I need to know whether NDEBUG is defined when specifying the noexcept specifier. I was thinking along the lines of this constexpr function:
constexpr inline bool is_defined() noexcept
{
return false;
}
constexpr inline bool is_defined(int) noexcept
{
return true;
}
Then use it like:
void f() noexcept(is_defined(NDEBUG))
{
// blah, blah
}
Does the standard library or the language already provide a facility for this so that I won't be reinventing the wheel?
Just use #ifdef?
#ifdef NDEBUG
using is_ndebug = std::true_type;
#else
using is_ndebug = std::false_type;
#endif
void f() noexcept(is_ndebug{}) {
// blah, blah
}
or a myriad of other similar ways: A constexpr function returning bool or std::true_type (conditionally). A static variable of one of two types. A traits class that takes an enum which lists various #define token-equivalents (eNDEBUG etc) which can be specialized for each such token it has support for, and generates errors if there is no such support. Using typedef instead of using (if your compiler has flaky support for using, I'm looking at you MSVC2013). I'm sure there can be others.
If you are only interested in NDEBUG, this is equivalent to testing whether assert() evaluates it's argument or not. In that case, you can use:
void f() noexcept(noexcept(assert((throw true,true))))
{
// ...
}
This is, of course, not necessarily an improvement :)
Related
I want to write a class that makes use of numerical quadrature. The quadrature order defines the size of some containers that I will use. I would like to make a type alias for such containers and it has to depend on the quadrature order.
The following code shows my trials. It feels suboptimal in the sense that I have to repeat the order in the type alias definition:
#include <array>
class Quadrature
{
public:
static constexpr unsigned int getOrder()
{
return 3;
}
// This line doesn't compile!
//
// using WeightsContainer = std::array<double, getOrder()>;
//
// g++ says "error: 'static constexpr unsigned int Quadrature::getOrder()'
// called in a constant expression before its definition is complete"
// This line compiles, but repeats the order. :-(
using WeightsContainer = std::array<double, 3>;
private:
WeightsContainer container;
};
One solution that I have found is introducing a template parameter Order. But actually I wanted to determine the quadrature order and introducing the template parameter would make it variable.
Is there a possibility to make the order a compile-time constant and use it within my type alias definition?
Edit:
For completeness, I could of course use a preprocessor define. But that feels old-fashioned. :-)
Edit 2:
Okay, I have found another possibility. I could add a function outside the class scope like this:
constexpr unsigned int order()
{
return 3;
}
But that feels wrong, because this is a property of the class and therefore should be within class scope!
One thing you can do is to move the value into a member variable:
class Quadrature
{
private:
static constexpr unsigned int _order = 3;
public:
static constexpr unsigned int getOrder()
{
return _order;
}
using WeightsContainer = std::array<double, _order>;
// ...
};
If you need more complicated computations instead of just return 3, under C++17 you can use a lambda as #Quentin mentioned:
class Quadrature
{
public:
static constexpr auto getOrder = []()
{
return ...;
};
using WeightsContainer = std::array<double, getOrder()>;
// ...
};
Otherwise, you will need to pull the function outside of class scope for reasons mentioned here.
I would like to know if it is possible to have a for loop in compile time with runtime or even compile time limit condition in c++11?
I start with a silly try to find out what I need.
for (uint32_t i = 0; i < n; ++i)
{
templated_func<i>();
}
consider I have a class with a private member variable n, and I want to call a template function with a different number that iterates from 0 to n (for the case of runtime limit condition)
I've had studies on the "Template Metaprogramming" and "Constexpr If" (c++17) but I have not gotten any results, can anyone help me?
You can't have a for loop, but you can call N lots of templated_func
namespace detail {
template <template<uint32_t> class F, uint32_t... Is>
void static_for_impl(std::integer_sequence<uint32_t, Is...>)
{
F<Is>{}()...;
}
}
template <template<uint32_t> class F, uint32_t N>
void static_for()
{
detail::static_for_impl<F>(std::make_integer_sequence<uint32_t, N>{});
}
template <uint32_t I>
struct templated_caller
{
void operator()() { templated_func<I>(); }
}
int main()
{
static_for<templated_caller, 10>();
return 0;
}
Note that this is more general than what you asked for. You can simplify it to just
template <uint32_t... Is>
void call_templated_func(std::integer_sequence<uint32_t, Is...>)
{
templated_func<Is>()...;
}
int main()
{
call_templated_func(std::make_integer_sequence<uint32_t, N>{});
return 0;
}
but that's lengthy to repeat multiple times, and you can't pass a function template as a template parameter.
As you said you only had C++11 then you will not have std::make_index_sequence and will have to provide it. Also, the fold expression in Caleth's answer is not available until C++17.
Providing your own implementation of index_sequence and a fold expression in c++11 can be done in the following way,
#include <iostream>
template <size_t... Is>
struct index_sequence{};
namespace detail {
template <size_t I,size_t...Is>
struct make_index_sequence_impl : make_index_sequence_impl<I-1,I-1,Is...> {};
template <size_t...Is>
struct make_index_sequence_impl<0,Is...>
{
using type = index_sequence<Is...>;
};
}
template<size_t N>
using make_index_sequence = typename detail::make_index_sequence_impl<N>::type;
template<size_t I>
void templated_func()
{
std::cout << "templated_func" << I << std::endl;
}
template <size_t... Is>
void call_templated_func(index_sequence< Is...>)
{
using do_ = int[];
do_ {0,(templated_func<Is>(),0)...,0};
}
int main()
{
call_templated_func(make_index_sequence< 10>());
return 0;
}
This is essentially the same as the answer by #Caleth , but with the missing bits provided and will compile on c++11.
demo
demo on c++11 compiler
I would like to know if it is possible to have a for loop in compile time with runtime or even compile time limit condition in c++11
I don't know a reasonable way to have such loop with a runtime condition.
With a compile time condition... If you can use at least C++14, you can use a solution based on std::integer_sequence/std::make_integer_sequence (see Caleth answer) or maybe std::index_sequence/std::make_index_sequence (just a little more synthetic).
If you're limited with C++11, you can create a surrogate for std::index_sequence/std::make_index_sequence or you can create a recursive template struct with static function (unfortunately you can partially specialize a template function but you can partially specialize classes and structs).
I mean... something as follows
template <std::size_t I, std::size_t Top>
struct for_loop
{
static void func ()
{
templated_func<I>();
for_loop<I+1u, Top>::func();
}
};
template <std::size_t I>
struct for_loop<I, I>
{ static void func () { } };
that you can call
constexpr auto n = 10u;
for_loop<0, n>::func();
if you want to call templated_func() with values from zero to n-1u.
Unfortunately this solution is recursive so you can have troubles with compilers recursion limits. That is... works only if n isn't high.
I'm coming from C# and trying to implement a simple Events/EventHandler pattern in c++11 which i believe the common name is Observers and signals, i know there are boost library and others but i dont want to use any external libs.
While searching online I found a simple implementation for what I need, so I took and modified the code and it works ok.
My problem is that the parameters are passed when registering events/observers, and not when raising/signaling/notifying which I find a bit awkward.
class EventManager
{
private:
static std::map<EventType, std::vector<std::function<void()>>> _eventHandlers;
public:
EventManager() = default;
template <typename EventHandler>
static void RegisterEventHandler(EventType&& eventType, EventHandler&& eventHandler)
{
EventManager::_eventHandlers[std::move(eventType)].push_back(std::forward<EventHandler>(eventHandler));
}
static void Raise(const EventType& event)
{
for (const auto& eventHandler : EventManager::_eventHandlers.at(event))
{
eventHandler();
}
}
// disallow copying and assigning
EventManager(const EventManager&) = delete;
EventManager& operator=(const EventManager&) = delete;
};
Can anyone help me to extend the following code by adding the functionality to accept parameters when raising the event as well ?
I believe this solves your question:
// g++ -std=c++11 -o /tmp/events /tmp/events.cpp && /tmp/events
// handler=1 arg=1
// handler=2 arg=1
// handler=1 arg=2
// handler=2 arg=2
#include <functional>
#include <map>
#include <vector>
template<class EventType, class... HandlerArgs>
class EventManager
{
public:
using EventHandler = std::function< void(HandlerArgs...) >;
void register_event(EventType&& event, EventHandler&& handler)
{
_handlers[std::move(event)].push_back(std::forward<EventHandler>(handler));
}
void raise_event(const EventType& event, HandlerArgs&&... args)
{
for (const auto& handler: EventManager::_handlers.at(event)) {
handler(std::forward<HandlerArgs>(args)...);
}
}
private:
std::map<EventType, std::vector<EventHandler>> _handlers;
};
int main(int argc, char **argv)
{
EventManager<int, int> m;
m.register_event(1, [](int arg) { printf("handler=%d arg=%d\n", 1, arg); });
m.register_event(1, [](int arg) { printf("handler=%d arg=%d\n", 2, arg); });
m.raise_event(1, 1);
m.raise_event(1, 2);
}
PS: I removed all the code regarding non-copiability and such, since it is not relevant to this question.
Since i havent got any answers on this, i figured a way to do so but i dont like it since i wanted a better way but well creating a static class that has static variables for each event, before raising the event , the caller will set those variables and the handler will read then reset them . this is dangerous approach especially with multi-threading since one or more threads might change the values while raising same event by mutli threads, so i had to implement some locking features to ensure thread safety.
Yes i know its not the best approach but as i'm not an expert in C++ and this question didnt get any comments nor answers, so this is the approach im following.
Say I have a struct (in real life, that's an automaton):
struct automaton
{
bool get_final() const { return final; }
void set_final() { final = true; }
bool final = false;
};
for which I want to provide a view that sees it transposed (iow, reversed, or mirrored). Because I have more than just a single automaton class, I have a class template that wraps my automaton (I really want composition, not inheritance), and bounces all the function calls to the wrapped automaton, reversing what needs to be. For sake of simplicity, here, it just forwards the calls.
By hand, I'd get
template <typename Aut>
struct transposed_by_hand
{
Aut& aut;
auto get_final() const -> bool
{
return aut.get_final();
}
auto set_final() -> void
{
aut.set_final();
}
};
But there are many functions, and I don't want to hard-code so much information (the function signature) in the wrapper. Thanks to variadic templates and perfect forwarding for the incoming arguments, and decltype for the result, it's quite easy to have one macro to factor the definition of all the const member-functions, and another macro for non-const member functions (the difference being precisely the const). Basically, in this case it boils down to this:
template <typename Aut>
struct transposed_with_decltype
{
Aut& aut;
auto get_final() const -> decltype(aut.get_final())
{
return aut.get_final();
}
auto set_final() -> decltype(aut.set_final())
{
aut.set_final();
}
};
This works well for non-const automata, but breaks if I wrap a const automaton:
int main()
{
const automaton aut;
transposed_by_hand<const automaton> trh = { aut };
transposed_with_decltype<const automaton> trd = { aut };
}
My compilers complain that (G++ 4.9):
f.cc: In instantiation of 'struct transposed_with_decltype<const automaton>':
f.cc:44:49: required from here
f.cc:34:12: error: passing 'const automaton' as 'this' argument of 'void automaton::set_final()' discards qualifiers [-fpermissive]
auto set_final() -> decltype(aut.set_final())
^
and (Clang++ 3.3):
f.cc:42:23: error: default initialization of an object of const type 'const automaton' requires a user-provided default constructor
const automaton aut;
^
f.cc:34:36: error: member function 'set_final' not viable: 'this' argument has type 'const automaton', but function is not marked const
auto set_final() -> decltype(aut.set_final())
^~~
f.cc:44:49: note: in instantiation of template class 'transposed_with_decltype<const automaton>' requested here
transposed_with_decltype<const automaton> trd = { aut };
^
f.cc:6:12: note: 'set_final' declared here
void set_final() { final = true; }
^
2 errors generated.
And they are right! The expression in the decltype is breaking the const-ness of the wrapped automaton. Yet, I am not going to use this function, I swear. Just like I will not use the corresponding one wrapped by hand.
So my question is: is there a means to write the definition of the wrapping set_final so that I don't have to spell out its signature (input and output)? I have tried to use std::enable_if, but it changes nothing to the problem here. And anyway, it would need that the compiler be lazy, and accepts not to evaluate the second parameter of std::enable_if if it does not need to...
template <typename Aut>
struct transposed_with_decltype
{
Aut& aut;
auto get_final() const -> decltype(aut.get_final())
{
return aut.get_final();
}
auto set_final() -> typename std::enable_if<!std::is_const<Aut>::value,
decltype(aut.set_final())>::type
{
aut.set_final();
}
};
Thanks in advance.
I can get it to work (using GCC 4.6) just by using the advice Xeo mentions in his comment. That is, I turn the function into a trivial template, like so:
template<typename = void>
auto set_final() -> decltype(aut.set_final()) {
return aut.set_final();
}
== EDIT ==
Luc Danton comments below that a more recent GCC may reject the code, complaining that the decltype isn't dependent. I only have 4.6 here, but perhaps this could be worked around using something like this:
template<typename KLUDGE = int>
auto set_final() -> decltype((&aut+KLUDGE(0))->set_final()) {
return aut.set_final();
}
YMMV.
In MSVC 2008, I have the following code:
class Foo {
// Be a little smarter about deriving the vertex type, to save the user some typing.
template<typename Vertex> inline void drawVertices(
Elements vCount, RenPrim primitiveType, PixMaterial *mtl, Vertex const *vertices)
{
this->drawVertices(vCount, primitiveType, mtl, vertices, Vertex::VertexType);
}
virtual void drawVertices(
Elements vCount,
RenPrim primitiveType,
PixMaterial *mtl,
void const *vertices,
uint vertexType) = 0;
};
I use it something like:
struct RenFlexibleVertexPc
{
enum { VertexType = RenVbufVertexComponentsPc };
float x;
float y;
float z;
GraVideoRgba8 c; // Video format, not external!
};
PixMaterial *material;
struct Pc : RenFlexibleVertexPc
{
void set(Triple t, uint cl) { x = (float)t.x_; y = (float)t.y_; z = (float)t.z_; c = cl; }
} vpc[4];
...
Foo *renderer;
renderer->drawVertices(4, RenPrimTriangleFan, material, vpc);
This works fine in MSVC 2008 SP1. However, GCC (3.4 and 4.1,2) throws a "no matching function for call to function" error, apparently not seeing the template when there is a non-template function with more arguments.
Is GCC broken, or is my code broken, and if so, why?
There is no problem with overloading or inheritance:
#include <iostream>
#include <memory>
namespace {
struct A {
virtual void f()
{
std::cout<<"inside A's f()\n";
}
template <typename T> void f(T t)
{
std::cout<<T::i<<'\t';
this->f();
}
};
struct B : A {
void f()
{
std::cout<<"hello\t";
A::f();
}
};
struct C {
static const unsigned int i = 5;
};
struct D {
enum { i = 6 };
};
}
int main()
{
std::auto_ptr<A> b(new B());
b->f(C());
b->f(D());
}
Works correctly. On the other hand, the smallest example I can find that exhibits your problem does not have inheritance or overloading:
#include <iostream>
namespace {
struct A {
template<class C> void print(C c)
{
c.print();
}
};
}
int main()
{
struct B {
void print()
{
std::cout << "whee!\n";
}
};
A a;
B b;
a.print(b);
}
Note that if struct B is defined in a namespace (whether it's an unnamed namespace, or a completely different namespace, or the global namespace) instead of inside main() that this compiles without error.
I don't know enough of the standard to say if this is a bug, but it appears to be one. I've gone ahead and reported it to the GCC bug database.
And here's your answer from the GCC developers (from the link above): "Local classes cannot be template arguments."
So the code is broken. Not that it's a bad idea. In fact, C++0x removes this restriction.
I noticed the line
Note that the code works in GCC if I explicitly cast vpc to (RenFlexibleVertexPc *)
And since RenFlexibleVertexPc is not a local class this makes sense. However Pc is a local class/struct, so it is not allowed.
#OP: Specifying the template parameter is a valid approach.
renderer->drawVertices<RenFlexibleVertexPc>(4, RenPrimTriangleFan, material, vpc);
With Pete's additions, you code also compiles on Apple's GCC 4.0.1, so I suspect there's something your posted code is missing that's causing the problem.
#Max: GCC's treatment of your source is standard. Struct B is local to main(), so B (and thus main()::B::print()) is not visible outside main(). As you're probably aware, moving the definition of B outside of main() and it will compile.
The definition of VertexType is already in the code (an enum). Elements is an unsigned long. Note that the code works in GCC if I explicitly cast vpc to (RenFlexibleVertexPc *)
If it's an enum why pass an object of type array 4 of struct? What is RenFlexibleVertexPc? The last argument to drawVertices should either be a constant pointer to a Vertex object or a const* to an object of a class derived from Vertex.
Foo *renderer;
renderer->drawVertices(4, RenPrimTriangleFan, material, vpc);
You are calling a function on an uninitialized pointer. I hope this is not the real code. \