I have MyClass which is a template class. I wanted to provide an initialize r list constructor so that I can conveniently write:
MyClass<int> Arr0{ 1, 2, 3, 4, 5, 8 };
On the other hand, I do not want to have duplicates in this list as this class meant to have only unique user inputs. I have seen many ways to check the duplicates in the array and I came up with the has_duplicates() following function.
I tried to combine the idea of checking, whether the std::initializer_list<T>ed temporary elements(or array) contains any duplicate elements in the member initializer list itself; if it contains static_assert() the template instantiation and thereby no object of this class will be constructed.
Following is the minimal example of my code.
#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
#include <iterator>
#include <initializer_list>
template <typename Iterator> // function to check duplicates(which works fine)
constexpr bool has_duplicates(Iterator start, Iterator end)
{
if (start == end) return false;
using Type = typename std::remove_reference_t<decltype(*end)>;
std::map<Type, std::size_t> countMap;
for (; start != end; ++start)
{
countMap[*start]++;
if (countMap[*start] >= 2) return true;
}
return false;
}
template <typename T> class MyClass
{
private:
std::vector<T> m_vec;
public:
MyClass(std::initializer_list<T> a)
: (has_duplicates(a.begin(), a.end()) //-----> here is the problem
? static_assert(false, " the array has duplicates....")
: m_vec(a)
)
{
std::cout << "Constriction successful....";
}
};
int main()
{
std::vector<int> test{ 1, 2, 3, 4, 1 };
std::cout << std::boolalpha
<< has_duplicates(test.begin(), test.end()) << std::endl; // works
MyClass<int> Arr0{ 1, 2, 3, 4 }; // error
return 0;
}
Upon compiling in MSVC 16.0(C++17 flag), this gives me the error:
error C2059: syntax error: 'static_assert'
note: while compiling class template member function 'MyClass<int>::MyClass(std::initializer_list<_Ty>)'
with
[
_Ty=int
]
note: see reference to function template instantiation 'MyClass<int>::MyClass(std::initializer_list<_Ty>)' being compiled
with
[
_Ty=int
]
note: see reference to class template instantiation 'MyClass<int>' being compiled
error C2143: syntax error: missing ';' before '}'
error C2059: syntax error: ')'
error C2447: '{': missing function header (old-style formal list?)
It says an simple syntax error, but I do not see any as per static_assert.
Can anybody help me find out the error?
What is the correct way to prevent construction of std::initializer_list<T> constutor arguments, in the above case?
What you're trying to do, a static assert to check the arguments of a constructor, is (as far I know) simply impossible.
A static_assert() works compile time where MyClass object is initialized (can be initialized) run-time.
The best I can imagine is a make_MyClass() function that receive a list of arguments as template parameters
template <auto v0, auto ... vs>
auto make_MyClass ()
{
static_assert( false == has_duplicates<v0, vs...>() );
return MyClass<decltype(v0)>{ v0, vs... };
}
so you can perform a static_assert() because now you know compile time the values; I've rewritten the has_duplicates() function as follows because your original function can't be effectively constexpr (because std::map isn't)
template <typename = void>
constexpr bool has_duplicates ()
{ return false; }
template <auto v0, auto ... vs>
constexpr bool has_duplicates ()
{ return ((v0 == vs) || ... ) || has_duplicates<vs...>(); }
The following is a full compiling example
#include <iostream>
#include <vector>
#include <initializer_list>
template <typename = void>
constexpr bool has_duplicates ()
{ return false; }
template <auto v0, auto ... vs>
constexpr bool has_duplicates ()
{ return ((v0 == vs) || ... ) || has_duplicates<vs...>(); }
template <typename T> class MyClass
{
private:
std::vector<T> m_vec;
public:
MyClass(std::initializer_list<T> a) : m_vec{a}
{ std::cout << "Constriction successful...."; }
};
template <auto v0, auto ... vs>
auto make_MyClass ()
{
static_assert( false == has_duplicates<v0, vs...>() );
return MyClass<decltype(v0)>{ v0, vs... };
}
int main ()
{
std::cout << std::boolalpha
<< has_duplicates<1, 2, 3, 4, 1>() << std::endl;
auto mc0 = make_MyClass<1, 2, 3, 4, 5>(); // compile
//auto mc1 = make_MyClass<1, 2, 3, 4, 1>(); // static_assert error
}