在C ++中注册存储说明符

Question

I am curious to know to behavior of register storage specifier in C/C++ . Following program valid in C++ , but in case of C it's not valid.

int main()
{
        register int i;
        int* b = &i; // Valid in C++ but not in C
}

So, My question I, Why C++ allowed address operator(&) to a register variable?

PS : I know register storage specifier is deprecated in modern compiler.

Answer 1

Your answer can be found here:

http://en.cppreference.com/w/cpp/language/storage_duration

In C, the address of a register variable cannot be taken, but in C++, a variable declared register is semantically indistinguishable from a variable declared without any storage class specifiers.

Answer 2

C and C++ diverged before the C language was standardized. Both adopted the register keyword but only C added the restriction on the & operator.

Digging into the history, the C++98 standard says,

A register specifier has the same semantics as an auto specifier together with a hint to the implementation that the object 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 object is taken. — end note ]

Subsequently it was deprecated, which is an opposite direction from strengthening it with semantic restrictions. It is scheduled for removal in the upcoming C++17 standard.

( auto was later repurposed in C++11. Its old meaning was the same as the default storage of a local variable.)

Answer 3

So, My question I, Why C++ allowed address operator(&) to a register variable?

The answer is:
Because C and C++ are different languages and follow different standards as pointed out in the other answers.

Answer 4

The difference is explained by this quote from the C++ Standard (7.1.1 Storage class specifiers)

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 (see D.2). —end note]