如何在C的头文件中正确编写extern数组的声明？

Question

First of all, let me confess that pointer-to-an-array always gets me confused. Hence, I am asking this question.

Suppose I want to share a global array of data across my program, for example:

static const char * EnumStrings_WIP_Selection_Box[] = { "Rectangular", "Siemens Gauss", "My_Gauss", "Planck-Taper", "Invalid"};

When I try to declare corresponding “extern” command

extern static const char * EnumStrings_WIP_Selection_Box[];

or

extern static const char * EnumStrings_WIP_Selection_Box[5];

I get the following error

error C2159: more than one storage class specified

Also, can someone also tell me the different between these?

Foo *array[10];
Foo (*array1)[10]; 

Any help would be appreciated.

Answer 1

I've assumed you want you array to be "totally" constant, as many people aren't aware of the two-consts on a pointer. This means not only can the text pointed to by the array not change, the pointers in the array also cannot be changed to point to new text.

You should make sure the data in your array is defined in a source file, or you risk a copy of the data in memory for every source file that includes the header.

Header file

extern const char *const MyText[];

Source file

extern const char *const MyText[] = { "Hello", "There"; };

Alternatively, the C++ approach

Header file

extern const std::vector<std::string> MyText;

Source file

extern const std::vector<std::string> MyText = { "Hello", "There"; };

To force it all in one file (not recommended in a header file)

const char *const MyText[] = { "Hello", "There"; };

Answer 2

extern and static are both storage classes, and are mutually exclusive.

Don't specify them together. You want:

extern const char * EnumStrings_WIP_Selection_Box[];

The static conflicts with the extern .

Answer 3

Try this:

.cpp file:

/* not static */
const char * EnumStrings_WIP_Selection_Box[] = { "Rectangular",
    "Siemens Gauss", "My_Gauss", "Planck-Taper", "Invalid"};

.h[pp] file:

extern const char * EnumStrings_WIP_Selection_Box[];

That said, for the sake of future maintainers, please consider defining your access in terms of methods, not exposed pointers (ie consider adding a char const * const GetWipSelectionByIndex(const std::size_t index) to your public API, instead of exposing an array).