Global const char pointer array in C++

Question

I have defined a const char pointer array in header file. Array contains multiple strings. Now I have to find a string at particular index. I don't know how should I access this array from another files. Please see my code:

common.h

extern const char *lookup_str[] = {"test Str0", "test Str1", "test Str2", "test Str3"};

file1.cpp

int ret = 3;
std::string r = lookup_str[ret];

Can I use this way in all my C files? Let me know if you can see any alternative approach. Any suggestion/help is much appreciated. Thank you in advance !

Answer 1

Declare the variable without initializing it in the .h file:

extern const char *lookup_str[];

Initialize it in exactly one source file.

const char *lookup_str[] = {"test Str0", "test Str1", "test Str2", "test Str3"};

To use it in a .cpp file, use:

#include "common.h"

...

int ret = 3;
std::string r = lookup_str[ret];

---

A better alternative, IMO, would be to provide a purely functional interface to the data.

The .h file:

std::string const& lookup_str(size_t index);

The .cpp file:

std::string const& lookup_str(size_t index)
{
   static std::vector<std::string> str = {"test Str0", "test Str1", "test Str2", "test Str3"};
   return str.at(index);  // This will throw exception if index is not
                          // within bounds.
}

and then use it as:

#include "common.h"

...

size_t ret = 3;
std::string r = lookup_str(ret);

Update

A C++98 implementation of lookup_str() would be:

std::string const& lookup_str(size_t index)
{
   static std::vector<std::string> str;
   if ( str.empty() )
   {
      str.push_back("test Str0");
      str.push_back("test Str1");
      str.push_back("test Str2");
      str.push_back("test Str3");
   }
   return str.at(index);
}

Answer 2

You should avoid having variable definitions in header files. Instead, you can have a declaration , which simply informs the compiler of a symbol, which can be defined elsewhere, and compiled separately.

So, your header should look like this:

extern const char *lookup_str[]; // defined elsewhere

And you should add this to a source (.c) file (doesn't matter which one, as long as it's linked into the final executable):

const char *lookup_str[] = { ... };

Then to use it, simply include the aforementioned header, and use it like you would any array.

Answer 3

Since you are doing this in c++ , dont do it with c-strings , do it the c++ way:

common.cpp

std::array<std::string, 4> lookup_str = {"test Str0", "test Str1", "test Str2", "test Str3"};

common.h

extern std::array<std::string, 4> lookup_str;

Then you can use it in the same way:

int ret = 3;
std::string r = lookup_str[ret];

Or in awesome c++ ways:

auto stringInArr = std::find(lookup_str.begin(), lookup_str.end(), "test Str3");

Here is a live example:

http://ideone.com/UFfIyV

Answer 4

A cleaner design IMO would be to declare the extern and definition in a header file separated by macro definition

strdef.h

#ifdef STRING_INSTANCE_DEFINE
const char *lookup_str[] = {"test Str0", "test Str1", "test Str2", "test Str3"};
#else
extern const char *lookup_str[];
#endif

strdef.c

#define STRING_INSTANCE_DEFINE
#include <strdef.h>

All other files

#include <strdef.h>

Remeber to define the macro STRING_INSTANCE_DEFINE only in 1 .c file. All other files using the array could simply include the header file and it would be available to them as extern.