address of register variable in C and C++

Question

I know the concept of register variable and it's use cases but there are few questions in my mind based on what I have tried.

1. I cannot access the address of a register variable in C though I can do it C++! Why? Is there any issue in accessing the addressing of a register variable?

2. Suppose if I declare a string variable in C++ as register, then where will that variable be stored? What is the point in declaring the storage class of non-numeric data types such as 'string' in C++ to be register??

UPDATE: I thought that C++ allows us to fetch the address of a register variable, as I was not getting any error in my program which is as follows:

#include<iostream>
#include<time.h>

using namespace std;

clock_t beg, en;

int main(){

    int j, k=0;

    beg=clock();
    for(register int i=0;i<10000000;i++){
        /*if(k==0){
            cout<<&i<<endl;    // if this code is uncommented, then C++ rejects the recommendation to make 'i' as register
            k++;
        }*/
    }
    en=clock();

    cout<<en-beg<<endl;

    cout<<&j<<endl<<&k;

    return 0;
}

What I have observed is, if I make the variable 'i' as register and don't try to print the address using '&i' then C++ accepts the recommendation and stores 'i' in register, this can be infered from running time of for loop which will always be around 4-12 ms if 'i' is in register. But if I try to print address of variable 'i' then though I don't get any error but C++ rejects the recommendation and this can be infered from the time of execution of loop which is always more than 25 if i is not register!!

So, basically I cannot fetch address of a variable with storage class as register in both C as well as C++!! WHY?

Answer 1

C and C++ are different languages.

• In C, you cannot take the address of a variable with register storage. Cf. C11 6.7.1/6:

  A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible. The extent to which such suggestions are effective is implementation-defined.

  Footnote: The implementation may treat any register declaration simply as an auto declaration. [...]

• In C++, register is a deprecated, meaningless keyword that has no effect (except perhaps serve as a compiler hint), and variables declared as register still just have automatic storage. In particular, C++ doesn't have a "register" storage class. (It just has the storage class specifier, inherited from C.) Cf. C++11, 7.1.1/3:

  A register specifier is a hint to the implementation that the variable so declared will be heavily used. [ Note: The hint can be ignored and in most implementations it will be ignored if the address of the variable is taken. This use is deprecated [...]

Even in C nothing is actually guaranteed about how register storage is implemented (implementations are free to treat register as auto), but the language rules apply regardless.