Question: What is difference between 

```
int x=7;

```

and 

```
register int x=7;

```

?

I am using C++.

Answer: 
`register` is deprecated in C++11. It is unused and reserved in C++17.

Source: <http://en.cppreference.com/w/cpp/keyword/register>
The `register` keyword was useful for:

* Inline assembly.
* Expert C/C++ programming.
* Cacheable variables declaration.

An example of a productive system, where the `register` keyword was required:

```
typedef unsigned long long Out;
volatile Out out,tmp;
Out register rax asm("rax");
asm volatile("rdtsc":"=A"(rax));
out=out*tmp+rax;

```

It has been deprecated since C++11 and is unused and reserved in [C++17](http://en.cppreference.com/w/cpp/keyword/register).