I want to do identical processing to a bunch of arguments of a function. Is there a way to loop over all arguments ? I am doing it the way represented in following code, but want to see if there is a compact way to do this.,
void methodA(int a1, int a2, int b1, double b2){
//.. some code
methodB(a1, f(a1));
methodB(a2, f(a2));
methodB(b1, f(b1));
methodB(b2, f(b2));
// more code follows ...
}
int f(int a){
// some function.
return a*10;
}
double f(double b){
return b/2.0;
}
You could use variadic templates:
template <typename T, typename ...Args>
void methodAHelper(T && t, Args &&... args)
{
methodB(t, f(t));
methodAHelper(std::forward<Args>(args)...);
}
void methodAHelper() { }
template <typename ...Args>
void methodA(Args &&... args)
{
// some code
methodAHelper(std::forward<Args>(args)...);
// some other code
}
You can possibly get rid of the && and the forwarding if you know that your methodB call doesn't know about rvalue references, that would make the code a bit simpler (you'd have const Args &... instead), for example:
methodAHelper(const T & t, const Args &... args)
{
methodB(t, f(t));
methodAHelper(args...);
}
You might also consider changing methodB: Since the second argument is a function of the first argument, you might be able to only pass the first argument and perform the call to f() inside the methodB(). That reduces coupling and interdependence; for example, the entire declaration of f would only need to be known to the implementation of methodB. But that's depends on your actual situation.
Alternatively, if there is only one overload of methodB whose first argument is of type T, then you could just pass a std::vector<T> to methodA and iterate over it:
void methodA(const std::vector<T> & v)
{
// some code
for (auto it = v.cbegin(), end = v.cend(); it != end; ++it)
methodB(*it, f(*it));
// some more code
}
int main() { methodA(std::vector<int>{1,2,3,4}); }
Yes there is, the concept you're looking for is called a variadic function.
Depending on what you are trying to do. The simplest thing might to revisit your function and see if it can take an array or std::vector as an argument. Might be much simpler that going the variadic route
Related
I have written the following short code in C++11 of a variable template function and store the arguments into a vector of type boost::any. It is working perfectly, but I don't want to use the boost::any library (due to some limitation).
#include <boost/any.hpp>
template <class Var, class... Args>
void cpp_for(Var *variable, uint32_t numParams, Args... args)
{
std::vector<boost::any> arguments{args...};
if(arguments.size() != numParams)
throw std::runtime_error("mismatch");
for(uint32_t i = 0; i < numParams; ++i)
variable[i] = *(boost::unsafe_any_cast<Var>(&arguments[i]));
}
And I call the function like this:
cpp_for(myObj->var, 3, 0x56, 0x23, 0x10);
Or
cpp_for(myObj2->var, 2, myObj2->var2, myObj2->var3);
Is there any way to store the arguments and process them one by one without the need for boost::any?
Edit 1: my arguments are all of the same type.
Edit 2: Since the goal of the code above is assignment, then creating an extra data structure (vector) is useless. Check 'Nir Friedman''s answer for a more efficient solution.
You could use std::common_type, e.g.:
template <class Var, class... Args>
void CPP_FOR(Var *variable, uint32_t numParams, Args... args)
{
std::vector<std::common_type_t<Args...>> arguments{args...};
// do stuff with arguments
}
You can also remove numParams and the runtime check because this will fail at compile time if there is no common type. And if you only want to iterate over the arguments, a vector is maybe overkill... so something like:
template <class Var, class... Args>
void CPP_FOR(Var *variable, Args... args)
{
std::common_type_t<Args...> arguments[]{args...};
for(size_t i = 0; i < sizeof...(Args); ++i)
variable[i] = /* ... */;
}
Note that both of these will fails if sizeof... (Args) is 0, i.e. you are calling with only a Var* - You may want to handle this case separately if necessary.
Assuming that your goal is really just to perform assignments, you don't need a vector at all.
template <class Var, class... Args>
void CPP_FOR(Var *variable, uint32_t numParams, Args... args)
{
if(sizeof...(Args) != numParams)
throw std::runtime_error("mismatch");
int i = 0;
int temp [] = {(variable[i++] = args, 0)...};
}
Live example: http://coliru.stacked-crooked.com/a/710a09332bf2c965
Not only is this zero overhead compared to creating a vector that is hard to optimize away, but it will allow implicit conversions in the most natural way. Other approaches may have surprises.
If you know that there's going to be at least one argument, you can write your function slightly differently to do so.
template <typename Arg, typename... Args>
void cpp_for(Arg arg, Args... args) {
std::vector<Arg> vec { arg, args... };
// ...
}
However, this will fail if the argument list is empty. The easiest solution to this is simply to add a second overload of cpp_for which takes no arguments.
void cpp_for() {
std::vector<SomeDefaultType> vec; // Empty vector
// ...
}
Of course, you only need to do so if it makes sense to call your function with zero arguments.
Bear in mind that this approach will give positively miserable error messages if Args and Arg don't all end up being the same type. This can be remedied with some careful use of static_assert.
I try to pass to a variadic template function a list of references and pass it to another function. The code that I wrote is the following:
template <typename T>
void fun(cv::Point_<T> & pt) { pt.x++; pt.y++; }
template <class ... args>
void caller(args & ... list) {
typedef typename std::tuple_element<0, std::tuple<args...> >::type T;
std::array<std::reference_wrapper<T>, sizeof...(list)> values {list ... };
for(int i=0; i<values.size(); i++)
fun(values[i]);
}
then I call the function caller in this way:
cv::Point2f a, b, c;
caller(a, b, c);
the compiler give me the following error:
No matching function for call to 'fun'
Candidate template ignored: could not match 'Point_' against 'reference_wrapper'
what I missing?
Although std::reference_wrapper<T> has an implicit conversion to T&, you cannot use both an implicit conversion and template argument deduction at the same time, and template argument deduction is necessary to call fun.
Try
fun(values[i].get());
Even simpler is
template <typename...Args>
void caller(Args&...args)
{
auto tmp = { (func(args),0)..., 0 };
}
This uses the fact that parameter pack expansion can occur in braced init lists. Since func() returns void, we cannot simply use { func(args)... }, but use (func(args),0) to have an int. Finally, the last 0 is to ensure that the code compiles (and does nothing) in case of an empty parameter pack.
You can generalise this and write a template that calls a given generic function for every element of a pack:
template <typename Func, typename...Args>
void call_for_each(Func &&func, Args&&...args)
{
auto unused = { (func(std::forward<Args>(args)),0)...,0 };
}
which may be used like this (C++14)
int main()
{
int a=1;
double b=2.4;
auto func = [](auto&x) { std::cout<<' '<<x++; };
call_for_each(func,a,b);
std::cout<<'\n';
call_for_each(func,a,b);
std::cout<<'\n';
}
This uses a C++14 lambda (taking an auto argument). Note that the parameter pack must come last among the template parameters of call_for_each.
Since the goal of this might be to iterate over all args, here's a more generic solution. We are going to implement for_pack:
template<typename... Args, typename F>
void for_pack(F function, Args&&... args) {
using expand = int[];
(void)expand{(function(std::forward<Args>(args)), void(), 0)..., 0};
}
This will execute function for every args in Args.
Now, your function caller is much more trivial to implement:
template <typename... args>
void caller(args&... list) {
for_pack([&](cv::Point_<T>& arg){
fun(arg);
}, list...);
}
Since a google search for "c++ pass reference parameters to variadic template" gives this as first result, I'll put this generic solution here.
struct HH { /*...*/ void change_me() { /*...*/ } };
template<typename...T> void parms_r_refs() {}
template<typename H, typename...T> void parms_r_refs(H &h, T&...t) { h.change_me(); parms_r_refs(t...); }
template<typename...T> void parms_r_refs(T&...t) { parms_r_refs(t...); }
HH a, b, c;
..
parms_r_refs(a, b, c);
..
I'm trying to programming in C++ a framework where the user can indicates a set of functions inside its program where he wants to apply a memoization strategy.
So let's suppose that we have 5 functions in our program f1...f5 and we want to avoid the (expensive) re-computation for the functions f1 and f3 if we already called them with the same input. Notice that each function can have different return and argument types.
I found this solution for the problem, but you can use only double and int.
MY SOLUTION
Ok I wrote this solution for my problem, but I don't know if it's efficient, typesafe or can be written in any more elegant way.
template <typename ReturnType, typename... Args>
function<ReturnType(Args...)> memoize(function<ReturnType(Args...)> func)
{
return ([=](Args... args) mutable {
static map<tuple<Args...>, ReturnType> cache;
tuple<Args...> t(args...);
auto result = cache.insert(make_pair(t, ReturnType{}));
if (result.second) {
// insertion succeeded so the value wasn't cached already
result.first->second = func(args...);
}
return result.first->second;
});
}
struct MultiMemoizator
{
map<string, boost::any> multiCache;
template <typename ReturnType, typename... Args>
void addFunction(string name, function < ReturnType(Args...)> func) {
function < ReturnType(Args...)> cachedFunc = memoize(func);
boost::any anyCachedFunc = cachedFunc;
auto result = multiCache.insert(pair<string, boost::any>(name,anyCachedFunc));
if (!result.second)
cout << "ERROR: key " + name + " was already inserted" << endl;
}
template <typename ReturnType, typename... Args>
ReturnType callFunction(string name, Args... args) {
auto it = multiCache.find(name);
if (it == multiCache.end())
throw KeyNotFound(name);
boost::any anyCachedFunc = it->second;
function < ReturnType(Args...)> cachedFunc = boost::any_cast<function<ReturnType(Args...)>> (anyCachedFunc);
return cachedFunc(args...);
}
};
And this is a possible main:
int main()
{
function<int(int)> intFun = [](int i) {return ++i; };
function<string(string)> stringFun = [](string s) {
return "Hello "+s;
};
MultiMemoizator mem;
mem.addFunction("intFun",intFun);
mem.addFunction("stringFun", stringFun);
try
{
cout << mem.callFunction<int, int>("intFun", 1)<<endl;//print 2
cout << mem.callFunction<string, string>("stringFun", " World!") << endl;//print Hello World!
cout << mem.callFunction<string, string>("TrumpIsADickHead", " World!") << endl;//KeyNotFound thrown
}
catch (boost::bad_any_cast e)
{
cout << "Bad function calling: "<<e.what()<<endl;
return 1;
}
catch (KeyNotFound e)
{
cout << e.what()<<endl;
return 1;
}
}
How about something like this:
template <typename result_t, typename... args_t>
class Memoizer
{
public:
typedef result_t (*function_t)(args_t...);
Memoizer(function_t func) : m_func(func) {}
result_t operator() (args_t... args)
{
auto args_tuple = make_tuple(args...);
auto it = m_results.find(args_tuple);
if (it != m_results.end())
return it->second;
result_t result = m_func(args...);
m_results.insert(make_pair(args_tuple, result));
return result;
}
protected:
function_t m_func;
map<tuple<args_t...>, result_t> m_results;
};
Usage is like this:
// could create make_memoizer like make_tuple to eliminate the template arguments
Memoizer<double, double> memo(fabs);
cout << memo(-123.456);
cout << memo(-123.456); // not recomputed
It's pretty hard to guess at how you're planning to use the functions, with or without memoisation, but for the container-of-various-function<>s aspect you just need a common base class:
#include <iostream>
#include <vector>
#include <functional>
struct Any_Function
{
virtual ~Any_Function() {}
};
template <typename Ret, typename... Args>
struct Function : Any_Function, std::function<Ret(Args...)>
{
template <typename T>
Function(T& f)
: std::function<Ret(Args...)>(f)
{ }
};
int main()
{
std::vector<Any_Function*> fun_vect;
auto* p = new Function<int, double, double, int> { [](double i, double j, int z) {
return int(i + j + z);
} };
fun_vect.push_back(p);
}
The problem with this is how to make it type-safe. Look at this code:
MultiMemoizator mm;
std::string name = "identity";
mm.addFunction(name, identity);
auto result = mm.callFunction(name, 1);
Is the last line correct? Does callFunction have the right number of parameters with the right types? And what is the return type?
The compiler has no way to know that: it has no way of understanding that name is "identity" and even if it did, no way to associate that with the type of the function. And this is not specific to C++, any statically-typed language is going to have the same problem.
One solution (which is basically the one given in Tony D's answer) is to tell the compiler the function signature when you call the function. And if you say it wrong, a runtime error occurs. That could look something like this (you only need to explicitly specify the return type, since the number and type of parameters is inferred):
auto result = mm.callFunction<int>(name, 1);
But this is inelegant and error-prone.
Depending on your exact requirements, what might work better is to use "smart" keys, instead of strings: the key has the function signature embedded in its type, so you don't have to worry about specifying it correctly. That could look something like:
Key<int(int)> identityKey;
mm.addFunction(identityKey, identity);
auto result = mm.callFunction(identityKey, 1);
This way, the types are checked at compile time (both for addFunction and callFunction), which should give you exactly what you want.
I haven't actually implemented this in C++, but I don't see any reason why it should be hard or impossible. Especially since doing something very similar in C# is simple.
you can use vector of functions with signature like void someFunction(void *r, ...) where r is a pointer to result and ... is variadic argument list. Warning: unpacking argument list is really inconvenient and looks more like a hack.
At first glance, how about defining a type that has template arguments that differ for each function, i.e.:
template <class RetType, class ArgType>
class AbstractFunction {
//etc.
}
have the AbstractFunction take a function pointer to the functions f1-f5 with template specializations different for each function. You can then have a generic run_memoized() function, either as a member function of AbstractFunction or a templated function that takes an AbstractFunction as an argument and maintains a memo as it runs it.
The hardest part will be if the functions f1-f5 have more than one argument, in which case you'll need to do some funky things with arglists as template parameters but I think C++14 has some features that might make this possible. An alternative is to rewrite f1-f5 so that they all take a single struct as an argument rather than multiple arguments.
EDIT: Having seen your problem 1, the problem you're running into is that you want to have a data structure whose values are memoized functions, each of which could have different arguments.
I, personally, would solve this just by making the data structure use void* to represent the individual memoized functions, and then in the callFunction() method use an unsafe type cast from void* to the templated MemoizedFunction type you need (you may need to allocate MemoizedFunctions with the "new" operator so that you can convert them to and from void*s.)
If the lack of type safety here irks you, good for you, in that case it may be a reasonable option just to make hand-written helper methods for each of f1-f5 and have callFunction() dispatch one of those functions based on the input string. This will let you use compile-time type checking.
EDIT #2: If you are going to use this approach, you need to change the API for callFunction() slightly so that callFunction has template args matching the return and argument types of the function, for example:
int result = callFunction<int, arglist(double, float)>("double_and_float_to_int", 3.5, 4);
and if the user of this API ever types the argument type or return types incorrectly when using callFunction... pray for their soul because things will explode in very ugly ways.
EDIT #3: You can to some extent do the type checking you need at runtime using std::type_info and storing the typeid() of the argument type and return type in your MemoizedFunction so that you can check whether the template arguments in callFunction() are correct before calling - so you can prevent the explosion above. But this will add a bit of overhead every time you call the function (you could wrap this in a IF_DEBUG_MODE macro to only add this overhead during testing and not in production.)
If I have a function which is required to produce a hook from an input object, should I try to do that without casting to std::function? With these 2 options, which should I pick and is option 2 a meaningful improvement?
std::function<void()> CreateHook(std::function<void()> f)
{
return []()
{
return Hook(f);
};
}
// option 1:
void Hook(std::function<void()> f)
{
// do something
}
// option 2:
<typename F>
void Hook(F f)
{
// do something
}
Type erase only when you need to. Either:
template<class F>
std::function<void()> CreateHook(F f) { return []() { return Hook(f); }; }
or even in C++1y:
template<class F>
auto CreateHook(F f) { return [f=std::move(f)]() { return Hook(f); }; }
but the second is probably overkill (the f=std::move(f) is not, but auto is, as I am guessing you will just store the return value in a std::function anyhow).
And if your Hook function is simple and stable:
template <typename F> void Hook(F f) {
// do something
}
but if it is large and complex:
void Hook(std::function<void()> f) {
// do something
}
because it lets you split interface from implementation.
The downside to this strategy is that some compilers suck at eliminating identical code in different functions causing binary size bloat, and it can cause some compile time bloat.
But if you defer type erasure in both cases, you can eliminate both two virtual function calls, and allow the compiler to inline the passed in f within the Hook body.
If however the interface for CreateHook cannot be changed, and Hook is only called from it, the template version of Hook is pointless, as it is only called with a std::function anyhow.
In your example the template is pointless, because it is only ever instantiated for T = std::function<void()> - the template overload is chosen by static type of f, not the runtime type.
I'm trying to find a method to iterate over an a pack variadic template argument list.
Now as with all iterations, you need some sort of method of knowing how many arguments are in the packed list, and more importantly how to individually get data from a packed argument list.
The general idea is to iterate over the list, store all data of type int into a vector, store all data of type char* into a vector, and store all data of type float, into a vector. During this process there also needs to be a seperate vector that stores individual chars of what order the arguments went in. As an example, when you push_back(a_float), you're also doing a push_back('f') which is simply storing an individual char to know the order of the data. I could also use a std::string here and simply use +=. The vector was just used as an example.
Now the way the thing is designed is the function itself is constructed using a macro, despite the evil intentions, it's required, as this is an experiment. So it's literally impossible to use a recursive call, since the actual implementation that will house all this will be expanded at compile time; and you cannot recruse a macro.
Despite all possible attempts, I'm still stuck at figuring out how to actually do this. So instead I'm using a more convoluted method that involves constructing a type, and passing that type into the varadic template, expanding it inside a vector and then simply iterating that. However I do not want to have to call the function like:
foo(arg(1), arg(2.0f), arg("three");
So the real question is how can I do without such? To give you guys a better understanding of what the code is actually doing, I've pasted the optimistic approach that I'm currently using.
struct any {
void do_i(int e) { INT = e; }
void do_f(float e) { FLOAT = e; }
void do_s(char* e) { STRING = e; }
int INT;
float FLOAT;
char *STRING;
};
template<typename T> struct get { T operator()(const any& t) { return T(); } };
template<> struct get<int> { int operator()(const any& t) { return t.INT; } };
template<> struct get<float> { float operator()(const any& t) { return t.FLOAT; } };
template<> struct get<char*> { char* operator()(const any& t) { return t.STRING; } };
#define def(name) \
template<typename... T> \
auto name (T... argv) -> any { \
std::initializer_list<any> argin = { argv... }; \
std::vector<any> args = argin;
#define get(name,T) get<T>()(args[name])
#define end }
any arg(int a) { any arg; arg.INT = a; return arg; }
any arg(float f) { any arg; arg.FLOAT = f; return arg; }
any arg(char* s) { any arg; arg.STRING = s; return arg; }
I know this is nasty, however it's a pure experiment, and will not be used in production code. It's purely an idea. It could probably be done a better way. But an example of how you would use this system:
def(foo)
int data = get(0, int);
std::cout << data << std::endl;
end
looks a lot like python. it works too, but the only problem is how you call this function.
Heres a quick example:
foo(arg(1000));
I'm required to construct a new any type, which is highly aesthetic, but thats not to say those macros are not either. Aside the point, I just want to the option of doing:
foo(1000);
I know it can be done, I just need some sort of iteration method, or more importantly some std::get method for packed variadic template argument lists. Which I'm sure can be done.
Also to note, I'm well aware that this is not exactly type friendly, as I'm only supporting int,float,char* and thats okay with me. I'm not requiring anything else, and I'll add checks to use type_traits to validate that the arguments passed are indeed the correct ones to produce a compile time error if data is incorrect. This is purely not an issue. I also don't need support for anything other then these POD types.
It would be highly apprecaited if I could get some constructive help, opposed to arguments about my purely illogical and stupid use of macros and POD only types. I'm well aware of how fragile and broken the code is. This is merley an experiment, and I can later rectify issues with non-POD data, and make it more type-safe and useable.
Thanks for your undertstanding, and I'm looking forward to help.
If your inputs are all of the same type, see OMGtechy's great answer.
For mixed-types we can use fold expressions (introduced in c++17) with a callable (in this case, a lambda):
#include <iostream>
template <class ... Ts>
void Foo (Ts && ... inputs)
{
int i = 0;
([&]
{
// Do things in your "loop" lambda
++i;
std::cout << "input " << i << " = " << inputs << std::endl;
} (), ...);
}
int main ()
{
Foo(2, 3, 4u, (int64_t) 9, 'a', 2.3);
}
Live demo
(Thanks to glades for pointing out in the comments that I didn't need to explicitly pass inputs to the lambda. This made it a lot neater.)
If you need return/breaks in your loop, here are some workarounds:
Demo using try/throw. Note that throws can cause tremendous slow down of this function; so only use this option if speed isn't important, or the break/returns are genuinely exceptional.
Demo using variable/if switches.
These latter answers are honestly a code smell, but shows it's general-purpose.
If you want to wrap arguments to any, you can use the following setup. I also made the any class a bit more usable, although it isn't technically an any class.
#include <vector>
#include <iostream>
struct any {
enum type {Int, Float, String};
any(int e) { m_data.INT = e; m_type = Int;}
any(float e) { m_data.FLOAT = e; m_type = Float;}
any(char* e) { m_data.STRING = e; m_type = String;}
type get_type() const { return m_type; }
int get_int() const { return m_data.INT; }
float get_float() const { return m_data.FLOAT; }
char* get_string() const { return m_data.STRING; }
private:
type m_type;
union {
int INT;
float FLOAT;
char *STRING;
} m_data;
};
template <class ...Args>
void foo_imp(const Args&... args)
{
std::vector<any> vec = {args...};
for (unsigned i = 0; i < vec.size(); ++i) {
switch (vec[i].get_type()) {
case any::Int: std::cout << vec[i].get_int() << '\n'; break;
case any::Float: std::cout << vec[i].get_float() << '\n'; break;
case any::String: std::cout << vec[i].get_string() << '\n'; break;
}
}
}
template <class ...Args>
void foo(Args... args)
{
foo_imp(any(args)...); //pass each arg to any constructor, and call foo_imp with resulting any objects
}
int main()
{
char s[] = "Hello";
foo(1, 3.4f, s);
}
It is however possible to write functions to access the nth argument in a variadic template function and to apply a function to each argument, which might be a better way of doing whatever you want to achieve.
Range based for loops are wonderful:
#include <iostream>
#include <any>
template <typename... Things>
void printVariadic(Things... things) {
for(const auto p : {things...}) {
std::cout << p.type().name() << std::endl;
}
}
int main() {
printVariadic(std::any(42), std::any('?'), std::any("C++"));
}
For me, this produces the output:
i
c
PKc
Here's an example without std::any, which might be easier to understand for those not familiar with std::type_info:
#include <iostream>
template <typename... Things>
void printVariadic(Things... things) {
for(const auto p : {things...}) {
std::cout << p << std::endl;
}
}
int main() {
printVariadic(1, 2, 3);
}
As you might expect, this produces:
1
2
3
You can create a container of it by initializing it with your parameter pack between {}. As long as the type of params... is homogeneous or at least convertable to the element type of your container, it will work. (tested with g++ 4.6.1)
#include <array>
template <class... Params>
void f(Params... params) {
std::array<int, sizeof...(params)> list = {params...};
}
This is not how one would typically use Variadic templates, not at all.
Iterations over a variadic pack is not possible, as per the language rules, so you need to turn toward recursion.
class Stock
{
public:
bool isInt(size_t i) { return _indexes.at(i).first == Int; }
int getInt(size_t i) { assert(isInt(i)); return _ints.at(_indexes.at(i).second); }
// push (a)
template <typename... Args>
void push(int i, Args... args) {
_indexes.push_back(std::make_pair(Int, _ints.size()));
_ints.push_back(i);
this->push(args...);
}
// push (b)
template <typename... Args>
void push(float f, Args... args) {
_indexes.push_back(std::make_pair(Float, _floats.size()));
_floats.push_back(f);
this->push(args...);
}
private:
// push (c)
void push() {}
enum Type { Int, Float; };
typedef size_t Index;
std::vector<std::pair<Type,Index>> _indexes;
std::vector<int> _ints;
std::vector<float> _floats;
};
Example (in action), suppose we have Stock stock;:
stock.push(1, 3.2f, 4, 5, 4.2f); is resolved to (a) as the first argument is an int
this->push(args...) is expanded to this->push(3.2f, 4, 5, 4.2f);, which is resolved to (b) as the first argument is a float
this->push(args...) is expanded to this->push(4, 5, 4.2f);, which is resolved to (a) as the first argument is an int
this->push(args...) is expanded to this->push(5, 4.2f);, which is resolved to (a) as the first argument is an int
this->push(args...) is expanded to this->push(4.2f);, which is resolved to (b) as the first argument is a float
this->push(args...) is expanded to this->push();, which is resolved to (c) as there is no argument, thus ending the recursion
Thus:
Adding another type to handle is as simple as adding another overload, changing the first type (for example, std::string const&)
If a completely different type is passed (say Foo), then no overload can be selected, resulting in a compile-time error.
One caveat: Automatic conversion means a double would select overload (b) and a short would select overload (a). If this is not desired, then SFINAE need be introduced which makes the method slightly more complicated (well, their signatures at least), example:
template <typename T, typename... Args>
typename std::enable_if<is_int<T>::value>::type push(T i, Args... args);
Where is_int would be something like:
template <typename T> struct is_int { static bool constexpr value = false; };
template <> struct is_int<int> { static bool constexpr value = true; };
Another alternative, though, would be to consider a variant type. For example:
typedef boost::variant<int, float, std::string> Variant;
It exists already, with all utilities, it can be stored in a vector, copied, etc... and seems really much like what you need, even though it does not use Variadic Templates.
There is no specific feature for it right now but there are some workarounds you can use.
Using initialization list
One workaround uses the fact, that subexpressions of initialization lists are evaluated in order. int a[] = {get1(), get2()} will execute get1 before executing get2. Maybe fold expressions will come handy for similar techniques in the future. To call do() on every argument, you can do something like this:
template <class... Args>
void doSomething(Args... args) {
int x[] = {args.do()...};
}
However, this will only work when do() is returning an int. You can use the comma operator to support operations which do not return a proper value.
template <class... Args>
void doSomething(Args... args) {
int x[] = {(args.do(), 0)...};
}
To do more complex things, you can put them in another function:
template <class Arg>
void process(Arg arg, int &someOtherData) {
// You can do something with arg here.
}
template <class... Args>
void doSomething(Args... args) {
int someOtherData;
int x[] = {(process(args, someOtherData), 0)...};
}
Note that with generic lambdas (C++14), you can define a function to do this boilerplate for you.
template <class F, class... Args>
void do_for(F f, Args... args) {
int x[] = {(f(args), 0)...};
}
template <class... Args>
void doSomething(Args... args) {
do_for([&](auto arg) {
// You can do something with arg here.
}, args...);
}
Using recursion
Another possibility is to use recursion. Here is a small example that defines a similar function do_for as above.
template <class F, class First, class... Rest>
void do_for(F f, First first, Rest... rest) {
f(first);
do_for(f, rest...);
}
template <class F>
void do_for(F f) {
// Parameter pack is empty.
}
template <class... Args>
void doSomething(Args... args) {
do_for([&](auto arg) {
// You can do something with arg here.
}, args...);
}
You can't iterate, but you can recurse over the list. Check the printf() example on wikipedia: http://en.wikipedia.org/wiki/C++0x#Variadic_templates
You can use multiple variadic templates, this is a bit messy, but it works and is easy to understand.
You simply have a function with the variadic template like so:
template <typename ...ArgsType >
void function(ArgsType... Args){
helperFunction(Args...);
}
And a helper function like so:
void helperFunction() {}
template <typename T, typename ...ArgsType >
void helperFunction(T t, ArgsType... Args) {
//do what you want with t
function(Args...);
}
Now when you call "function" the "helperFunction" will be called and isolate the first passed parameter from the rest, this variable can b used to call another function (or something). Then "function" will be called again and again until there are no more variables left. Note you might have to declare helperClass before "function".
The final code will look like this:
void helperFunction();
template <typename T, typename ...ArgsType >
void helperFunction(T t, ArgsType... Args);
template <typename ...ArgsType >
void function(ArgsType... Args){
helperFunction(Args...);
}
void helperFunction() {}
template <typename T, typename ...ArgsType >
void helperFunction(T t, ArgsType... Args) {
//do what you want with t
function(Args...);
}
The code is not tested.
#include <iostream>
template <typename Fun>
void iteratePack(const Fun&) {}
template <typename Fun, typename Arg, typename ... Args>
void iteratePack(const Fun &fun, Arg &&arg, Args&& ... args)
{
fun(std::forward<Arg>(arg));
iteratePack(fun, std::forward<Args>(args)...);
}
template <typename ... Args>
void test(const Args& ... args)
{
iteratePack([&](auto &arg)
{
std::cout << arg << std::endl;
},
args...);
}
int main()
{
test(20, "hello", 40);
return 0;
}
Output:
20
hello
40