C++ best way to define cross-file constants

Question

I am working on a game and have an interesting question. I have some game-wide constant values that I want to implement in one file. Right now I have something like this:

constants.cpp

extern const int BEGINNING_HEALTH = 10;
extern const int BEGINNING_MANA = 5;

constants.hpp

extern const int BEGINNING_HEALTH;
extern const int BEGINNING_MANA;

And then files just #include "constants.hpp" This was working great, until I needed to use one of the constants as a template parameter, because externally-linked constants are not valid template parameters. So my question is, what is the best way to implement these constants? I am afraid that simply putting the constants in a header file will cause them to be defined in each translation unit. And I don't want to use macros.

Thanks

Tom · Accepted Answer · 2009-03-13T11:48:31.977

19

Get rid of the extern and you're set.

This code works perfectly fine in a header, because everything is "truly constant" and therefore has internal linkage:

const int BEGINNING_HEALTH = 10;
const int BEGINNING_MANA = 5;
const char BEGINNING_NAME[] = "Fred";
const char *const BEGINNING_NAME2 = "Barney";

This code cannot safely be put in a header file because each line has external linkage (either explicitly or because of not being truly constant):

extern const int BEGINNING_HEALTH = 10;
extern const int BEGINNING_MANA = 5;
const char *BEGINNING_NAME = "Wilma";  // the characters are const, but the pointer isn't

edited Mar 13 '09 at 11:48

answered Mar 13 '09 at 04:04

Tom

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i suspect a copy/paste bug in last BEGINNING_NAME[] one. did you want to write it as BEGINNING_NAME ? – Johannes Schaub - litb Mar 13 '09 at 04:45
You need "static", otherwise you do not get internal linkage or, more ideally, the equivalent of a "#define" with the values inlined. – Jim Buck Mar 18 '09 at 05:07
@Jim Buck constants in C++ have internal linkage - no need for "static" – Jan 23 '10 at 13:09

score 10 · Answer 2 · answered Mar 13 '09 at 03:53

10

How about enums?

constants.hpp

  enum {
    BEGINNING_HEALTH = 10,
    BEGINNING_MANA = 5
  }

answered Mar 13 '09 at 03:53

Tommy Hui

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score 7 · Answer 3 · answered Mar 13 '09 at 04:33

7

Use "static const int" in your .hpp file, and put nothing in the .cpp file (except whatever other code you have there of course).

answered Mar 13 '09 at 04:33

Jim Buck

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score 4 · Answer 4 · answered Mar 13 '09 at 12:18

4

make use of namespaces:

namespace GameBeginning {
    const int HEALTH = 10;
    const int MANA   = 5; 
};

then u can use as player.health = GameBeginning::HEALTH;

answered Mar 13 '09 at 12:18

user77682

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score 0 · Answer 5 · answered Mar 13 '09 at 04:06

0

Most compilers simply don't allocate space for const POD values. They optimize them out and treat them as if they had been #defined, don't they?

answered Mar 13 '09 at 04:06

greyfade

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Close, but still not quite the same. Example where macros work but constants don't: #define FOO "foo"; \ const char *str = "bar" FOO; C and C++ allow concatenation of string literal tokens, but not string constants. – Tom Mar 13 '09 at 04:40
This is only true for integral constants. Floats, doubles, char*s, and other types will be allocated storage if optimization is disabled. – Adam Rosenfield Mar 13 '09 at 04:46
@Tom: That's not what I meant. I meant for POD constants, not preprocessor majicks. I meant in the sense that the symbol and the space for the value don't exist unless you attempt to take its address. – greyfade Mar 13 '09 at 06:12
I think POD officially includes floats and doubles, but floats and doubles are generally not optimized in this way. – Max Lybbert Mar 13 '09 at 08:52

score 0 · Answer 6 · answered Mar 13 '09 at 06:51

0

What ever happened to a simple:

#define BEGINNING_HEALTH 10

Man, those were the days.
Oh wait, those still are the days!

answered Mar 13 '09 at 06:51

slacy

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That might not do what you're expecting in the context of template parameters... :) rlbond needs that. – Mihai Limbășan Mar 13 '09 at 07:29
5

Some of us like our debuggers to show 'BEGINNING_HEALTH' instead of '10'. – JoeG Mar 13 '09 at 12:13
When needs debuggers when you have printf()? – slacy Mar 13 '09 at 20:53
2

printf, #define, what next. This is C++. – Sebastian Mach Sep 29 '11 at 14:43

score -2 · Answer 7 · answered Mar 13 '09 at 03:53

-2

perhaps something along the lines of a static class?

class CONSTANTS {
public:
static inline int getMana() { return 10;};
};

answered Mar 13 '09 at 03:53

MighMoS

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That doesn't help. Function values cannot be used as template parameters. C++0x keyword `constexpr` is supposed to work around that. Besides, C++ has namespaces, which are superioer to a static class for "namespacing" constants. – Tom Mar 13 '09 at 04:06

jeffD · Answer 8 · 2009-03-13T06:33:22.747

-5

As a quick answer to the title question, a singleton pattern is a possible best, C++ way to define cross-file constants and insure only one instance of the object.

As far as the template parameter problem, you need to pass a type not a value. Your type is "int".

edited Mar 13 '09 at 06:33

answered Mar 13 '09 at 04:10

jeffD

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This seems like overkill for a set of simple constants, and is not likely to solve his problems with template instantiation. – Eclipse Mar 13 '09 at 04:15
Why wouldn't is solve his problem with template instantiation, since it wouldn't be an extern, it would be a local. Overkill is relative, what is the quality perspective $100M, $100K, or $100 game budget? – jeffD Mar 13 '09 at 06:26
jeffD, the problem is that your answer does not make sense. do you want to make an "int" a singleton? an int is comprised by value, not by identity. another thing that probably made people downvote you is that he doesn't want to pass a type, but a value to his template. Why do you say he can't? – Johannes Schaub - litb Mar 13 '09 at 06:44
The title to the question is a good question by itself. Maybe I do need to see more of his code. See http://www.cplusplus.com/doc/tutorial/templates.html for a ref. "A template parameter is a special kind of parameter that can be used to pass a type as argument". Wish comments allowed hyperlinks. – jeffD Mar 13 '09 at 06:55
Nice, the http address automatically becomes hyperized. – jeffD Mar 13 '09 at 07:02
jeffD, unfortunately, the tutorial at cplusplus.com isn't quite good - too incomplete and sometimes wrong. for example template declarations are not "expressions" as they say, they are declarations. blurs out the difference between a specialization and an explicit specialization on their page too. – Johannes Schaub - litb Mar 13 '09 at 07:47
better get a good book (there are some good threads on SO about C++ books) rather than relying on cplusplus.com . even for a reference, it has some quite blatantly wrong pages (i wonder whether they still maintain them?). and please read the very end on that linked page (about non-type parameters). – Johannes Schaub - litb Mar 13 '09 at 07:49
For the record, you can pass integral values to templates (Boost.Array uses this ability, I believe). – Max Lybbert Mar 13 '09 at 08:54
jeffD, I have a Matrix class that is uses a static array. Its class signature is template class Matrix. This is perfectly legal C++ code. – rlbond Mar 13 '09 at 16:09
I stand by my answer to the title question. You may be right about what he's doing w/ templates, w/o seeing the code and error it's unsure. There are really two questions here, I think I'll pose one as a seperate question. – jeffD Mar 13 '09 at 17:05

C++ best way to define cross-file constants

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