C Template Syntax for Mixed Class and Method Templates-CodePudding

I am having trouble getting an inline method implementation to reference a generic template instance method [ check nomenclature ] because I don't know how to refer to it (or can't). I can make it work inline, but want to know if/how to do defer the definition.

I've found similar questions with the same setup (class/struct template parameter and a method instance parameter), but they are always solving a different problem (I think).

#include <functional>

template <typename T>
struct Vec {
  T elem;
  Vec(T t) : elem(t) { }

  // this works, but I want to defer the definition to later
  template <typename R>
  Vec<R> fmap(std::function<R(T)> apply) const {
    return Vec<R>(apply(elem));
  }

  // but I want to defer the definition to later in the file
  //  template <typename R>
  //  Vec<R> fmap2(std::function<R(T)> apply) const;
};

// This FAILS: how do I refer to this?
// template <typename T,typename R>
// inline Vec<R> Vec<T>::fmap2(std::function<R(T)> apply) const {
//  return Vec<R>();
// }

Uncommenting fmap2 on GCC gives.

no declaration matches ‘Vec<R> Vec<T>::fmap2(std::function<R(T)>) const’

Visual Studio 2019 gives

unable to match function definition to an existing declaration

The inline definition works, but I want to define that templated member later in the compilation unit if possible.

Thanks in advance.

CodePudding user response：

You need to declare typename T for Vec first, and then typename R for fmap2, like this

template <typename T>
template <typename R>
Vec<R> Vec<T>::fmap2(std::function<R(T)> apply) const {
  return Vec<R>();
}