4

Consider the class

template <typename T>
struct Foo {
   Foo(const Foo<T>& other) {}
};

For the constructor argument type, is const Foo<T>& and const Foo& the same in this context? I always assumed not, thinking that the latter can be called for Foo<int> f = Foo<float>(), and the former cannot. But now I'm not sure if that is so.

Violet Giraffe
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    Possible duplicate of [Difference when omitting the C++ template argument list](http://stackoverflow.com/questions/3690506/difference-when-omitting-the-c-template-argument-list) – LogicStuff Aug 10 '16 at 08:03
  • as the answers already say, yes that's legal. It's not ambigous because you cannot have both `class Foo` and `class Foo` – peterchen Aug 10 '16 at 08:09

2 Answers2

8

Within a class template, class template parameters have one unique meaning for every instantiation. That means that Foo<int> has T==int, and thus the templated ctor is Foo<int>::Foo(const Foo<int>& other).

It is possible to have additional template parameters, though :

template <typename T>
struct Foo {
   template <typename U>
   Foo(const Foo<U>& other) {}
};

Now T can be different from U.

MSalters
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6

Yes, it's the same.

This is due to the injected-class-name. The class name is inserted into the scope of all classes so that name lookup acts sensibly. When the injected-class-name is used as a type name in a template class, it's equivalent to the template name followed by the template parameters enclosed in <> ([temp.local]/1), so Foo is equivalent to Foo<T> in that context.

TartanLlama
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