Is there a workaround for template template arguments being invisible?

user_700825 0
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Consider the following code:

template<typename... Args1> 
constexpr size_t direct = sizeof... (Args1);

template<template<typename... Args1> typename A1> 
constexpr size_t nested = sizeof... (Args1); // compile error ☹️

Nested line does not compile since (I presume) Args1 is a nested argument.

Is there a way to work around this in a non-intrusive way? By intrusive I mean modifying types passed, for example adding

constexpr size_t sizeof_args = sizeof... (Args...);

to each template passed to this template.

Note: I am fine with using C++20 concepts, but AFAIK they do not offer improvements in this space.

c++20 template-templates c++
1 Answers

You can write a primary variable template, and then specialize the variable template for template template parameters.

This allows you to explicitly name the template parameters of the template template parameter separately:

template<typename ...> 
constexpr size_t nested = -1; // any value will do, 
                              // but prefer one that is invalid for specializations

template<template<typename ...> typename A1, typename... Args1> 
                // name the parameters here  ^_______________^
constexpr size_t nested<A1<Args1...>> = sizeof...(Args1);
    // specialize here ^____________^

The concepts feature added in C++20 doesn't help in any way here, since there's nothing being constrained.

Here's a demo.

user_83728530
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