I've defined a base class template:
template<class actual_class>
class base
{
public:
static const int value;
}
and the definition of value depends on the actual_class tparam.
Next, I have a bunch of derived classes from base, let's call them a and b. Let's also say that a::value = 5 and b::value = 10.
Now, in a method template where I need to access the static values from a parameter pack. I'd like to have them in a vector.
template<class... derived_from_bases>
void foo(irrelevant_class<derived_from_bases...> irrelevant)
{
// std::vector<int> values = { ... }
...
}
For the function called with < a, b > tparams I'd like the values vector to look like this:
std::vector<int> values = {5 /* a::value */, 10 /* b::value */};
Also having an std::array instead of an std::vector would be a nice touch.
Thank you for your help in advance.
For a vector, you just need
std::vector<int> values = { derived_from_bases::value... };
If you have C++17, you can get a std::array the same way like
std::array values = { derived_from_bases::value... };
and CTAD will deduce the type and size of the array for you. If you do not have C++17, then you can use
std::array<int, sizeof...(derived_from_bases)> values = { derived_from_bases::value... };
I am quite new to c++ and I try to create an Array class in c++17 with the use of templates. In this class I overload the + operator, in such a way that it can add Arrays of multiple types. It does work so far and I am able to add arrays of different e.g. float and int type together. However, I am having some trouble with how to define the type of the new array, which is the result of the addition.
Let's say the arrays which I add are of type float and int. Then the new array should also be float. However, on forehand I dont know which array has the float type, the first one or the second one, so I can't create a new array with typename T or U.
Also, if due to coindicdence two float arrays add up together to only int values (e.g. 1.5 + 3.5 = 5(int) ), then the new array should be of type int.
Basically in summary, I try to define the type of the new array based on the type of the content after addition.
I came across some solutions that include decltype. However I can't manage to find a way how to include this for multiple values, since the array has more than one value. In my current code I create the new array based on the type T. However, if in a case T is of type int and U of type float, the result is not correct.
Any advice or tips are much appreciated.
Thanks in advance,
template <typename T>
class Array {
public:
T* content;
int length;
// Default Constructor
Array() : content(nullptr), length(0) {}
// Constructor when length is provided
Array(int length) : content(new T[length]), length(length) {}
// Constructor (using initializer list)
Array(std::initializer_list<T> list) : Array((int)list.size()) {
std::uninitialized_copy(list.begin(), list.end(), content);
}
// Obtain content at index i
float& operator[](int i) { return content[i]; }
// Adding arrays
template <typename U>
Array& operator+(Array<U>& other) {
Array<T>* new_array = new Array(other.length);
for (auto i = 0; i < other.length; i++)
new_array->content[i] = this->content[i] + other.content[i];
return *new_array;
}
};
With decltype, your operator + might look like:
template<typename U>
auto operator+(const Array<U>& rhs)
-> Array<std::decay_t<decltype((*this)[0] + rhs[0])>>
{
Array<std::decay_t<decltype((*this)[0] + rhs[0])>> res(rhs.length);
for (auto i = 0; i != rhs.length; i++) {
res[i] = (*this)[i] + rhs[i];
}
return res;
}
Demo
well i cant find how do this, basically its a variable union with params, basic idea, (writed as function)
Ex1
union Some (int le)
{
int i[le];
float f[le];
};
Ex2
union Some
{
int le;
int i[le];
float f[le];
};
obs this don't works D:
maybe a way to use an internal variable to set the lenght but don't works too.
Thx.
No, this is not possible: le would need to be known at compile-time.
One solution would be to use a templated union:
template <int N> union Some
{
int i[N];
float f[N];
};
N, of course, is compile-time evaluable.
Another solution is the arguably more succinct
typedef std::vector<std::pair<int, float>> Some;
or a similar solution based on std::array.
Depending on your use case you could try to simulate a union.
struct Some
{
//Order is important
private:
char* pData;
public:
int* const i;
float* const f;
public:
Some(size_t len)
:pData(new char[sizeof(int) < sizeof(float) ? sizeof(float) : sizeof(int)])
,i ((int*)pData)
,f ((float*)pData)
{
}
~Some()
{
delete[] pData;
}
Some(const Some&) = delete;
Some& operator=(const Some&) = delete;
};
Alternative solution using templates, unique_ptr and explicit casts:
//max_size_of<>: a recursive template struct to evaluate the
// maximum value of the sizeof function of all types passed as
// parameter
//The recursion is done by using the "value" of another
// specialization of max_size_of<> with less parameter types
template <typename T, typename...Args>
struct max_size_of
{
static const std::size_t value = std::max(sizeof(T), max_size_of<Args...>::value);
};
//Specialication for max_size_of<> as recursion stop
template <typename T>
struct max_size_of<T>
{
static const std::size_t value = sizeof(T);
};
//dataptr_auto_cast<>: a recursive template struct that
// introduces a virtual function "char* const data_ptr()"
// and an additional explicit cast operator for a pointer
// of the first type. Due to the recursion a cast operator
// for every type passed to the struct is created.
//Attention: types are not allowed to be duplicate
//The recursion is done by inheriting from of another
// specialization of dataptr_auto_cast<> with less parameter types
template <typename T, typename...Args>
struct dataptr_auto_cast : public dataptr_auto_cast<Args...>
{
virtual char* const data_ptr() const = 0; //This is needed by the cast operator
explicit operator T* const() const { return (T*)data_ptr(); } //make it explicit to avoid unwanted side effects (manual cast needed)
};
//Specialization of dataptr_auto_cast<> as recursion stop
template <typename T>
struct dataptr_auto_cast<T>
{
virtual char* const data_ptr() const = 0;
explicit operator T* const() const { return (T*)data_ptr(); }
};
//union_array<>: inherits from dataptr_auto_cast<> with the same
// template parameters. Also has a static const member "blockSize"
// that indicates the size of the largest datatype passed as parameter
// "blockSize" is used to determine the space needed to store "size"
// elements.
template <typename...Args>
struct union_array : public dataptr_auto_cast<Args...>
{
static const size_t blockSize = max_size_of<Args...>::value;
private:
std::unique_ptr<char[]> m_pData; //std::unique_ptr automatically deletes the memory it points to on destruction
size_t m_size; //The size/no. of elements
public:
//Create a new array to store "size" elements
union_array(size_t size)
:m_pData(new char[size*blockSize])
,m_size(size)
{
}
//Copy constructor
union_array(const union_array<Args...>& other)
:m_pData(new char[other.m_size*blockSize])
,m_size(other.m_size)
{
memcpy(m_pData.get(), other.m_pData.get(), other.m_size);
}
//Move constructor
union_array(union_array<Args...>&& other)
:m_pData(std::move(other.m_pData))
,m_size(std::move(other.m_size))
{
}
union_array& operator=(const union_array<Args...>& other)
{
m_pData = new char[other.m_size*blockSize];
m_size = other.m_size;
memcpy(m_pData.get(), other.m_pData.get(), other.m_size);
}
union_array& operator=(union_array<Args...>&& other)
{
m_pData = std::move(other.m_pData);
m_size = std::move(other.m_size);
}
~union_array() = default;
size_t size() const
{
return m_size;
}
//Implementation of dataptr_auto_cast<>::data_ptr
virtual char* const data_ptr() const override
{
return m_pData.get();
}
};
int main()
{
auto a = union_array<int, char, float, double>(5); //Create a new union_array object with enough space to store either 5 int, 5 char, 5 float or 5 double values.
((int*)a)[3] = 3; //Use as int array
auto b = a; //copy
((int*)b)[3] = 1; //Change a value
auto c = std::move(a);// move a to c, a is invalid beyond this point
// std::cout << ((int*)a)[3] << std::endl; //This will crash as a is invalid due to the move
std::cout << ((int*)b)[3] << std::endl; //prints "1"
std::cout << ((int*)c)[3] << std::endl; //prints "3"
}
Explanation
template <typename T, typename...Args>
struct max_size_of
{
static const std::size_t value = std::max(sizeof(T), max_size_of<Args...>::value);
};
template <typename T>
struct max_size_of<T>
{
static const std::size_t value = sizeof(T);
};
max_size_of<> is used to get the largest sizeof() value of all types passed as template paremeters.
Let's have a look at the simple case first.
- max_size_of<char>::value: value will be set to sizeof(char).
- max_size_of<int>::value: value will be set to sizeof(int).
- and so on
If you put in more than one type it will evaluate to the maximum of the sizeof of these types.
For 2 types this would look like this: max_size_of<char, int>::value: value will be set to std::max(sizeof(char), max_size_of<int>::value).
As described above max_size_of<int>::value is the same as sizeof(int), so max_size_of<char, int>::value is the same as std::max(sizeof(char), sizeof(int)) which is the same as sizeof(int).
template <typename T, typename...Args>
struct dataptr_auto_cast : public dataptr_auto_cast<Args...>
{
virtual char* const data_ptr() const = 0;
explicit operator T* const() const { return (T*)data_ptr(); }
};
template <typename T>
struct dataptr_auto_cast<T>
{
virtual char* const data_ptr() const = 0;
explicit operator T* const() const { return (T*)data_ptr(); }
};
dataptr_auto_cast<> is what we use as a simple abstract base class.
It forces us to implement a function char* const data_ptr() const in the final class (which will be union_array).
Let's just assume that the class is not abstract and use the simple version dataptr_auto_cast<T>:
The class implements a operator function that returns a pointer of the type of the passed template parameter.
dataptr_auto_cast<int> has a function explicit operator int* const() const;
The function provides access to data provided by the derived class through the data_ptr()function and casts it to type T* const.
The const is so that the pointer isn't altered accidentially and the explicit keyword is used to avoid unwanted implicit casts.
As you can see there are 2 versions of dataptr_auto_cast<>. One with 1 template paremeter (which we just looked at) and one with multiple template paremeters.
The definition is quite similar with the exception that the multiple parameters one inherits dataptr_auto_cast with one (the first) template parameter less.
So dataptr_auto_cast<int, char> has a function explicit operator int* const() const; and inherits dataptr_auto_cast<char> which has a function explicit operator char* const() const;.
As you can see there is one cast operator function implemented with each type you pass.
There is only one exception and that is passing the same template parameter twice.
This would lead in the same operator function being defined twice within the same class which doesn't work.
For this use case, using this as a base class for the union_array, this shouldn't matter.
Now that these two are clear let's look at the actual code for union_array:
template <typename...Args>
struct union_array : public dataptr_auto_cast<Args...>
{
static const size_t blockSize = max_size_of<Args...>::value;
private:
std::unique_ptr<char[]> m_pData;
size_t m_size;
public:
//Create a new array to store "size" elements
union_array(size_t size)
:m_pData(new char[size*blockSize])
,m_size(size)
{
}
//Copy constructor
union_array(const union_array<Args...>& other)
:m_pData(new char[other.m_size*blockSize])
,m_size(other.m_size)
{
memcpy(m_pData.get(), other.m_pData.get(), other.m_size);
}
//Move constructor
union_array(union_array<Args...>&& other)
:m_pData(std::move(other.m_pData))
,m_size(std::move(other.m_size))
{
}
union_array& operator=(const union_array<Args...>& other)
{
m_pData = new char[other.m_size*blockSize];
m_size = other.m_size;
memcpy(m_pData.get(), other.m_pData.get(), other.m_size);
}
union_array& operator=(union_array<Args...>&& other)
{
m_pData = std::move(other.m_pData);
m_size = std::move(other.m_size);
}
~union_array() = default;
size_t size() const
{
return m_size;
}
virtual char* const data_ptr() const override
{
return m_pData.get();
}
};
As you can see union_array<> inherits from dataptr_auto_cast<> using the same template arguments.
So this gives us a cast operator for every type passed as template paremeter to union_array<>.
Also at the end of union_array<> you can see that the char* const data_ptr() const function is implemented (the abstract function from dataptr_auto_cast<>).
The next interesting thing to see is static const size_t blockSize which is initilialized with the maximum sizeof value of the template paremeters to union_array<>.
To get this value the max_size_of is used as described above.
The class uses std::unique_ptr<char[]> as data storage, as std::unique_ptr automatically will delete the space for us, once the class is destroyed.
Also std::unique_ptr is capable of move semantics, which is used in the move assign operator function and the move constructor.
A "normal" copy assign operator function and a copy constructor are also included and copy the memory accordingly.
The class has a constructor union_array(size_t size) which takes the number of elements the union_array should be able to hold.
Multiplying this value with blockSize gives us the space needed to store exactly size elements of the largest template type.
Last but not least there is an access method to ask for the size() if needed.
C++ requires that the size of a type be known at compile time.
The size of a block of data need not be known, but all types have known sizes.
There are three ways around it.
I'll ignore the union part for now. Imagine if you wanted:
struct some (int how_many) {
int data[how_many];
};
as the union part adds complexity which can be dealt with separately.
First, instead of storing the data as part of the type, you can store pointers/references/etc to the data.
struct some {
std::vector<int> data;
explicit some( size_t how_many ):data(how_many) {};
some( some&& ) = default;
some& operator=( some&& ) = default;
some( some const& ) = default;
some& operator=( some const& ) = default;
some() = default;
~some() = default;
};
here we store the data in a std::vector -- a dynamic array. We default copy/move/construct/destruct operations (explicitly -- because it makes it clearer), and the right thing happens.
Instead of a vector we can use a unique_ptr:
struct some {
std::unique_ptr<int[]> data;
explicit some( size_t how_many ):data(new int[how_many]) {};
some( some&& ) = default;
some& operator=( some&& ) = default;
some() = default;
~some() = default;
};
this blocks copying of the structure, but the structure goes from being size of 3 pointers to being size of 1 in a typical std implementation. We lose the ability to easily resize after the fact, and copy without writing the code ourselves.
The next approach is to template it.
template<std::size_t N>
struct some {
int data[N];
};
this, however, requires that the size of the structure be known at compile-time, and some<2> and some<3> are 'unrelated types' (barring template pattern matching). So it has downsides.
A final approach is C-like. Here we rely on the fact that data can be variable in size, even if types are not.
struct some {
int data[1]; // or `0` in some compilers as an extension
};
some* make_some( std::size_t n ) {
Assert(n >= 1); // unless we did `data[0]` above
char* buff = new some[(n-1)*sizeof(int) + sizeof(some)]; // note: alignment issues on some platforms?
return new(buff) some(); // placement new
};
where we allocate a buffer for some of variable size. Access to the buffer via data[13] is practically legal, and probably actually so as well.
This technique is used in C to create structures of variable size.
For the union part, you'll want to create a buffer of char with the right size std::max(sizeof(float), sizeof(int))*N, and expose functions:
char* data(); // returns a pointer to the start of the buffer
int* i() { return reinterpret_cast<int*>(data()); }
float* f() { return reinterpret_cast<float*>(data()); }
you may also need to properly initialize the data as the proper type; in theory, a char buffer of '\0's may not correspond to defined float values or ints that are zero.
I would like to suggest a different approach: Instead of tying the number of elements to the union, tie it outside:
union Some
{
int i;
float f;
};
Some *get_Some(int le) { return new Some[le]; }
Don't forget to delete[] the return value of get_Some... Or use smart pointers:
std::unique_ptr<Some[]> get_Some(int le)
{ return std::make_unique<Some[]>(le); }
You can even create a Some_Manager:
struct Some_Manager
{
union Some
{
int i;
float f;
};
Some_Manager(int le) :
m_le{le},
m_some{std::make_unique<Some[]>(le)}
{}
// ... getters and setters...
int count() const { return m_le; }
Some &operator[](int le) { return m_some[le]; }
private:
int m_le{};
std::unique_ptr<Some[]> m_some;
};
Take a look at the Live example.
It's not possible to declare a structure with dynamic sizes as you are trying to do, the size must be specified at run time or you will have to use higher-level abstractions to manage a dynamic pool of memory at run time.
Also, in your second example, you include le in the union. If what you were trying to do were possible, it would cause le to overlap with the first value of i and f.
As was mentioned before, you could do this with templating if the size is known at compile time:
#include <cstdlib>
template<size_t Sz>
union U {
int i[Sz];
float f[Sz];
};
int main() {
U<30> u;
u.i[0] = 0;
u.f[1] = 1.0;
}
http://ideone.com/gG9egD
If you want dynamic size, you're beginning to reach the realm where it would be better to use something like std::vector.
#include <vector>
#include <iostream>
union U {
int i;
float f;
};
int main() {
std::vector<U> vec;
vec.resize(32);
vec[0].i = 0;
vec[1].f = 42.0;
// But there is no way to tell whether a given element is
// supposed to be an int or a float:
// vec[1] was populated via the 'f' option of the union:
std::cout << "vec[1].i = " << vec[1].i << '\n';
}
http://ideone.com/gjTCuZ
For a user defined class which already has an explicit constructor accepting a single argument, Is it possible to implement the Conversion Constructor behavior in some other way.
The class in question -
class Foo
{
explicit Foo(int size);
}
Is it possible to make this code still valid -
Foo a = 3;
No. This is not possible. A constructor can only be used for one purpose, and you can't have two constructors with the same arguments that do different things.
If Foo were an array of ints like you said in your comments, it probably would be a poor design choice to have a constructor with a single argument that fills the first element of the array. It's poor design for a couple of reasons:
How do you know what size the array is going to be when Foo is initialized? Even if there were some default value, it would be a non-obvious implementation.
It's confusing. You should use the element of least surprise when designing code. I've never, ever seen a class that wraps an array have a single argument constructor that fills it's first element. It doesn't really make sense.
In C++ 11, you can use an initializer list as the second constructor.
#include <initializer_list>
#include <cstring>
class Foo {
private:
int* arr;
size_t len;
public:
explicit Foo(size_t size)
: len(size) {
arr = new int[len];
}
Foo(std::initializer_list<int> list) // Initializer list constructor
: len(list.size()) {
arr = new int[len];
size_t pos = 0;
for(auto it = list.begin(), e = list.end(); it != e; ++it) {
arr[pos++] = *it; // copy the list into your array
}
}
Foo(const Foo& other)
: len(other.len) {
arr = new int[len];
memcpy(arr, other.arr, len * sizeof*arr);
}
Foo& operator=(const Foo& other) {
if (&other != this) {
delete[] arr;
len = other.len;
arr = new int[len];
memcpy(arr, other.arr, len * sizeof*arr);
}
return *this;
}
~Foo() {
delete[] arr;
}
};
That way, you could use it like this to initialize 1 element:
Foo a = { 3 };
Or like this to create an array with "3" ints:
Foo a(3);
Or even create the whole array at once:
Foo a = { 3, 7, 10, 12, 13 };
I realize that you just used an int-array as an example, but this just shows that there are often better ways of designing code then what you propose.
Yes. Right after the definition of class Foo simply write:
struct Bar {
Bar(int){};
};
#define Foo Bar
and the required code now compiles, and calls a conversion constructor.
This seems like a bad idea, but it is possible.
I'd like to instanciate my object with template parameters only. One of the parameters is a pointer to array, and I'm looking for the correct syntax.
const MyCustomType* array[2] = { &object1, &object2 };
OBJ1 < 10, 10, array > myobj1;
Below, a sample of the class OBJ1.
template < int a, int b, /* help ! */ >
class OBJ1
{
public:
OBJ1();
~OBJ1();
private:
//methods
};
What's the right syntax to use the third template parameter ? Is that even possible ?
For an array of type int of size n the syntax is
int (*paramname)[N];
Or with a helper type alias
template<typename T>
using type = T;
Then
type<int[N]> *paramname;
In your case you pass a pointer to the arrays first element though, and not a pointer to the array. You need to prefix the array name with & to do the latter when passing the array.