Is there a reason to declare something "volatile const" in C, but only "volatile" in C ++?

Question

I used the header file in my project, which had the following definitions (s):

#ifdef __cplusplus extern "C" { #endif #ifdef __cplusplus #define __I volatile /*!< Defines 'read only' permissions*/ #else #define __I volatile const /*!< Defines 'read only' permissions*/ #endif 

__I used in the following header file as follows:

  typedef struct { // more members before __I uint32_t CR; /*!< GPIO Commit*/ // more members after } GPIOA_Type; #define GPIOF_BASE 0x40025000UL #define GPIOF ((GPIOA_Type *) GPIOF_BASE) 

My question is, why __I make const in C, but not in C ++? You can still change the value that CR points to, since you have an address, but just wondering why the __I definition __I different.

For someone interesting, this is for or from, __I is determined from the IAR Embedded Workbench ARM for Cortex-M4, and the structure is from CMSIS Texas Instruments LM4F120H5QR files.

Answer 1

In C ++, the default const variables in the default file area are associated with static linking, which is undesirable for GPIO with memory mapping. The “correct” fix for this is the extern keyword, but it cannot be used here, since it is obvious that __I should work with class members as well. Thus, removing const makes the default extern binding.