Memory usage: #define vs. static const for uint8_t












1















I'm writing an Arduino library to communicate with an I2C device, and I'm wondering what the best way is to declare all the register addresses so as to save memory.



Using #defines:



#define REGISTER_MOTOR_1_MODE 0x44

#define REGISTER_MOTOR_2_MODE 0x47


Or using static const:



static const uint8_t REGISTER_MOTOR_1_MODE = 0x44;

static const uint8_t REGISTER_MOTOR_2_MODE = 0x47;


(Obviously I have more than just two registers I need to declare, but I thought two would illustrate the point just fine)






c++ memory-usage







share|improve this question























  • define's are just "find and replace"s, and can bit you, if you do anything other than numbers. For example #define LED_MASK 0x01<<2. A safe(r) way to write that would be #define LED_MASK (0x01<<2). See also stackoverflow.com/questions/6542270/…

    – Gerben
    4 hours ago
















1















I'm writing an Arduino library to communicate with an I2C device, and I'm wondering what the best way is to declare all the register addresses so as to save memory.



Using #defines:



#define REGISTER_MOTOR_1_MODE 0x44

#define REGISTER_MOTOR_2_MODE 0x47


Or using static const:



static const uint8_t REGISTER_MOTOR_1_MODE = 0x44;

static const uint8_t REGISTER_MOTOR_2_MODE = 0x47;


(Obviously I have more than just two registers I need to declare, but I thought two would illustrate the point just fine)






c++ memory-usage







share|improve this question























  • define's are just "find and replace"s, and can bit you, if you do anything other than numbers. For example #define LED_MASK 0x01<<2. A safe(r) way to write that would be #define LED_MASK (0x01<<2). See also stackoverflow.com/questions/6542270/…

    – Gerben
    4 hours ago














1












1








1








I'm writing an Arduino library to communicate with an I2C device, and I'm wondering what the best way is to declare all the register addresses so as to save memory.



Using #defines:



#define REGISTER_MOTOR_1_MODE 0x44

#define REGISTER_MOTOR_2_MODE 0x47


Or using static const:



static const uint8_t REGISTER_MOTOR_1_MODE = 0x44;

static const uint8_t REGISTER_MOTOR_2_MODE = 0x47;


(Obviously I have more than just two registers I need to declare, but I thought two would illustrate the point just fine)






c++ memory-usage







share|improve this question














I'm writing an Arduino library to communicate with an I2C device, and I'm wondering what the best way is to declare all the register addresses so as to save memory.



Using #defines:



#define REGISTER_MOTOR_1_MODE 0x44

#define REGISTER_MOTOR_2_MODE 0x47


Or using static const:



static const uint8_t REGISTER_MOTOR_1_MODE = 0x44;

static const uint8_t REGISTER_MOTOR_2_MODE = 0x47;


(Obviously I have more than just two registers I need to declare, but I thought two would illustrate the point just fine)






c++ memory-usage




c++ memory-usage






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 6 hours ago









Android DevAndroid Dev

1084




1084













  • define's are just "find and replace"s, and can bit you, if you do anything other than numbers. For example #define LED_MASK 0x01<<2. A safe(r) way to write that would be #define LED_MASK (0x01<<2). See also stackoverflow.com/questions/6542270/…

    – Gerben
    4 hours ago



















  • define's are just "find and replace"s, and can bit you, if you do anything other than numbers. For example #define LED_MASK 0x01<<2. A safe(r) way to write that would be #define LED_MASK (0x01<<2). See also stackoverflow.com/questions/6542270/…

    – Gerben
    4 hours ago

















define's are just "find and replace"s, and can bit you, if you do anything other than numbers. For example #define LED_MASK 0x01<<2. A safe(r) way to write that would be #define LED_MASK (0x01<<2). See also stackoverflow.com/questions/6542270/…

– Gerben
4 hours ago





define's are just "find and replace"s, and can bit you, if you do anything other than numbers. For example #define LED_MASK 0x01<<2. A safe(r) way to write that would be #define LED_MASK (0x01<<2). See also stackoverflow.com/questions/6542270/…

– Gerben
4 hours ago










2 Answers
2






active

oldest

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6














You'll find no noticeable difference memory-wise between the two.



The only real difference is that the const method also imposes a type to the value, which can be useful for function overloading or mathematical operations.






share|improve this answer































    3














    A #define is a preprocessor macro. As Gerben says in his comment, it's just an automated find-and-replace.



    If you use it to hold things like C string constants, e.g. #define ERROR_STRING "You messed up, bud!" it could actually cause your program to take more memory, since that same string literal will be duplicated every time you reference it. In that case a static const would be both type-safe AND reduce memory usage.






    share|improve this answer
























    • Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

      – juhist
      1 hour ago






    • 1





      @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

      – Mooing Duck
      1 hour ago











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    2 Answers
    2






    active

    oldest

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    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    6














    You'll find no noticeable difference memory-wise between the two.



    The only real difference is that the const method also imposes a type to the value, which can be useful for function overloading or mathematical operations.






    share|improve this answer




























      6














      You'll find no noticeable difference memory-wise between the two.



      The only real difference is that the const method also imposes a type to the value, which can be useful for function overloading or mathematical operations.






      share|improve this answer


























        6












        6








        6







        You'll find no noticeable difference memory-wise between the two.



        The only real difference is that the const method also imposes a type to the value, which can be useful for function overloading or mathematical operations.






        share|improve this answer













        You'll find no noticeable difference memory-wise between the two.



        The only real difference is that the const method also imposes a type to the value, which can be useful for function overloading or mathematical operations.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 6 hours ago









        Majenko♦Majenko

        68.1k43277




        68.1k43277























            3














            A #define is a preprocessor macro. As Gerben says in his comment, it's just an automated find-and-replace.



            If you use it to hold things like C string constants, e.g. #define ERROR_STRING "You messed up, bud!" it could actually cause your program to take more memory, since that same string literal will be duplicated every time you reference it. In that case a static const would be both type-safe AND reduce memory usage.






            share|improve this answer
























            • Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

              – juhist
              1 hour ago






            • 1





              @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

              – Mooing Duck
              1 hour ago
















            3














            A #define is a preprocessor macro. As Gerben says in his comment, it's just an automated find-and-replace.



            If you use it to hold things like C string constants, e.g. #define ERROR_STRING "You messed up, bud!" it could actually cause your program to take more memory, since that same string literal will be duplicated every time you reference it. In that case a static const would be both type-safe AND reduce memory usage.






            share|improve this answer
























            • Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

              – juhist
              1 hour ago






            • 1





              @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

              – Mooing Duck
              1 hour ago














            3












            3








            3







            A #define is a preprocessor macro. As Gerben says in his comment, it's just an automated find-and-replace.



            If you use it to hold things like C string constants, e.g. #define ERROR_STRING "You messed up, bud!" it could actually cause your program to take more memory, since that same string literal will be duplicated every time you reference it. In that case a static const would be both type-safe AND reduce memory usage.






            share|improve this answer













            A #define is a preprocessor macro. As Gerben says in his comment, it's just an automated find-and-replace.



            If you use it to hold things like C string constants, e.g. #define ERROR_STRING "You messed up, bud!" it could actually cause your program to take more memory, since that same string literal will be duplicated every time you reference it. In that case a static const would be both type-safe AND reduce memory usage.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 4 hours ago









            Duncan CDuncan C

            1,7091617




            1,7091617













            • Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

              – juhist
              1 hour ago






            • 1





              @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

              – Mooing Duck
              1 hour ago



















            • Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

              – juhist
              1 hour ago






            • 1





              @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

              – Mooing Duck
              1 hour ago

















            Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

            – juhist
            1 hour ago





            Would it be duplicated? I mean, isn't it the job of the compiler to ensure "You messed up, bud!" occurs exactly once in the string constant table?

            – juhist
            1 hour ago




            1




            1





            @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

            – Mooing Duck
            1 hour ago





            @juhist: C++ explicitly states that the compiler can choose to duplicate identical strings, or not. Most compilers have a flag to either do this, or not. I think, by default, most compilers do not merge strings, in case the app relies on them being unique.

            – Mooing Duck
            1 hour ago


















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