how to define a constant array in c/c++?

c++ c arrays constants

107,721

Solution 1

In C++ source file

extern "C" const int array[] = { 1, 2, 3 };

In header file to be included in both C and C++ source file

#ifdef __cplusplus
extern "C" {
#endif
extern const int array[];
#ifdef __cplusplus
}
#endif

Solution 2

In C++, the most common way to define a constant array should certainly be to, erm, define a constant array:

const int my_array[] = {5, 6, 7, 8};

Do you have any reason to assume that there would be some problem on that embedded platform?

Solution 3

In C++

const int array[] = { 1, 2, 3 };

That was easy enough but maybe I'm not understanding your question correctly. The above will not work in C however, please specify what language you are really interested in. There is no such language as C/C++.

Solution 4

It's impossible to define array constant using the define directive.

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107,721

Author by

Andrey Pesoshin

Computer fan

Updated on July 20, 2022

Comments

Andrey Pesoshin almost 2 years
How to define constant 1 or 2 dimensional array in C/C++? I deal with embedded platform (Xilinx EDK), so the resources are limited.

I'd like to write in third-party header file something like
```
#define MYCONSTANT 5
```
but for array. Like
```
#define MYARRAY(index) { 5, 6, 7, 8 }
```
What is the most common way to do this?
- Ken Wayne VanderLinde almost 13 years
  
  A macro based solution as used in the first example wouldn't work. Somewhere, an actual array must be allocated in memory, as in sbi's answer
- A. K. almost 13 years
  
  Please elaborate a bit. Does constant array mean that you don't want the array to be changed at a later stage or something else.
- Andrey Pesoshin almost 13 years
  
  @Aditya Kumar - that's correct. I want to define the set of constants in separate file(s) and access them using integer index from main source file.
- A. K. almost 13 years
  
  Then @jahhaj's answer seems to be the most appropriate
- Dan almost 13 years
  
  The Xilinx tag isn't going to help you. I would suggest dropping it, and adding "const" and "array" tags, although at this point the question has been answered sufficiently anyway.
sbi almost 13 years

Why doesn't this work in C? I thought C had copied const from C++ by, I think C89?
jahhaj almost 13 years

If this was put in a header file and that header file was included in more than one source file, in C you would get multiple definition errors, in C++ you would not. Const does not have exactly the same meaning in C and C++.
Andrey Pesoshin almost 13 years

okay. And where to define it - header/source file? When i define it in third-party h file (it's a separate driver for microprocessor system) - i get an error - something like "this variable cannot be defined multiple times".
Andrey Pesoshin almost 13 years

when i define it in source file - it cannot be accessed from main.cc file (in which i include mydriver.h)
jahhaj almost 13 years

So you are writing in C? You have managed to pick on the the few areas where the same code will work differently in C and C++, so please specify which language
jahhaj almost 13 years

in source file const int array[] = { 1, 2, 3 }; in header file extern const int array[]; you need both (assuming you are writing C)
Andrey Pesoshin almost 13 years

to be specific - my application consists in general of 2 parts - main program written in C++ and peripheral core software driver written in pure C (as it is compiled as a part of board support package).
Andrey Pesoshin almost 13 years

yes, external h/source files are in C and main source code is in C++ - that is the way EDK offers to do
jahhaj almost 13 years

Well that's another complication. Mixed C and C++ means you are also going to need extern "C" declarations, see the new answer I added
sbi almost 13 years

@jahhaj: Thanks! I never really worked in C, so I didn't know that.