I'm a newbie at using the STL Algorithms and am currently stuck on a syntax error. My overall goal of this is to filter the source list like you would using Linq in c#. There may be other ways to do this in C++, but I need to understand how to use algorithms.
My user-defined function object to use as my function adapter is
struct is_Selected_Source : public std::binary_function<SOURCE_DATA *, SOURCE_TYPE, bool>
{
bool operator()(SOURCE_DATA * test, SOURCE_TYPE ref)const
{
if (ref == SOURCE_All)
return true;
return test->Value == ref;
}
};
And in my main program, I'm using as follows -
typedef std::list<SOURCE_DATA *> LIST;
LIST; *localList = new LIST;;
LIST* msg = GLOBAL_DATA->MessageList;
SOURCE_TYPE _filter_Msgs_Source = SOURCE_TYPE::SOURCE_All;
std::remove_copy(msg->begin(), msg->end(), localList->begin(),
std::bind1st(is_Selected_Source<SOURCE_DATA*, SOURCE_TYPE>(), _filter_Msgs_Source));
What I'm getting the following error in Rad Studio 2010. The error means "Your source file used a typedef symbol where a variable should appear in an expression. "
"E2108 Improper use of typedef 'is_Selected_Source'"
Edit -
After doing more experimentation in VS2010, which has better compiler diagnostics, I found the problem is that the definition of remove_copy only allows uniary functions. I change the function to uniary and got it to work.
(This is only relevant if you didn't accidentally omit some of your code from the question, and may not address the exact problem you're having)
You're using is_Selected_Source as a template even though you didn't define it as one. The last line in the 2nd code snippet should read std::bind1st(is_Selected_Source()...
Or perhaps you did want to use it as a template, in which case you need to add a template declaration to the struct.
template<typename SOURCE_DATA, typename SOURCE_TYPE>
struct is_Selected_Source : public std::binary_function<SOURCE_DATA *, SOURCE_TYPE, bool>
{
// ...
};
At a guess (though it's only a guess) the problem is that std::remove_copy expects a value, but you're supplying a predicate. To use a predicate, you want to use std::remove_copy_if (and then you'll want to heed #Cogwheel's answer).
I'd also note that:
LIST; *localList = new LIST;;
Looks wrong -- I'd guess you intended:
LIST *locallist = new LIST;
instead.
Related
This is mainly to clean up a bunch of code from my constructor. I have around 20+ lines of connect(object, func1, this, func2) in the constructor alone and I am trying to clean up the code by having a std::vector<std::tuple<QObject*,std::function<void()>,std::function<void>>>> connections;
It would work out quite nicely if I could do something like:
std::vector<std::tuple<QObject*,std::function<void()>,std::function<void>>>> connections = {
std::make_tuple(mySlider, std::bind(&QSlider::sliderReleased,mySlider, std::bind(&Foo::onSliderChanged,this)),
.
.
.
};
And then call it like this:
for(auto &&e : connections)
connect(std::get<0>(e),std::get<1>(e),this,std::get<2>(e));
However, when I do this I get an error that there is a substitution failure and a std::function<void()> cannot be converted into a function pointer. So decide to change it up and create actual function pointers like the following:
typename void(Foo::*fooFunc)();
typename void(QSlider::*sliderFunc)();
std::vector<std::tuple<QObject*,sliderFunc,fooFunc>> sliderConnections = {
std::make_tuple(mySlider, &QSlider::sliderReleased, &Foo::onSliderChanged),
.
.
.
};
And same thing, I then try to call it:
for(auto &&e : sliderConnections)
connect(std::get<0>(e),std::get<1>(e),this,std::get<2>(e));
However this also provides a similar error where there are no conversions. Which doesn't make any sense because now I am actually using a function pointer. Which according to the connection documentation it should be able to take a function pointer to connect them. So either I am passing it in incorrectly. Or what I am trying to achieve is not possible.
Any help would be appreciated!
After looking at G.M's comment I realized they were correct. A QObject* is not a QSlider* and therefore when trying to call the function QSlider::sliderReleased it couldn't connect the two because QObject does not have a slider. So once I changed that in the vector of tuples the code compiled just fine.
ex:
typedef void(Foo::*fooFunc)();
typedef void(QSlider::*sliderFunc)();
typedef void(QSpinBox::*spinFunc)();
const std::vector<std::tuple<QSlider*, sliderFunc, fooFunc>> sliderConnections = {
std::make_tuple(slider1, &QSlider::sliderReleased, &Foo::onSlider1Changed),
std::make_tuple(slider2, &QSlider::sliderReleased, &Foo::onSlider2Changed),
std::make_tuple(slider3, &QSlider::sliderReleased, &Foo::onSlider3Changed)
};
const std::vector<std::tuple<QSpinBox*, spinFunc, fooFunc>> spinConnections = {
std::make_tuple(spin1, &QSpinBox::editingFinished, &Foo::onSpin1Changed),
std::make_tuple(spin2, &QSpinBox::editingFinished, &Foo::onSpin2Changed),
std::make_tuple(spin3, &QSpinBox::editingFinished, &Foo::onSpin3Changed)
};
These will be private members in whatever class you are in charge of. And then in the constructor, instead of having 6 lines of connect(object,SIGNAL,object,SLOT), you can then put them into a function and call them like:
for(auto && tup : sliderConnections)
connect(std::get<0>(tup),std::get<1>(tup),this,std::get<2>(tup));
This successfully connects all the objects to their appropriate functions. Again, it's personal preference. I was just wondering if there was a way and G.M pointed me in the correct direction.
On this subject, I have read few relevant SO questions/answers/comments. Found only one relevant but somewhat buried question/answer here. Allow me to try and clearly show the issue in question/answer manner. For the benefit of others.
Let the code speak. imagine you design this template.
// value holder V1.0
// T must not be reference or array or both
template<typename T> struct no_arrf_naive
{
static_assert(!std::is_reference_v<T>, "\n\nNo references!\n\n");
static_assert(!std::is_array_v<T>, "\n\nNo arrays!\n\n");
using value_type = T;
T value;
};
Simple and safe, one might think. Some time after, other folks take this complex large API, where this is buried deep, and start using it. The struct above is deep inside. As usually, they just use it, without looking into the code behind.
using arf = int(&)[3];
using naivete = no_arrf_naive<arf>;
// the "test" works
constexpr bool is_ok_type = std::is_class_v< naivete >;
// this declaration will also "work"
void important ( naivete ) ;
But. Instantiations do not work
naivete no_compile;
static assert message does show all of a sudden. But how has the "test" compiled and passed? What is going on here?
The issue is that API is wrong. static_assert as class member does "kick-in" but not before instantiation.
First the offending API commented
template<typename T>
struct no_arrf_naive
{
// member declarations
// used only on implicit instantiation
// https://en.cppreference.com/w/cpp/language/class_template#Implicit_instantiation
static_assert(!std::is_reference_v<T>, "\n\nNo references!\n\n");
static_assert(!std::is_array_v<T>, "\n\nNo arrays!\n\n");
using value_type = T;
T value;
};
Users are here properly coding to transform from Template to Type, but, static_assert's do not kick-in:
using naivete = no_arrf_naive<arf>;
This might most worryingly go on unnoticed, until someone wants to use this. That will not compile and the message, API author has placed in there, will show at last. But alas, too late.
And on projects laboring on some large C++ source, problems that show up late, are the most notorious ones.
The solution is good old SFINAE. The API fixed is this:
// value holder
// references or arrays or both are excluded at compile time
template<typename T,
std::enable_if_t<
(!std::is_reference_v<T> && !std::is_array_v<T>), bool> = true
> struct no_arrf
{
using value_type = T;
T value;
};
The above will not compile immediately upon trying to create the type from template with either reference or array or both:
// reference to array of three int's
using arf = int(&)[3] ;
// no can do
using no_naivete = no_arrf<arf>;
(MSVC) error C2972: 'no_arrf':
template parameter 'unnamed-parameter':
the type of non-type argument is invalid
I might think this whole story might look like trivial or even useless to some. But, I am sure many good folks are coming to SO for badly needed standard C++ advice. For them, this is neither trivial nor useless.
Many thanks for reading.
I hope some one can help me a little here. I am relatively new to C++ and also to the concept of Templates.
I need to create a std::function based on some data that I am getting in a list.
The signature of the function should be according to the data available. I am looking for something like this
template <typename ret, typename... Args, typename newArg>
struct typeparser<ret(...Args)>{
typeparser<ret(...Args)> insertArg(newArg)
{
retrun typeparser <ret(...args, newArg) > ;
}
};
What I want to do is iterate through a vector of boost::variant and then based on the type of value i see, add it to the list of parameters once complete, create a std:function and load it from a lib, then execute it. Make any sense?
std::vector<boost::varient<int, char, std::string>> list;
arglist = typeparser<int()>; //all functions have int return, so start with return int and 0 args
for(boost::varient<int, char, std::string> a : list) {
if(a.type() == typeid(int)){
arglist.addArg(int); // now add int to list of args
} else
if(a.type()== typeid(char)) {
arglist.add(char);
} else
if (a.type()== typeid(bla)) {
arglist.add(bla);
}
} // end for
//now create the function here
std::function<arglist> f = //load from library;
Does this even seem possible? Maybe I am looking at the problem in the wrong way? Any thing will help at this time.
Thanks a lot!!
A std::function must have all its parameters specified at compile time -- what you're asking for would require the set of parameters not be known until runtime which is not allowed.
It would be theoretically possible to make what you're proposing by having something like std::function that contained a stack of parameters to call or something like that, but I don't believe that there is a portable way to do it.
It sounds like you would be better off asking for a solution to the problem you need this "runtime std::function" for.
I'm implementing Lua in a game engine. All of the functions being exported to Lua have headers that start with luavoid, luaint or luabool just for quick reference of the expected parameters, and so I can see at a glance that this function is being exported.
#define luavoid(...) void
luavoid(std::string s) TextMsg()
{
std::string s;
ExtractLuaParams(1, s);
::TextMsg(s.c_str());
}
To actually export a function to Lua, they're added to a dictionary. On startup, the map is used to call lua_register.
std::unordered_map<std::string, ScriptCall> _callMap = {
{ "TextMsg", TextMsg },
...
}
There will be a lot of functions exported. Rather than have to maintain this map manually, I'd like to automate its creation.
My first instinct was something with macros at compile-time. I gave up on it initially and started writing a program to parse the code (as a pre-build event), since all the functions can be text-matched with the luaX macros. It would create a header file with the map automatically generated.
Then I went back to doing it at compile-time after figuring out a way to do it. I came up with this solution as an example before I finally implement it in the game:
using MapType = std::unordered_map<std::string, int>;
template <MapType& m>
struct MapMaker
{
static int MakePair(std::string s, int n)
{
m[s] = n;
return n;
}
};
#define StartMap(map) MapType map
#define AddMapItem(map, s, n) int map##s = MapMaker<map>::MakePair(#s, n)
StartMap(myMap);
AddMapItem(myMap, abc, 1);
AddMapItem(myMap, def, 2);
AddMapItem(myMap, ghi, 3);
void main()
{
for (auto& x : myMap)
{
std::cout << x.first.c_str() << "->" << x.second << std::endl;
}
}
It works.
My question is, how horrible is this and can it be improved? All I want in the end is a list mapping a a string to a function. Is there a better way to create a map or should I just go with the text-parsing method?
Be gentle(-ish). This is my first attempt at coding with templates like this. I assume this falls under template metaprogramming.
how horrible is this and can it be improved?
Somewhere between hideous and horrendous. (Some questions better left unasked.) And yes...
All I want in the end is a list mapping a a string to a function. Is there a better way to create a map or should I just go with the text-parsing method?
The simplest thing to do is:
#define ADDFN(FN) { #FN, FN }
std::unordered_map<std::string, ScriptCall> _callMap = {
ADDFN(TextMsg),
...
};
This uses the macros to automate the repetition in the string literal function names and identifiers - there's nothing further substantive added by your implementation.
That said, you could experiment with automating things further than your implementation, perhaps something like this:
#define LUAVOID(FN, ...) \
void FN(); \
static auto addFN ## __LINE__ = myMap.emplace(#FN, FN); \
void FN()
LUAVOID(TextMsg, string s)
{
...
}
See it running here.
The idea here is that the macro generates a function declaration so that it can register the function, then a definition afterwards. __LINE__ likely suffices for uniqueness of the identifiers - assuming you have one file doing this, and that your compiler substitutes a numeric literal (which all compilers I've used do, but I can't remember if the Standard mandates that). The emplace function has a non-void return type so can be used directly to insert to the map.
Be gentle(-ish). This is my first attempt at coding with templates like this.
Sorry.
I assume this falls under template metaprogramming.
It's arguable. Many C++ programmers (myself included) think of "metaprogramming" as involving more advanced template usage - such as variable-length lists of parameters, recursive instantiations, and specialisation - but many others consider all template usage to be "metaprogramming" since the templates provide instructions for how to create instantiations, which is technically sufficient to constitute metaprogramming.
I'm making an input range to iterate over a custom container that holds data points that need to remain accurately paired as inputs and targets. I need different Ranges for returning training data (double[][]), inputs (double[]) and the targets (also double[]). I managed to get the following code to compile and work perfectly, but I don't know why.
public struct DataRange(string type)
if( type == "TrainingData" ||
type == "InputData" ||
type == "TargetData" )
{
private immutable(int) length;
private uint next;
private Data data;
this(Data d){
this.length = d.numPoints;
this.next = 0;
this.data = d;
}
#property bool empty(){return next == length;}
#property auto front(){
static if(type == "TrainingData")
return this.data.getTrainingData(next);
else static if(type == "InputData")
return this.data.getInputData(next);
else return this.data.getTargetData(next);
}
void popFront(){++next;}
}
static assert(isInputRange!(DataRange!"TrainingData"));
static assert(isInputRange!(DataRange!"InputData"));
static assert(isInputRange!(DataRange!"TargetData"));
I've been reading the "The D Programming Language" by Alexandrescu, and I have found parameterized structs of the form
struct S(T){...} // or
struct S(T[]){...}
but these take type parameters, not expressions like I've done. I haven't been able to find any similar examples on dlang.org with parameterized types.
This compiles and works on DMD 2.066 and GDC 4.9.0.
I don't even know why I tried this, and looking back at it I don't know why it works. Anybody know what I'm missing? Where is this documented?
Ok, I found the answer. Though this wasn't specifically mentioned or described in any of the tutorials or anywhere in the book, I was eventually able to find it at http://dlang.org.template.html. Basically there are two things going on here.
1.) Though my code says struct, this is really a template (that results in a struct). I have seen examples of this online and in the book, though it wasn't described as a template. It was a bit confusing because I didn't use the template keyword, and in the book they are described as "parameterized."
2.) From the website linked above...
Template parameters can be types, values, symbols, or tuples
So in my case my template parameter was a symbol. The examples in the book used types.
Digging into the language specifications on the website reveals there is a lot more going on than is covered in the book!
Alternatively you could use an enum to simplify the constraint in such a way that a wrong template instantiation is impossible (even if in your code the template constraint does it perfectly). example:
enum rangeKind{training, input, target};
public struct DataRange(rangeKind Kind)
{
}
void main(string args[])
{
DataRange!(rangeKind.training) dr;
}