typename T>
struct add_const_to_ref {
using type = T;
};
template<typename T>
struct add_const_to_ref<T&> {
using type = const T&;
};
template<typename T>
using add_const_to_ref_t = typename add_const_to_ref<T>::type;
template<class Func, class... Args>
add_const_to_ref_t<decltype(std::declval<Func>()(std::forward<Args>(std::declval<Args&&>())...))> ConstInvoke(Func f, Args&&... args)
{
return f(std::forward<Args>(args)...);
}
The following test:
#include <iostream>
int& foo(int& x) {
return x;
}
int bar() {
return 5;
}
int main() {
int x = 0;
std::cout << std::is_same_v<int&, decltype(ConstInvoke(foo, x))> << "\n";
std::cout << std::is_same_v<int, decltype(ConstInvoke(foo, x))> << "\n";
std::cout << std::is_same_v<const int&, decltype(ConstInvoke(foo, x))> << "\n";
std::cout << std::is_same_v<int&, decltype(ConstInvoke(bar))> << "\n";
std::cout << std::is_same_v<int, decltype(ConstInvoke(bar))> << "\n";
std::cout << std::is_same_v<const int&, decltype(ConstInvoke(bar))> << "\n";
};
Gives the expected result:
0
0
1
0
1
0
i.e.
ConstInvoke(foo, x) returns const int&,
ConstInvoke(bar) returns int
An alternative, to simplify the horrible ConstInvoke, one can try like this:
template<typename T>
decltype(auto) as_const_to_ref(T&& arg) {
return (add_const_to_ref_t<T>)(std::forward<T>(arg));
}
template<class Func, class... Args>
decltype(auto) ConstInvoke2(Func f, Args&&... args)
{
return as_const_to_ref(f(std::forward<Args>(args)...));
}
I've looking for how to cast class member to C-style callback.
Recentrly i found answer with special bind hack allows to bind class members to C-style callbacks:
https://stackoverflow.com/a/39524069/5405443
I have this working code to bind function MyClass::f to C function f:
But i want to avoid explicit passing cb_type as template parameter to c_bind function.
In provided example CB has type void (*)(int) and Func template parameter has void (MyClass::*)(int) type.
template<typename CB, typename Func, typename... Params>
CB* c_bind(std::_Bind<Func(Params...)> function) {
return Callback<typename ActualType<CB>::type, __COUNTER__, Func>::getCallback(function);
}
typedef void (cb_type)(int);
class MyClass {
public:
void f(int x) {
std::cout << "Hello from MyClass::f(int), value: " << x << std::endl;
}
};
int main() {
MyClass mc;
auto f = c_bind<cb_type>(std::bind(&MyClass::f, mc, std::placeholders::_1));
// ^ how to avoid explicit callback type declaration here?
f(10);
return 0;
}
Also i found this piece of code (https://gist.github.com/vikchopde/73b62314379f733e8938f11b246df49c) for "unwrapping" some kind of functions.
bool ok = fu::is_unwrappable<decltype(&MyClass::f)>::value; // always false
// fu::unwrap_function<decltype(&MyClass::f)>::type::function_ptr blah; // won't compile
but it won't work by unknown to me reason.
My question is there any workaround to extract return type and args list from type with class-memeber pointer like void (MyClass::*)(int) and contruct C-like type void (*)(int) ?
Thank you for any help!
Well, in C++17, you are allowed to pass an arbitrary non-type parameter to a class with template<auto>. Therefore, we could store MyClass::f as a template parameter and parse its type with decltype. After passing this type to another templated class, we are able to extract desired types using template specialization.
The code below shows how to construct a C-style function wrapper<>::func_type.
Since you seem to bind an object to its member function, I additionally write the demo code to do this by invoking wrapper<>::bind. Hope it helps.
class MyClass {
public:
void f(int x) {
std::cout << "Hello from MyClass::f(int), value: " << x << std::endl;
}
};
void f(int x) {
std::cout << "Hello from f(int), value: " << x << std::endl;
}
template<auto F>
struct wrapper
{
template<typename> struct inner;
template<class Cls, typename Ret, typename... Args>
struct inner<Ret(Cls::*)(Args...)>
{
using func_type = Ret(Args...);
static auto bind(Cls *obj)
{
return [=](Args ...args){
return (obj->*F)(std::forward<Args>(args)...);
};
}
};
using func_type = typename inner<decltype(F)>::func_type;
static const constexpr auto bind = inner<decltype(F)>::bind;
};
int main() {
MyClass mc;
auto h = wrapper<&MyClass::f>::bind(&mc);
h(10);
using func_t = typename wrapper<&MyClass::f>::func_type;
std::function<func_t> g = f;
g(1);
return 0;
}
First of all i would like to thank #Dappur for nice example. Using your guide i will rewrite my ugly bind interface with std::_Bind usage later. Also i want to thank #Sam Varshavchik for mentioning that set of C++ books. I'll start reading it to become C++ grandmaster like you to learn how "why i cannot cast it like this". But unfortunately with my poor c++ experience I can still do it now. Here is working code:
template<class T, unsigned int n, class CallerType>
struct CallbackWrapper;
template<class Ret, class... Params, unsigned int n, class CallerType>
struct CallbackWrapper<Ret(Params...), n, CallerType> {
static auto get(std::function<Ret(Params...)>&& fn) -> Ret(*)(Params...) {
func = fn;
return static_cast<Ret(*)(Params...)>(CallbackWrapper<Ret(Params...), n, CallerType>::callback);
}
private:
static std::function<Ret(Params...)> func;
static Ret callback(Params... args) {
return func(args...);
}
};
template<class Ret, class... Params, unsigned int n, class CallerType>
std::function<Ret(Params...)> CallbackWrapper<Ret(Params...), n, CallerType>::func;
template<typename T>
struct lambda_to_stdf {
using type = void;
};
template<typename Ret, typename Class, typename... Args>
struct lambda_to_stdf<Ret(Class::*)(Args...) const> {
using type = std::function<Ret(Args...)>;
};
template<class Ret, class Cls, class... Args1, class... Args2>
auto c_bind(std::_Bind<Ret(Cls::*(Cls, Args1...))(Args2...)> function) -> Ret(*)(Args2...) {
return CallbackWrapper<Ret(Args2...), __COUNTER__, Ret(Cls::*(Cls, Args1...))(Args2...)>::get(std::move(function));
}
template<class Ret, class... Args>
auto c_bind(std::function<Ret(Args...)> function) -> Ret(*)(Args...) {
return CallbackWrapper<Ret(Args...), __COUNTER__, std::function<Ret(Args...)>>::get(std::move(function));
}
template<class F>
auto c_bind(F function) -> decltype(c_bind((typename lambda_to_stdf<decltype(&F::operator())>::type)(function))) {
return c_bind((typename lambda_to_stdf<decltype(&F::operator())>::type)(function));
}
Usage:
class MyClass {
public:
void f(int x) {
std::cout << "Hello from MyClass::f(int), value: " << x << std::endl;
}
};
int main() {
MyClass mc;
auto f = c_bind(std::bind(&MyClass::f, mc, std::placeholders::_1));
f(10);
std::function<void(int)> stdf = [](int v) {
std::cout << "hello from std::function, value: " << v << std::endl;
};
auto f2 = c_bind(stdf);
f2(100);
auto f3 = c_bind([](int v) -> int {
std::cout << "hello from lambda, value: " << v << std::endl;
return 5.0f;
});
f3(1000);
return 0;
}
Hope it will be helpful for someone.
I'm trying to create a generic wrapper function that takes a function as a template argument and takes the same arguments as that function as its arguments. For example:
template <typename F, F func>
/* return type of F */ wrapper(Ts... Args /* not sure how to get Ts*/)
{
// do stuff
auto ret = F(std::forward<Ts>(args)...);
// do some other stuff
return ret;
}
The solution needs to be castable to a function pointer with the same type as func so that I can pass it to a C api. In other words, the solution needs to be a function and not a function object. Most importantly, I need to be able to do work in the wrapper function.
If the inline comments aren't clear, I'd like to be able to do something like the following:
struct c_api_interface {
int (*func_a)(int, int);
int (*func_b)(char, char, char);
};
int foo(int a, int b)
{
return a + b;
}
int bar(char a, char b, char c)
{
return a + b * c;
}
c_api_interface my_interface;
my_interface.func_a = wrapper<foo>;
my_interface.func_b = wrapper<bar>;
I looked for related posts and found these, but none of them are quite what I'm trying to do. Most of these posts concern function objects. Is what I'm trying to do even possible?
Function passed as template argument
Function wrapper via (function object) class (variadic) template
How does wrapping a function pointer and function object work in generic code?
How do I get the argument types of a function pointer in a variadic template class?
Generic functor for functions with any argument list
C++ Functors - and their uses
In response to the first 2 responses, I edited the question to make it clear that I need to be able to do work in the wrapper function (i.e. modify some global state before and after the call to the wrapped function)
template<class F, F f> struct wrapper_impl;
template<class R, class... Args, R(*f)(Args...)>
struct wrapper_impl<R(*)(Args...), f> {
static R wrap(Args... args) {
// stuff
return f(args...);
}
};
template<class F, F f>
constexpr auto wrapper = wrapper_impl<F, f>::wrap;
Use as wrapper<decltype(&foo), foo>.
#include <utility>
#include <iostream>
struct c_api_interface { int (*func_a)(int, int); int (*func_b)(char, char, char); };
int foo(int a, int b) { return a + b; }
int bar(char a, char b, char c) { return a + b * c; }
template<typename Fn, Fn fn, typename... Args>
typename std::result_of<Fn(Args...)>::type
wrapper(Args... args) {
std::cout << "and ....it's a wrap ";
return fn(std::forward<Args>(args)...);
}
#define WRAPIT(FUNC) wrapper<decltype(&FUNC), &FUNC>
int main() {
c_api_interface my_interface;
my_interface.func_a = WRAPIT(foo);
my_interface.func_b = WRAPIT(bar);
std:: cout << my_interface.func_a(1,1) << std::endl;
std:: cout << my_interface.func_b('a','b', 1) << std::endl;
return 0;
}
see http://rextester.com/ZZD18334
you may try something like that (Ugly, but works)
#include <iostream>
#include <functional>
struct wrapper_ctx
{
wrapper_ctx ()
{
std::cout << "Before" << std::endl;
}
~wrapper_ctx ()
{
std::cout << "after" << std::endl;
}
};
template <typename F, typename... Args>
auto executor (F&& f, Args&&... args) -> typename std::result_of<F(Args...)>::type
{
wrapper_ctx ctx;
return std::forward<F>(f)( std::forward<Args>(args)...);
}
template <typename F>
class wrapper_helper;
template<typename Ret, typename... Args>
class wrapper_helper <std::function<Ret(Args...)>>
{
std::function<Ret(Args...)> m_f;
public:
wrapper_helper( std::function<Ret(Args...)> f )
: m_f(f) {}
Ret operator()(Args... args) const
{
return executor (m_f, args...);
}
};
template <typename T>
wrapper_helper<T> wrapper (T f)
{
return wrapper_helper <T>(f);
}
int sum(int x, int y)
{
return x + y;
}
int main (int argc, char* argv [])
{
std::function<int(int, int)> f = sum;
auto w = wrapper (f);
std::cout << "Executing the wrapper" << std::endl;
int z = w(3, 4);
std::cout << "z = " << z << std::endl;
}
you probably need something like
template <typename F>
class Wrapper {
public:
Wrapper(F *func) : function(func) {}
operator F* () { return function; }
F *function;
};
Which you can use like void (*funcPtr)(int) = Wrapper<void(int)>(&someFunction);
I think that will be the concise way to do what you want:
template <typename F>
F* wrapper(F* pFunc)
{
return pFunc;
}
and use it like this:
my_interface.func_a = wrapper(foo);
my_interface.func_a(1, 3);
You may try this
template <class R, class... Args>
struct wrap
{
using funct_type = R(*)(Args...);
funct_type func;
wrap(funct_type f): func(f) {};
R operator()(Args&&... args)
{
//before code block
std::cout << "before calling\n";
R ret=func(std::forward<Args>(args)...);
//after code block
std::cout << "After calling\n";
}
};
use like this for example:
int somefunc(double &f, int x);
auto wrapped_somefunc=wrap{somefunc};
double f=1.0;
int x = 2;
auto result=wrapped_somefunc(f,x);
This one is for c++17 and newer uses auto template parameters:
template <auto func, class... Args>
auto wrap_func(Args... args)
{
std::cout << "before calling wrapped func\n";
auto ret = func(args...);
std::cout << "after calling wrapped func\n";
return ret;
}
use for example:
int some_func(int a, int b);
auto ret = wrap_func<some_func>(2, 3);
I'm trying to implement a generic curry function in C++14 that takes a callable object as an input parameter and allows currying.
Desired syntax:
auto sum3 = [](int x, int y, int z){ return x + y + z; };
int main()
{
assert(curry(sum3)(1)(1)(1) == 3);
auto plus2 = curry(sum3)(1)(1);
assert(plus2(1) == 3);
assert(plus2(3) == 5);
}
My implementation idea is as follows: have the curry function return an unary function that binds its argument to a future call to the original function, recursively. Call the bound original function on the "last recursive step".
Detecting the "last recursive step" is the problematic part.
My idea was detecting whether or not the current bound function (during the recursion) was legally callable with zero arguments, using a is_zero_callable type trait:
template <typename...>
using void_t = void;
template <typename, typename = void>
class is_zero_callable : public std::false_type
{
};
template <typename T>
class is_zero_callable<T, void_t<decltype(std::declval<T>()())>>
: public std::true_type
{
};
Unfortunately, I cannot seem to find a way to check the correct function for zero-argument "callability" - I need to somehow check if the function that will be returned from the currently bound function is zero-callable.
Here's what I've got so far (godbolt link):
template <typename TF, bool TLastStep>
struct curry_impl;
// Base case (last step).
// `f` is a function callable with no arguments.
// Call it and return.
template <typename TF>
struct curry_impl<TF, true>
{
static auto exec(TF f)
{
return f();
}
};
// Recursive case.
template <typename TF, bool TLastStep>
struct curry_impl
{
static auto exec(TF f)
{
// Bind `x` to subsequent calls.
return [=](auto x)
{
// This is `f`, with `x` bound as the first argument.
// (`f` is the original function only on the first recursive
// step.)
auto bound_f = [=](auto... xs)
{
return f(x, xs...);
};
// Problem: how to detect if we need to stop the recursion?
using last_step = std::integral_constant<bool, /* ??? */>;
// `is_zero_callable<decltype(bound_f)>{}` will not work,
// because `bound_f` is variadic and always zero-callable.
// Curry recursively.
return curry_impl<decltype(bound_f),
last_step{}>::exec(bound_f);
};
}
};
// Interface function.
template <typename TF>
auto curry(TF f)
{
return curry_impl<TF, is_zero_callable<decltype(f)>{}>::exec(f);
}
Is my approach/intuition viable? (Is it actually possible to stop the recursion by detecting whether or not we've reached a zero-arg-callable version of the original function?)
...or is there a better way of solving this problem?
(Please ignore the missing perfect forwarding and the lack of polish in the example code.)
(Note that I've tested this currying implementation using an user-specified template int TArity parameter to stop the recursion, and it worked properly. Having the user manually specify the arity of the original f function is unacceptable, however.)
The minimal changes required to make this work in Clang is
auto bound_f = [=](auto... xs) -> decltype(f(x, xs...))
// ^^^^^^^^^^^^^^^^^^^^^^^^^
{
return f(x, xs...);
};
using last_step = std::integral_constant<bool,
is_zero_callable<decltype(bound_f)>{}>;
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explicitly specifying the return type should make it SFINAE-friendly and capable of being detected by is_zero_callable. Unfortunately, GCC is unhappy with this, probably due to a bug.
A generic lambda is basically a class with a templated operator(), so we can just write it ourselves:
template<class F, class T>
struct bound_function {
F f;
T arg;
template<class... Args>
auto operator()(Args... args) const -> decltype(f(arg, args...)){
return f(arg, args...);
}
};
Note that I'm imitating the semantics of the generic lambda here and making the operator() const. A full-featured implementation will likely want to overload on constness and value categories.
Then
auto bound_f = bound_function<TF, decltype(x)>{f, x};
works in both GCC and Clang, but has a theoretical problem: when only f(arg) is valid (and not with extra arguments), then instantiating bound_function (which instantiates a declaration of its operator()) is ill-formed NDR because every valid specialization of operator()'s declaration requires an empty parameter pack.
To avoid this, let's specialize bound_function for the "no further arguments needed" case. And since we are computing this information anyway, let's just express it in a member typedef.
template<class F, class T, class = void>
struct bound_function {
using zero_callable = std::false_type;
F f;
T arg;
template<class... Args>
auto operator()(Args... args) const -> decltype(f(arg, args...)){
return f(arg, args...);
}
};
template<class F, class T>
struct bound_function<F, T, void_t<decltype(std::declval<const F&>()(std::declval<const T&>()))>> {
using zero_callable = std::true_type;
F f;
T arg;
decltype(auto) operator()() const {
return f(arg);
}
};
Then
auto bound_f = bound_function<TF, decltype(x)>{f, x};
using last_step = typename decltype(bound_f)::zero_callable;
under file check. please.
https://github.com/sim9108/Study2/blob/master/SDKs/curryFunction.cpp
// ConsoleApplication4.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <string>
#include <functional>
#include <type_traits>
// core function
template<typename FN, std::size_t N>
struct curry
{
FN fn_;
curry(FN fn) :fn_{ fn }
{
}
template<typename... TS, typename = std::enable_if_t< (N - sizeof...(TS)) != 0, int >>
auto operator()(TS... ts1) {
auto fn = [f = this->fn_, ts1...](auto... args) mutable {
return f(ts1..., args...);
};
return curry<decltype(fn), N - sizeof...(TS)>(fn);
}
template<typename... TS, typename Z = void, typename = std::enable_if_t< (N - sizeof...(TS)) == 0, int > >
auto operator()(TS... ts1) {
return fn_(ts1...);
}
};
//general make curry function
template<typename R, typename... Args>
auto make_curry(R(&f)(Args...)) {
auto fn = [&f](Args... args) {
return f(args...);
};
return curry<decltype(fn), sizeof...(Args)>(fn);
}
//general make curry member function
template<typename C, typename R, typename... Args>
auto make_curry(R(C::*f)(Args...), C c) {
auto fn = [f, c](Args... args) mutable {
return (c.*f)(args...);
};
return curry<decltype(fn), sizeof...(Args)>(fn);
}
template<typename C, typename R, typename... Args>
auto make_curry(R(C::*f)(Args...) const, C c) {
auto fn = [f, c](Args... args) mutable {
return (c.*f)(args...);
};
return curry<decltype(fn), sizeof...(Args)>(fn);
}
//general make curry lambda function
template<typename C>
auto make_curry(C&& c) {
using CR = std::remove_reference_t<C>;
return make_curry(&CR::operator(), c);
}
using std::string;
using std::function;
string func(string a, string b, string c) {
return "general function:" + a + b + c;
}
struct A {
string func(string a, string b, string c) {
return "member function:" + a + b + c;
};
};
int main(int argc, char* argv[])
{
{ //general function curry
auto c = make_curry(func);
auto m1 = c("t1")("t2")("t3");
auto m2 = c("test1")("test2")("test3");
auto m3 = c("m1");
auto m4 = m3("m2");
auto m5 = m4("m3");
std::cout << m5 << std::endl;
std::cout << m2 << std::endl;
std::cout << m5 << std::endl;
}
{ //member function curry
A a;
auto c = make_curry(&A::func, a);
auto m1 = c("t1")("t2")("t3");
auto m2 = c("test1")("test2")("test3");
auto m3 = c("m1");
auto m4 = m3("m2");
auto m5 = m4("m3");
std::cout << m1 << std::endl;
std::cout << m2 << std::endl;
std::cout << m5 << std::endl;
}
{ //lambda function curry
auto fn = [](string a, string b, string c) {
return "lambda function:" + a + b + c;
};
auto c = make_curry(fn);
auto m1 = c("t1")("t2")("t3");
auto m2 = c("test1")("test2")("test3");
auto m3 = c("m1");
auto m4 = m3("m2");
auto m5 = m4("m3");
std::cout << m1 << std::endl;
std::cout << m2 << std::endl;
std::cout << m5 << std::endl;
}
auto func3 = make_curry(func);
std::cout << func3("Hello, ")( "World!", " !hi") << std::endl;
std::cout << func3("Hello, ","World!")(" !hi") << std::endl;
std::cout << func3("Hello, ","World!", " !hi") << std::endl;
std::cout << func3()("Hello, ", "World!", " !hi") << std::endl;
return 0;
}
What is currying?
How can currying be done in C++?
Please Explain binders in STL container?
1. What is currying?
Currying simply means a transformation of a function of several arguments to a function of a single argument. This is most easily illustrated using an example:
Take a function f that accepts three arguments:
int
f(int a,std::string b,float c)
{
// do something with a, b, and c
return 0;
}
If we want to call f, we have to provide all of its arguments f(1,"some string",19.7f).
Then a curried version of f, let's call it curried_f=curry(f) only expects a single argument, that corresponds to the first argument of f, namely the argument a. Additionally, f(1,"some string",19.7f) can also be written using the curried version as curried_f(1)("some string")(19.7f). The return value of curried_f(1) on the other hand is just another function, that handles the next argument of f. In the end, we end up with a function or callable curried_f that fulfills the following equality:
curried_f(first_arg)(second_arg)...(last_arg) == f(first_arg,second_arg,...,last_arg).
2. How can currying be achieved in C++?
The following is a little bit more complicated, but works very well for me (using c++11)... It also allows currying of arbitrary degree like so: auto curried=curry(f)(arg1)(arg2)(arg3) and later auto result=curried(arg4)(arg5). Here it goes:
#include <functional>
namespace _dtl {
template <typename FUNCTION> struct
_curry;
// specialization for functions with a single argument
template <typename R,typename T> struct
_curry<std::function<R(T)>> {
using
type = std::function<R(T)>;
const type
result;
_curry(type fun) : result(fun) {}
};
// recursive specialization for functions with more arguments
template <typename R,typename T,typename...Ts> struct
_curry<std::function<R(T,Ts...)>> {
using
remaining_type = typename _curry<std::function<R(Ts...)> >::type;
using
type = std::function<remaining_type(T)>;
const type
result;
_curry(std::function<R(T,Ts...)> fun)
: result (
[=](const T& t) {
return _curry<std::function<R(Ts...)>>(
[=](const Ts&...ts){
return fun(t, ts...);
}
).result;
}
) {}
};
}
template <typename R,typename...Ts> auto
curry(const std::function<R(Ts...)> fun)
-> typename _dtl::_curry<std::function<R(Ts...)>>::type
{
return _dtl::_curry<std::function<R(Ts...)>>(fun).result;
}
template <typename R,typename...Ts> auto
curry(R(* const fun)(Ts...))
-> typename _dtl::_curry<std::function<R(Ts...)>>::type
{
return _dtl::_curry<std::function<R(Ts...)>>(fun).result;
}
#include <iostream>
void
f(std::string a,std::string b,std::string c)
{
std::cout << a << b << c;
}
int
main() {
curry(f)("Hello ")("functional ")("world!");
return 0;
}
View output
OK, as Samer commented, I should add some explanations as to how this works. The actual implementation is done in the _dtl::_curry, while the template functions curry are only convenience wrappers. The implementation is recursive over the arguments of the std::function template argument FUNCTION.
For a function with only a single argument, the result is identical to the original function.
_curry(std::function<R(T,Ts...)> fun)
: result (
[=](const T& t) {
return _curry<std::function<R(Ts...)>>(
[=](const Ts&...ts){
return fun(t, ts...);
}
).result;
}
) {}
Here the tricky thing: For a function with more arguments, we return a lambda whose argument is bound to the first argument to the call to fun. Finally, the remaining currying for the remaining N-1 arguments is delegated to the implementation of _curry<Ts...> with one less template argument.
Update for c++14 / 17:
A new idea to approach the problem of currying just came to me... With the introduction of if constexpr into c++17 (and with the help of void_t to determine if a function is fully curried), things seem to get a lot easier:
template< class, class = std::void_t<> > struct
needs_unapply : std::true_type { };
template< class T > struct
needs_unapply<T, std::void_t<decltype(std::declval<T>()())>> : std::false_type { };
template <typename F> auto
curry(F&& f) {
/// Check if f() is a valid function call. If not we need
/// to curry at least one argument:
if constexpr (needs_unapply<decltype(f)>::value) {
return [=](auto&& x) {
return curry(
[=](auto&&...xs) -> decltype(f(x,xs...)) {
return f(x,xs...);
}
);
};
}
else {
/// If 'f()' is a valid call, just call it, we are done.
return f();
}
}
int
main()
{
auto f = [](auto a, auto b, auto c, auto d) {
return a * b * c * d;
};
return curry(f)(1)(2)(3)(4);
}
See code in action on here. With a similar approach, here is how to curry functions with arbitrary number of arguments.
The same idea seems to work out also in C++14, if we exchange the constexpr if with a template selection depending on the test needs_unapply<decltype(f)>::value:
template <typename F> auto
curry(F&& f);
template <bool> struct
curry_on;
template <> struct
curry_on<false> {
template <typename F> static auto
apply(F&& f) {
return f();
}
};
template <> struct
curry_on<true> {
template <typename F> static auto
apply(F&& f) {
return [=](auto&& x) {
return curry(
[=](auto&&...xs) -> decltype(f(x,xs...)) {
return f(x,xs...);
}
);
};
}
};
template <typename F> auto
curry(F&& f) {
return curry_on<needs_unapply<decltype(f)>::value>::template apply(f);
}
In short, currying takes a function f(x, y) and given a fixed Y, gives a new function g(x) where
g(x) == f(x, Y)
This new function may be called in situations where only one argument is supplied, and passes the call on to the original f function with the fixed Y argument.
The binders in the STL allow you to do this for C++ functions. For example:
#include <functional>
#include <iostream>
#include <vector>
using namespace std;
// declare a binary function object
class adder: public binary_function<int, int, int> {
public:
int operator()(int x, int y) const
{
return x + y;
}
};
int main()
{
// initialise some sample data
vector<int> a, b;
a.push_back(1);
a.push_back(2);
a.push_back(3);
// here we declare a function object f and try it out
adder f;
cout << "f(2, 3) = " << f(2, 3) << endl;
// transform() expects a function with one argument, so we use
// bind2nd to make a new function based on f, that takes one
// argument and adds 5 to it
transform(a.begin(), a.end(), back_inserter(b), bind2nd(f, 5));
// output b to see what we got
cout << "b = [" << endl;
for (vector<int>::iterator i = b.begin(); i != b.end(); ++i) {
cout << " " << *i << endl;
}
cout << "]" << endl;
return 0;
}
Simplifying Gregg's example, using tr1:
#include <functional>
using namespace std;
using namespace std::tr1;
using namespace std::tr1::placeholders;
int f(int, int);
..
int main(){
function<int(int)> g = bind(f, _1, 5); // g(x) == f(x, 5)
function<int(int)> h = bind(f, 2, _1); // h(x) == f(2, x)
function<int(int,int)> j = bind(g, _2); // j(x,y) == g(y)
}
Tr1 functional components allow you to write rich functional-style code in C++. As well, C++0x will allow for in-line lambda functions to do this as well:
int f(int, int);
..
int main(){
auto g = [](int x){ return f(x,5); }; // g(x) == f(x, 5)
auto h = [](int x){ return f(2,x); }; // h(x) == f(2, x)
auto j = [](int x, int y){ return g(y); }; // j(x,y) == g(y)
}
And while C++ doesn't provide the rich side-effect analysis that some functional-oriented programming languages perform, const analysis and C++0x lambda syntax can help:
struct foo{
int x;
int operator()(int y) const {
x = 42; // error! const function can't modify members
}
};
..
int main(){
int x;
auto f = [](int y){ x = 42; }; // error! lambdas don't capture by default.
}
Hope that helps.
Have a look at Boost.Bind which makes the process shown by Greg more versatile:
transform(a.begin(), a.end(), back_inserter(b), bind(f, _1, 5));
This binds 5 to f's second argument.
It’s worth noting that this is not currying (instead, it’s partial application). However, using currying in a general way is hard in C++ (in fact, it only recently became possible at all) and partial application is often used instead.
Other answers nicely explain binders, so I won't repeat that part here. I will only demonstrate how currying and partial application can be done with lambdas in C++0x.
Code example: (Explanation in comments)
#include <iostream>
#include <functional>
using namespace std;
const function<int(int, int)> & simple_add =
[](int a, int b) -> int {
return a + b;
};
const function<function<int(int)>(int)> & curried_add =
[](int a) -> function<int(int)> {
return [a](int b) -> int {
return a + b;
};
};
int main() {
// Demonstrating simple_add
cout << simple_add(4, 5) << endl; // prints 9
// Demonstrating curried_add
cout << curried_add(4)(5) << endl; // prints 9
// Create a partially applied function from curried_add
const auto & add_4 = curried_add(4);
cout << add_4(5) << endl; // prints 9
}
If you're using C++14 it's very easy:
template<typename Function, typename... Arguments>
auto curry(Function function, Arguments... args) {
return [=](auto... rest) {
return function(args..., rest...);
}; // don't forget semicolumn
}
You can then use it like this:
auto add = [](auto x, auto y) { return x + y; }
// curry 4 into add
auto add4 = curry(add, 4);
add4(6); // 10
Some great answers here. I thought I would add my own because it was fun to play around with the concept.
Partial function application: The process of "binding" a function with only some of its parameters, deferring the rest to be filled in later. The result is another function with fewer parameters.
Currying: Is a special form of partial function application where you can only "bind" a single argument at a time. The result is another function with exactly 1 fewer parameter.
The code I'm about to present is partial function application from which currying is possible, but not the only possibility. It offers a few benefits over the above currying implementations (mainly because it's partial function application and not currying, heh).
Applying over an empty function:
auto sum0 = [](){return 0;};
std::cout << partial_apply(sum0)() << std::endl;
Applying multiple arguments at a time:
auto sum10 = [](int a, int b, int c, int d, int e, int f, int g, int h, int i, int j){return a+b+c+d+e+f+g+h+i+j;};
std::cout << partial_apply(sum10)(1)(1,1)(1,1,1)(1,1,1,1) << std::endl; // 10
constexpr support that allows for compile-time static_assert:
static_assert(partial_apply(sum0)() == 0);
A useful error message if you accidentally go too far in providing arguments:
auto sum1 = [](int x){ return x;};
partial_apply(sum1)(1)(1);
error: static_assert failed "Attempting to apply too many arguments!"
Other answers above return lambdas that bind an argument and then return further lambdas. This approach wraps that essential functionality into a callable object. Definitions for operator() allow the internal lambda to be called. Variadic templates allow us to check for someone going too far, and an implicit conversion function to the result type of the function call allows us to print the result or compare the object to a primitive.
Code:
namespace detail{
template<class F>
using is_zero_callable = decltype(std::declval<F>()());
template<class F>
constexpr bool is_zero_callable_v = std::experimental::is_detected_v<is_zero_callable, F>;
}
template<class F>
struct partial_apply_t
{
template<class... Args>
constexpr auto operator()(Args... args)
{
static_assert(sizeof...(args) == 0 || !is_zero_callable, "Attempting to apply too many arguments!");
auto bind_some = [=](auto... rest) -> decltype(myFun(args..., rest...))
{
return myFun(args..., rest...);
};
using bind_t = decltype(bind_some);
return partial_apply_t<bind_t>{bind_some};
}
explicit constexpr partial_apply_t(F fun) : myFun(fun){}
constexpr operator auto()
{
if constexpr (is_zero_callable)
return myFun();
else
return *this; // a callable
}
static constexpr bool is_zero_callable = detail::is_zero_callable_v<F>;
F myFun;
};
Live Demo
A few more notes:
I chose to use is_detected mainly for enjoyment and practice; it serves the same as a normal type trait would here.
There could definitely be more work done to support perfect forwarding for performance reasons
The code is C++17 because it requires for constexpr lambda support in C++17
And it seems that GCC 7.0.1 is not quite there yet, either, so I used Clang 5.0.0
Some tests:
auto sum0 = [](){return 0;};
auto sum1 = [](int x){ return x;};
auto sum2 = [](int x, int y){ return x + y;};
auto sum3 = [](int x, int y, int z){ return x + y + z; };
auto sum10 = [](int a, int b, int c, int d, int e, int f, int g, int h, int i, int j){return a+b+c+d+e+f+g+h+i+j;};
std::cout << partial_apply(sum0)() << std::endl; //0
static_assert(partial_apply(sum0)() == 0, "sum0 should return 0");
std::cout << partial_apply(sum1)(1) << std::endl; // 1
std::cout << partial_apply(sum2)(1)(1) << std::endl; // 2
std::cout << partial_apply(sum3)(1)(1)(1) << std::endl; // 3
static_assert(partial_apply(sum3)(1)(1)(1) == 3, "sum3 should return 3");
std::cout << partial_apply(sum10)(1)(1,1)(1,1,1)(1,1,1,1) << std::endl; // 10
//partial_apply(sum1)(1)(1); // fails static assert
auto partiallyApplied = partial_apply(sum3)(1)(1);
std::function<int(int)> finish_applying = partiallyApplied;
std::cout << std::boolalpha << (finish_applying(1) == 3) << std::endl; // true
auto plus2 = partial_apply(sum3)(1)(1);
std::cout << std::boolalpha << (plus2(1) == 3) << std::endl; // true
std::cout << std::boolalpha << (plus2(3) == 5) << std::endl; // true
Currying is a way of reducing a function that takes multiple arguments into a sequence of nested functions with one argument each:
full = (lambda a, b, c: (a + b + c))
print full (1, 2, 3) # print 6
# Curried style
curried = (lambda a: (lambda b: (lambda c: (a + b + c))))
print curried (1)(2)(3) # print 6
Currying is nice because you can define functions that are simply wrappers around other functions with pre-defined values, and then pass around the simplified functions. C++ STL binders provide an implementation of this in C++.
I implemented currying with variadic templates as well (see Julian's answer). However, I did not make use of recursion or std::function. Note: It uses a number of C++14 features.
The provided example (main function) actually runs at compile time, proving that the currying method does not trump essential optimizations by the compiler.
The code can be found here: https://gist.github.com/Garciat/c7e4bef299ee5c607948
with this helper file: https://gist.github.com/Garciat/cafe27d04cfdff0e891e
The code still needs (a lot of) work, which I may or may not complete soon. Either way, I'm posting this here for future reference.
Posting code in case links die (though they shouldn't):
#include <type_traits>
#include <tuple>
#include <functional>
#include <iostream>
// ---
template <typename FType>
struct function_traits;
template <typename RType, typename... ArgTypes>
struct function_traits<RType(ArgTypes...)> {
using arity = std::integral_constant<size_t, sizeof...(ArgTypes)>;
using result_type = RType;
template <size_t Index>
using arg_type = typename std::tuple_element<Index, std::tuple<ArgTypes...>>::type;
};
// ---
namespace details {
template <typename T>
struct function_type_impl
: function_type_impl<decltype(&T::operator())>
{ };
template <typename RType, typename... ArgTypes>
struct function_type_impl<RType(ArgTypes...)> {
using type = RType(ArgTypes...);
};
template <typename RType, typename... ArgTypes>
struct function_type_impl<RType(*)(ArgTypes...)> {
using type = RType(ArgTypes...);
};
template <typename RType, typename... ArgTypes>
struct function_type_impl<std::function<RType(ArgTypes...)>> {
using type = RType(ArgTypes...);
};
template <typename T, typename RType, typename... ArgTypes>
struct function_type_impl<RType(T::*)(ArgTypes...)> {
using type = RType(ArgTypes...);
};
template <typename T, typename RType, typename... ArgTypes>
struct function_type_impl<RType(T::*)(ArgTypes...) const> {
using type = RType(ArgTypes...);
};
}
template <typename T>
struct function_type
: details::function_type_impl<typename std::remove_cv<typename std::remove_reference<T>::type>::type>
{ };
// ---
template <typename Args, typename Params>
struct apply_args;
template <typename HeadArgs, typename... Args, typename HeadParams, typename... Params>
struct apply_args<std::tuple<HeadArgs, Args...>, std::tuple<HeadParams, Params...>>
: std::enable_if<
std::is_constructible<HeadParams, HeadArgs>::value,
apply_args<std::tuple<Args...>, std::tuple<Params...>>
>::type
{ };
template <typename... Params>
struct apply_args<std::tuple<>, std::tuple<Params...>> {
using type = std::tuple<Params...>;
};
// ---
template <typename TupleType>
struct is_empty_tuple : std::false_type { };
template <>
struct is_empty_tuple<std::tuple<>> : std::true_type { };
// ----
template <typename FType, typename GivenArgs, typename RestArgs>
struct currying;
template <typename FType, typename... GivenArgs, typename... RestArgs>
struct currying<FType, std::tuple<GivenArgs...>, std::tuple<RestArgs...>> {
std::tuple<GivenArgs...> given_args;
FType func;
template <typename Func, typename... GivenArgsReal>
constexpr
currying(Func&& func, GivenArgsReal&&... args) :
given_args(std::forward<GivenArgsReal>(args)...),
func(std::move(func))
{ }
template <typename... Args>
constexpr
auto operator() (Args&&... args) const& {
using ParamsTuple = std::tuple<RestArgs...>;
using ArgsTuple = std::tuple<Args...>;
using RestArgsPrime = typename apply_args<ArgsTuple, ParamsTuple>::type;
using CanExecute = is_empty_tuple<RestArgsPrime>;
return apply(CanExecute{}, std::make_index_sequence<sizeof...(GivenArgs)>{}, std::forward<Args>(args)...);
}
template <typename... Args>
constexpr
auto operator() (Args&&... args) && {
using ParamsTuple = std::tuple<RestArgs...>;
using ArgsTuple = std::tuple<Args...>;
using RestArgsPrime = typename apply_args<ArgsTuple, ParamsTuple>::type;
using CanExecute = is_empty_tuple<RestArgsPrime>;
return std::move(*this).apply(CanExecute{}, std::make_index_sequence<sizeof...(GivenArgs)>{}, std::forward<Args>(args)...);
}
private:
template <typename... Args, size_t... Indices>
constexpr
auto apply(std::false_type, std::index_sequence<Indices...>, Args&&... args) const& {
using ParamsTuple = std::tuple<RestArgs...>;
using ArgsTuple = std::tuple<Args...>;
using RestArgsPrime = typename apply_args<ArgsTuple, ParamsTuple>::type;
using CurryType = currying<FType, std::tuple<GivenArgs..., Args...>, RestArgsPrime>;
return CurryType{ func, std::get<Indices>(given_args)..., std::forward<Args>(args)... };
}
template <typename... Args, size_t... Indices>
constexpr
auto apply(std::false_type, std::index_sequence<Indices...>, Args&&... args) && {
using ParamsTuple = std::tuple<RestArgs...>;
using ArgsTuple = std::tuple<Args...>;
using RestArgsPrime = typename apply_args<ArgsTuple, ParamsTuple>::type;
using CurryType = currying<FType, std::tuple<GivenArgs..., Args...>, RestArgsPrime>;
return CurryType{ std::move(func), std::get<Indices>(std::move(given_args))..., std::forward<Args>(args)... };
}
template <typename... Args, size_t... Indices>
constexpr
auto apply(std::true_type, std::index_sequence<Indices...>, Args&&... args) const& {
return func(std::get<Indices>(given_args)..., std::forward<Args>(args)...);
}
template <typename... Args, size_t... Indices>
constexpr
auto apply(std::true_type, std::index_sequence<Indices...>, Args&&... args) && {
return func(std::get<Indices>(std::move(given_args))..., std::forward<Args>(args)...);
}
};
// ---
template <typename FType, size_t... Indices>
constexpr
auto curry(FType&& func, std::index_sequence<Indices...>) {
using RealFType = typename function_type<FType>::type;
using FTypeTraits = function_traits<RealFType>;
using CurryType = currying<FType, std::tuple<>, std::tuple<typename FTypeTraits::template arg_type<Indices>...>>;
return CurryType{ std::move(func) };
}
template <typename FType>
constexpr
auto curry(FType&& func) {
using RealFType = typename function_type<FType>::type;
using FTypeArity = typename function_traits<RealFType>::arity;
return curry(std::move(func), std::make_index_sequence<FTypeArity::value>{});
}
// ---
int main() {
auto add = curry([](int a, int b) { return a + b; });
std::cout << add(5)(10) << std::endl;
}
These Links are relevant:
The Lambda Calculus page on Wikipedia has a clear example of currying
http://en.wikipedia.org/wiki/Lambda_calculus#Motivation
This paper treats currying in C/C++
http://asg.unige.ch/site/papers/Dami91a.pdf
C++20 provides bind_front for doing currying.
For older C++ version it can be implemented (for single argument) as follows:
template <typename TFunc, typename TArg>
class CurryT
{
private:
TFunc func;
TArg arg ;
public:
template <typename TFunc_, typename TArg_>
CurryT(TFunc_ &&func, TArg_ &&arg)
: func(std::forward<TFunc_>(func))
, arg (std::forward<TArg_ >(arg ))
{}
template <typename... TArgs>
auto operator()(TArgs &&...args) const
-> decltype( func(arg, std::forward<TArgs>(args)...) )
{ return func(arg, std::forward<TArgs>(args)...); }
};
template <typename TFunc, typename TArg>
CurryT<std::decay_t<TFunc>, std::remove_cv_t<TArg>> Curry(TFunc &&func, TArg &&arg)
{ return {std::forward<TFunc>(func), std::forward<TArg>(arg)}; }
https://coliru.stacked-crooked.com/a/82856e39da5fa50d
void Abc(std::string a, int b, int c)
{
std::cerr << a << b << c << std::endl;
}
int main()
{
std::string str = "Hey";
auto c1 = Curry(Abc, str);
std::cerr << "str: " << str << std::endl;
c1(1, 2);
auto c2 = Curry(std::move(c1), 3);
c2(4);
auto c3 = Curry(c2, 5);
c3();
}
Output:
str:
Hey12
Hey34
Hey35
If you use long chains of currying then std::shared_ptr optimization can be used to avoid copying all previous curried parameters to each new carried function.
template <typename TFunc>
class SharedFunc
{
public:
struct Tag{}; // For avoiding shadowing copy/move constructors with the
// templated constructor below which accepts any parameters.
template <typename... TArgs>
SharedFunc(Tag, TArgs &&...args)
: p_func( std::make_shared<TFunc>(std::forward<TArgs>(args)...) )
{}
template <typename... TArgs>
auto operator()(TArgs &&...args) const
-> decltype( (*p_func)(std::forward<TArgs>(args)...) )
{ return (*p_func)(std::forward<TArgs>(args)...); }
private:
std::shared_ptr<TFunc> p_func;
};
template <typename TFunc, typename TArg>
SharedFunc<
CurryT<std::decay_t<TFunc>, std::remove_cv_t<TArg>>
>
CurryShared(TFunc &&func, TArg &&arg)
{
return { {}, std::forward<TFunc>(func), std::forward<TArg>(arg) };
}
https://coliru.stacked-crooked.com/a/6e71f41e1cc5fd5c