Class method with number of arguments specified by integer template parameter - c++

Question

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Suppose
template<typename Type, int Size> class vector
{
Type data[Size];
}
Is it possible to replace a constructor which takes Size number of arguments in template specializations like this
template<typename Type> class vector3<Type,3>
{
Type data[3];
public:
vector3( Type, Type, Type );
}
with something in the non-specialized template class? Like a "varargs constructor" that produces a constructor with Size number of arguments of type Type?
A solution involving C++0x features is fine.

Answer 1

In C++0x you have template typedef finally available!
Disclaimer: nothing has been compiled...
From Wikipedia's article:
template< typename second>
using TypedefName = SomeType<OtherType, second, 5>;
which in your case would yield
template <class Type>
using vector3 = vector<Type, 3>;
I can't tell you how much I craved for this ;)
However it doesn't solve the parameters issue. As mentioned, you could try and use variadic templates here, however I am unsure as to their application in this case. The normal use is with recursive methods and you would need to throw a static_assert in the midst.
Edited to take the comments into account.
template <class Type, size_t Size>
class vector
{
public:
template <class... Args>
vector(Args... args): data({args...})
{
// Necessary only if you wish to ensure that the exact number of args
// is passed, otherwise there could be less than requested
BOOST_MPL_ASSERT_RELATION(sizeof...(Args), ==, Size);
}
private:
T data[Size];
};
Another possibility that is already available is to combine Preprocessor generation with boost::enable_if.
template <class Type, size_t Size>
class vector
{
public:
vector(Type a0, typename boost::enable_if_c< Size == 1 >::type* = 0);
vector(Type a0, Type a1, typename boost::enable_if_c< Size == 2 >::type* = 0);
// ...
};
Using Boost.Preprocessor for the generation makes this easier.
BOOST_PP_REPEAT(MAX_COUNT, CONSTRUCTOR_MACRO, ~);
// where MAX_COUNT is defined to the maximum size you wish
// and CONSTRUCTOR_MACRO actually generates the constructor
#define CONSTRUCTOR_MACRO(z, n, data) \
vector( \
BOOST_PP_ENUM_PARAMS(n, Type a), \
typename boost::enable_if_c< Size == n >::type* = 0 \
);
The implementation of the constructor is left as an exercise for the reader. It's another call to BOOST_PP_REPEAT.
As you can see, it soon gets ugly, so you'll be better off if you can use the variadic template version.

Answer 2

There's a further solution to your problem: Using variadic template parameters in the initializer list
template<typename T, unsigned int N>
struct vector {
T data[N];
template<typename... Args>
vector(Args... args) : data({args...}) { }
};
However the number of arguments only needs to be less or equal than N and their types only needs to be convertible to T.

Answer 3

Is it possible to replace a constructor which takes Size number of arguments in template specializations like this
Not without a ton of repetitive, mechanical code, and the maximum size will be limited by the number of times you repeat yourself. Case in point: boost::tuple (which might have exactly the functionality what you want).
In C++0x this won't be a problem thanks to variadic templates.

Answer 4

First you should consider using std::array. It doesn't meet all your requirements but it's close enough that if the differences don't matter you can save yourself a lot of work. The problem is this cheap version will have construct that accepts 2 arguments as well as 3.
template< typename T>
using Vector3 = std::array<T,3>;
Vector3 v1{1,2,3};
Vector3 v2{1,2}; // it sounds like you want to disallow this case.
Otherwise you can create a custom class that acts a lot like std::array, except with a more picky constructor.
template<typename T, std::size_t SIZE>
class Vector
{
public:
template< typename ... Args >
Vector( Args ... args ) :
data({args...})
{
static_assert( sizeof...(Args) == SIZE,
"Incorrect number of arguments passed to Vector constructor");
}
/* lots of extra code here to add std::array -like methods */
private:
// could use std::array here as well.
T data[3];
};
template< typename T >
using Vector3 = Vector<T,3>;
Vector3 v1(1,2,3);
Vector3 v2(1,2); // error at compile time.