I'm trying to use GMP with C++11, but apparently it's not allowed to use mpz\_class in constexpr functions because mpz\_class is not a literal type.

```
#include <iostream>
#include <gmpxx.h>
using namespace std;

constexpr mpz_class factorial(mpz_class n)
{
    if (n == 0) return 1;
    else return n * factorial(n - 1);
}

int main()
{
    cout << factorial(20);
}

```

Is it possible?
['https://Stackoverflow.com/questions/26499775', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/4168350/']
Instagram says:

> 
> Note that we do not include an expiry time. Our access\_tokens have no explicit expiry, though your app should handle the case that either the user revokes access or we expire the token after some period of time. In this case, your response’s meta will contain an “error\_type=OAuthAccessTokenError”. In other words: do do not assume your access\_token is valid forever.
> 
> 
> 

If the token is somehow expired then they request for a new one, but that should be a rare case as instagram says they do no include an expiry time.

For more [info read this](http://instagram.com/developer/authentication/). Hope this helps.. :)
First you have to get the access token and save that access token in your app, access token is not valid forever, so you should implement and handle this on your application follow this tutorial for login with instagram using oauth and its implementation on app side
<http://codegerms.com/login-with-instagram-ios-application-objective-c/>