CODEWITHSUNDEEP
cmacros
Yuval Levy
Yuval Levy

Reputation: 2516

Macro with new arguments

I wrote this macro for searching number in array:

#define FIND(VALUE,ARR,INDEX){\
    int i;\
    INDEX = -1;\
    for(i = 0;i <sizeof(ARR)/sizeof(VALUE); i++){\
        if(ARR[i] == VALUE){\
            INDEX = i;\
        }\
    }\
\
}

My questions is, will it be a problem if I define int i in the macro and also in the main?

Upvotes: 0

Views: 64

Answers (5)

cmaster - reinstate monica
cmaster - reinstate monica

Reputation: 40625

Of course, the i is only a problem, if you get i passed in to your macro as a parameter, because macros do pure text substitution.

Apart from that, there are several issues with your code:

  1. The idiomatic way to wrap a void-function like macro is to use a do { ... } while(0) loop. Only that swallows semicolons correctly.

  2. You have not wrapped all your parameters in parentheses as you should. If you pass 7 & foo as VALUE, the expression if(ARR[i] == VALUE){ will expand to if(ARR[i] == 7 & foo){, which won't do what you expect.

  3. Your macro parameters are evaluated several times, so your code will explode if you pass something like foo++ for any of them.

  4. The element count calculation sizeof(ARR)/sizeof(VALUE) is plain wrong. There is no guarantee that the type of VALUE is the same as the element type of the array. If you calculate an array element count in a macro, always used sizeof(ARR)/sizeof(*ARR).

Upvotes: 1

Balu
Balu

Reputation: 2447

It will not be a problem, even you redefine i in main.

#define FIND(VALUE,ARR,INDEX){\ int i;\ INDEX = -1;\ for(i = 0;i <sizeof(ARR)/sizeof(VALUE); i++){\ if(ARR[i] == VALUE){\ INDEX = i;\ }\ }\ \ } Since you defined a own scope '{' within the macro.

Upvotes: 0

herohuyongtao
herohuyongtao

Reputation: 50667

It's OK. The i in the macro will be only visible in the macro, and will also hide outer i within this macro scope.

Upvotes: 0

Gene
Gene

Reputation: 46960

No. The inner block will hide the outer. However there will be a problem if the INDEX passed in is i. This is why you frequently see weird variable names like _i_ used in macros. It's an attempt to avoid name clashes.

With modern compilers and hardware, there is little reason to fiddle with macros in his manner. Just use a function.

Upvotes: 2

alk
alk

Reputation: 70931

The i in the macro will hide any other i declared either in the surrounding context or globally. Whether one regards this as a problem or not depends on the need to be able to access the hidden i.

Upvotes: 4

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