Compound template

Question

I'm trying to make "compound" template type. Something like this

template <typename A, typename T>
class configurator
{
public:
    configurator(const A<T> & adapter) : m_adapter(adapter) {}
private:
    A<T> m_adapter;
};

Compiler complains with

error: expected ')'
    configurator(const A<T> & adapter
                        ^

Why this is not working? Is it possible to make it work?

songyuanyao · Answer 1 · 2018-05-10T10:15:49.473

5

A is declared as type template parameter; you can't use it as template-name and specify template argument for it.

You want template template parameter. e.g.

template <template <typename> typename A, typename T>
class configurator

BTW if A is supposed to work with multiple template arguments you could specify A with template parameter pack:

template <template <typename...> typename A, typename T>
class configurator

edited May 10 '18 at 10:15

answered May 10 '18 at 09:55

songyuanyao

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The OP should beware that this will only work if `A` has *exactly* one typename argument. It won't work for (eg) `std::vector`, because although that is usually used with one template argument, it actually has an allocator, and may have additional, implementation specific, parameters. – Martin Bonner supports Monica May 10 '18 at 10:04
1

@MartinBonner [It works since C++17](http://coliru.stacked-crooked.com/a/c458b4fd6cea62d1). – HolyBlackCat May 10 '18 at 10:07
@HolyBlackCat Do you have more links about it? It seems [clang](https://wandbox.org/permlink/0fuyRoXsb1IuK7vT) doesn't support that.. – songyuanyao May 10 '18 at 10:14
@songyuanyao I found [this](https://stackoverflow.com/questions/48645226/template-template-parameter-and-default-values), so maybe clang just doesn't support it yet? – HolyBlackCat May 10 '18 at 10:18
@HolyBlackCat Thanks for that. – songyuanyao May 10 '18 at 10:20
@HolyBlackCat Oooh! That's shiny. Excellent. – Martin Bonner supports Monica May 10 '18 at 13:43

Compound template

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