Can we send pair a template argumrent another pair

Question

Can we use pair as an argument for another pair in C++.

Here is a test program to do that.

#include <iostream>
#include <utility>

int main()
{
    std::pair<int,int> m;
    m=std::make_pair(1,3);
    int r = 3;
    int *q = &r;
    int **p =&q;
    int **t = p;
    std::pair<int**,<std::pair<int,int> > > i(p,m);
    std::cout<<i[t];

    return 0;
}

This is the error, I am getting.

Test.cpp: In function ‘int main()’:
Test.cpp:12:45: error: template argument 2 is invalid
  std::pair<int**,<std::pair<int,int>>> i(p,m);
                                             ^

If I changed the format of declaration and wrote the program in the following way,

#include <iostream>
#include <utility>

int main()
{
    std::pair<int,int> m;
    m=std::make_pair(1,3);
    int r = 3;
    int *q = &r;
    int **p =&q;
    int **t = p;
    std::pair<
                int**,
                <
                    std::pair<
                                int,
                                int
                                >
                            >
                        > i(p,m);
    std::cout<<i[t];

    return 0;
}

There is an additional error.

Test.cpp: In function ‘int main()’:
Test.cpp:20:7: error: template argument 2 is invalid
       > i(p,m);
       ^
Test.cpp:20:14: error: expression list treated as compound expression in initializer [-fpermissive]
       > i(p,m);

What might be the issue and how to solve it?

---

On a side note, I did a program and compiled it in an very old Dev-c++ compiler on a windows 7 machine which used a code similar to the above and it ran perfectly fine. It was a lab program in my college which had only that compiler.

The above code, I ran on both windows 7 and Ubuntu 16.04 using GNU g++ 5.4.0

Ideone: Link to the actual program for reference

You might also have a look at the actual error, I faced in the above link.

Answer 1

There are two problems with your code.

1. You have a superfluous pair of angle brackets in your template instantiation.
2. The cout parameter makes no sense

---

#include <iostream>
#include <utility>

int main()
{
    std::pair<int,int> m;
    m=std::make_pair(1,3);
    int r = 3;
    int *q = &r;
    int **p =&q;
    int **t = p;
    std::pair<
                int**,
                // < Omit this
                    std::pair<
                                int,
                                int
                                >
                            // > also this
                        > i(p,m);
    // std::cout<<i.[t]; indexing with a pointer value doesn't make sense
    // i. for a pair doesn't make sense, specify either first or second
    std::cout<<i.first[0]; // <<<< Did you mean something like this?

    return 0;
}

Check the compiling version live

Answer 2

This is incorrect:

std::pair<int**,<std::pair<int,int> > > i(p,m);

Just remove the extra <>:

std::pair<int**, std::pair<int,int > > i(p,m);

Also, I'm not sure what you're trying to do in the cout part, but pair doesn't have a [] operator. You can access the elements with first and second:

i.first
i.second
i.second.first
i.second.second

You may also be interested in std::tuple if you don't want to nest one pair into another:

std::tuple<int**, int, int> i;