CODEWITHSUNDEEP
c++c++11templates
Solrac
Solrac

Reputation: 58

Why does template argument deduction/substitution fail with std::tuple and template derived classes?

In the next code, the template argument deduction fails when I try to pass a std::tuple of reference derived classes as argument to the function that receive a std::tuple of reference base classes. Why the compiler can not be deduced the template arguments T1 and T2? and How can I fix it?

// Example program
#include <iostream>
#include <tuple>

template<typename T>
struct Base {};

template<typename T>
struct Derived1 : Base<T> {};

template<typename T>
struct Derived2 : Base<T> {};

template<typename T1, typename T2>
void function(std::tuple<Base<T1>&,Base<T2>&> arg)
{
    std::cout << "Hello\n";
}

int main()
{
    Derived1<int> d1;
    Derived2<double> d2;

    //function(std::tie(d1, d2));  /* In this case the template argument deduction/substitution failed */
    function<int,double>(std::tie(d1, d2)); /* here works */

    Base<int>& b1 = d1;
    Base<double>& b2 = d2;

    function(std::tie(b1, b2)); /* but, in this case also works */
}

This is the compile error for the line code function(std::tie(d1, d2));:

 In function 'int main()':
25:30: error: no matching function for call to 'function(std::tuple<Derived1<int>&, Derived2<double>&>)' 
25:30: note: candidate is: 
15:6: note: template<class T1, class T2> void function(std::tuple<Base<T>&, Base<T2>&>) 
15:6: note: template argument deduction/substitution failed: 
25:30: note: mismatched types 'Base<T>' and 'Derived1<int>' 
25:30: note: 'std::tuple<Derived1<int>&, Derived2<double>&>' is not derived from 'std::tuple<Base<T>&, Base<T2>&>'

Upvotes: 3

Views: 1563

Answers (1)

skypjack
skypjack

Reputation: 50550

Deduction doesn't work this way. It pulls in before any conversion or whatever. Here the compiler expects a Base<T> from which to deduce T and you are trying to pass a DerivedN<T>. They are completely different beasts from the point of view of the type system and the function is discarded when trying to find a good match for the call.
Look at the error, it's quite clear.

How can I fix it?

You can use something like this to have them accepted and still force the fact that they derive from Base:

#include<type_traits>

// ...

template<template<typename> class C1, template<typename> class C2, typename T1, typename T2>
std::enable_if_t<(std::is_base_of<Base<T1>, C1<T1>>::value and std::is_base_of<Base<T2>, C2<T2>>::value)>
function(std::tuple<C1<T1>&, C2<T2>&> arg)
{
    std::cout << "Hello\n";
}

// ...

See it up and running on wandbox.

Upvotes: 1

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