LPCOpen: #define vs const usage

[karthikvenkates]

Hi,

In LPCOpen for LPC812, I could find two conventions being following while returning value from a function.

For example, Chip_Clock_GetIntOscRate() function returns "SYSCTL_IRC_FREQ" which is defined as 12000000 using #define as shown below:

But in function Chip_Clock_GetMainOscRate(), "OscRateIn" is returned where OscRateInn is a const variable with value 12000000, as shown below.

I understand the difference between #define and const.

#define - does not allocate memory. Just a text replacment. Cannot be passed as pointer in the function argument.

const - allocates dedicated memory in Flash and the value cannot be changed. Address of this can be used as argument of a function.

But my question is, why cant NXP could have simple used #define OscRateIn  (12000000) instead of const uint32_t OscRateIn = 12000000 ?

[marcprager]

Interestingly, nowhere you found the use of enum - which is the only C-language approach without 'namespace' conflict potential or memory wasting potential  .-)

Yes, if we have a good, optimizing compiler/linker, then the result is the same, for all approaches.

If someone offers me code with more than a handful of #defines I dismiss both the code and the person as a programmer.

[kerryzhou]

Hello Marc Prager,

   Some description from the website:

#define is a preprocessor directive. Things defined by #define are replaced by the preprocessor before compilation begins.

const variables are actual variables like other normal variable.

The big advantage of const over #define is type checking. We can also have pointers to const variables, we can pass them around, typecast them and any other thing that can be done with a normal variable. One disadvantage that one could think of is extra space for variable which is immaterial due to optimizations done by compilers.

In general const is a better option if we have a choice. There are situations when #define cannot be replaced by const. For example, #define can take parameters (See this for example). #define can also be used to replace some text in a program with another text.

A #define is used as immediate constant or as a macro. Where as the constant is a variable whose value can not change.

Pointer can be declared to a constant, but not for #define.

#define can not define externally, and there is no way to use it for availability to the linker. Where as the constant can be global .

Wish it helps you!

Have a great day,
Kerry

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[kerryzhou]

Hello karthik venkatesh ,

    I think it is just the code write habit question.

    Maybe the lpcopen code is not finished by one engineer, some people like to use the define, some like constant.

    But they work for different file, so there has inconsistent problem.

    But I think this is not the problem, in the practical usage, you can use the method which you like.

Have a great day,
Kerry

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