Objective-C 中的#define vs const

#define vs const in Objective-C(Objective-C 中的#define vs const)

问题描述

I'm new to Objective-C, and I have a few questions regarding const and the preprocessing directive #define.

First, I found that it's not possible to define the type of the constant using #define. Why is that?

Second, are there any advantages to use one of them over the another one?

Finally, which way is more efficient and/or more secure?

First, I found that its not possible to define the type of the constant using #define, why is that?

Why is what? It's not true:

#define MY_INT_CONSTANT ((int) 12345)

Second, are there any advantages to use one of them over the another one?

Yes. #define defines a macro which is replaced even before compilation starts. const merely modifies a variable so that the compiler will flag an error if you try to change it. There are contexts in which you can use a #define but you can't use a const (although I'm struggling to find one using the latest clang). In theory, a const takes up space in the executable and requires a reference to memory, but in practice this is insignificant and may be optimised away by the compiler.

consts are much more compiler and debugger friendly than #defines. In most cases, this is the overriding point you should consider when making a decision on which one to use.

Just thought of a context in which you can use #define but not const. If you have a constant that you want to use in lots of .c files, with a #define you just stick it in a header. With a const you have to have a definition in a C file and

// in a C file
const int MY_INT_CONST = 12345;

// in a header
extern const int MY_INT_CONST;

in a header. MY_INT_CONST can't be used as the size of a static or global scope array in any C file except the one it is defined in.

However, for integer constants you can use an enum. In fact that is what Apple does almost invariably. This has all the advantages of both #defines and consts but only works for integer constants.

// In a header
enum
{
    MY_INT_CONST = 12345,
};

Finally, which way is more efficient and/or more secure?

#define is more efficient in theory although, as I said, modern compilers probably ensure there is little difference. #define is more secure in that it is always a compiler error to try to assign to it

#define FOO 5

// ....

FOO = 6;   // Always a syntax error

consts can be tricked into being assigned to although the compiler might issue warnings:

const int FOO = 5;

// ...

(int) FOO = 6;     // Can make this compile

Depending on the platform, the assignment might still fail at run time if the constant is placed in a read only segment and it's officially undefined behaviour according to the C standard.

Personally, for integer constants, I always use enums for constants of other types, I use const unless I have a very good reason not to.

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