What exactly is a variable template

Question

I am getting c++11 warnings about variable templates. I'm not entirely sure that I need them, because I'm not entirely sure what they are. References that I've found don't bother defining the term before using it. My immediate thought was that it was an attempt to declare a variable that is of the templated type T, but this code compiles fine:

template <class T>
image<T> *image<T>::copy() const {
  image<T> *im = new image<T>(w, h, false);
  memcpy(im->data, data, w * h * sizeof(T));
  return im;
}

What is a thorough definition of a variable template, and why does this NOT have a variable template problem

Answer 1

A class template is a template that defines a series of classes, based on one or more template parameters. A function template is a template that defines a series of functions, based on one or more template parameters. vector is a template class; vector<int> is a specific class instantiated from that template.

A variable template is therefore a template that defines a series of variables, based on one or more template parameters:

template<typename T>
T variable_name{};

That is a variable template. You would specify which one you want just like any other template: variable_name<int> will be of type int. variable_name is a template; variable_name<int> is actually a variable.

Of course, you can do more complex things:

template<typename T>
vector<T> vector_var{};

vector_var<int> is a vector<int>.

Variable templates can only be introduced at namespace/global scope and as static members of classes. At namespace/global scope, they're usually declared constexpr (and/or in C++17, inline). They're useful for making constants:

template<typename T>
inline constexpr bool is_default_constructible_v = std::is_default_constructible<T>::value;

So if you want to tell if something is default constructible, you don't need the slightly awkward ::value syntax.

Variable templates are a C++14 feature, which is why your compiler probably warned you about using them in a C++11 mode.

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image<T> *im = new image<T>(w, h, false);

This is not a variable template. This is a regular variable whose type is very well defined: image<T>. There is exactly one variable named "im". You don't use im<T> to get a variable.

That statement does not define a family of variables. What you have is a family of functions, where each of them contain a variable called im. The function is the template here, not the variable.