Search code examples
c++templatesvariadic

C++ Getting a number of elements in variadic template

So I'm trying to get a number of elements in my variadic template like so:

Test(const T& t...);

template<typename T>
inline Class<T>::Test(const T& t...){
     int num = sizeof...(t);
     ...
}

However it doesn't compile and I get the following errors:

error C3523: 'sizeof...' requires as its argument an unexpanded parameter pack

't' is not a valid operand for 'sizeof...'. Did you mean to use 'sizeof'?

What is the correct way to get the number of arguments in t...?

Solution

  • If T were a parameter pack, you could use the following syntax to declare a function that takes a sequence of parameters with types corresponding to those in the parameter pack:

    inline Class<T>::Test(const T&... t) { /* ... */ }

    However, it appears that T isn't a parameter pack. It's just a single type parameter. Plus, you put the ... in the wrong place. So what you really did was declare a function that takes a single parameter of type T, plus a C-style ellipsis. (Yes, the comma before the ellipsis is optional!)

    So when you write sizeof...(t), the compiler complains because t is not a pack. It's just a normal parameter.

    Perhaps you wanted to declare Test to be a function that takes an arbitrary number of arguments, but all of type const T&? Unfortunately, there is no easy way to do that in current C++. See Specifying one type for all arguments passed to variadic function or variadic template function w/out using array, vector, structs, etc? for solutions.