Tag: register keyword in C

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Is it good useful meaningful worthwhile to use register variables in C++?

The register keyword has been largely ignored by compilers for decades. In C, it prevents you from taking the address of a variable. But otherwise, most compilers ignore the hint it offers.

STANDARD C++ does not support register variables in 2022. In C register variables serve as a hint to the compiler to use a register rather than a memory address. Prior to C++17, it could also be used as a hint to the compiler in C++ as well, but only some conforming compilers did anything as a result. Starting in C++17, register has become an unused but reserved keyword.

Some compilers still permit it and some may even honor the request. Your programs, however, will not be cross-platform or written in true, standards-compliant C++ if you use the register keyword.

NOTE:

The situation in C (as opposed to C++) is quite different.

“Is it good?”

Usually the answer to that question is that it’s the wrong question, and that you should instead ask “Is it useful/meaningful/worthwhile…?” For example, if you had asked about using the register keyword in C, I would say “There’s nothing wrong about it. It won’t harm your program. It’s just a waste of time.”

It turns out C++ deprecated the register keyword in C++11, and removed it in C++17. As of C++17, the keyword register is now a reserved word with no meaning.

So, the answer to “is it good?” is “No.” Your program will fail to compile as a C++17 program.

By the end of 90s, register was the “R-type” decal of the C programming world. It did about as much for the speed of your program as the decal did for your car. (Possible exception for weak compilers on some embedded platforms.)

In any case, it doesn’t make sense to have a register qualifier on a parameter in a function prototype. It is ignored on any function declaration that is not a definition, similar to top level const.

That is, all four of these are equivalent:

Reference: here

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