Template template arguments that do not match template template arguments

Question

The following code cannot compile with an error template template argument has different template parameters than its corresponding template template parameter:

#include <tuple>
#include <vector>
#include <string>
#include <iostream>

template<template<typename, typename> class Blah, typename KeyType, typename ValueType>
void print_tuploid(const Blah<KeyType, ValueType>& tup) {
    std::cout << "first value: " << std::get<0>(tup) << " second value: " << std::get<1>(tup) << std::endl;
}

int main() {
  auto stuff = std::make_tuple(1, 2);
  print_tuploid(stuff);
}

The original intent of this code does not matter, at this point I am just trying to understand why this is considered unacceptable.

If I changed the call to std::make_tupleon std::make_pair, the code will compile and run correctly, which leads me to believe that there is something strange going on with respect std::tuple.

I initially thought that it std::tuplemight have some additional default template arguments that I didn’t know about, because if I changed the definition print_tuploidto the following, the code FORKES for both std::make_tupleand std::make_pair:

template<template<typename...> class Blah, typename KeyType, typename ValueType>
void print_tuploid(const Blah<KeyType, ValueType>& tup) {
    std::cout << "first value: " << std::get<0>(tup) << " second value: " << std::get<1>(tup) << std::endl;
}

But when I tried to infer the type being deduced stuffusing the following code:

#include <tuple>

template<typename T>
class TypePrinter;

int main() {
  auto stuff = std::make_tuple(1, 2);
  TypePrinter<decltype(stuff)> error;
}

: implicit instantiation of undefined template 'TypePrinter<std::__1::tuple<int, int> >', , .

, , ? :

#include <iostream>

template<template<typename, typename> class Blah, typename KeyType, typename ValueType>
void print_tuploid(Blah<KeyType, ValueType>&& tup) {
    std::cout << "first value: " << std::get<0>(tup) << " second value: " << std::get<1>(tup) << std::endl;
}

int main() {
  auto stuff = std::make_pair(1, 2);
  print_tuploid(stuff);
}

: no known conversion from 'std::__1::pair<int, int>' to 'pair<int, int> &&' for 1st argument.

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Answer 1

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