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To add - the preprocessor is a pretty dated thing that merely uses text substitution, so you have no easy way to debug the code that the preprocessor passes to
by tediousdemise 6y ago
To add - the preprocessor is a pretty dated thing that merely uses text substitution, so you have no easy way to debug the code that the preprocessor passes to the compiler. Defines such as function-like macros throw away type safety and can have extremely unexpected behavior. For example, you might think this code will do what you want.
#define SQ(x) x*x
cout << SQ(2) // prints 4 since 2*2 = 4
cout << SQ(1+3) // prints 7 since 1+3*1+3 = 7
Macros are pretty much entirely unnecessary and should be avoided. Compilers inline nowadays, so you don’t really gain anything from using a macro when you could have just made a function that gives you type safety and intuitive behavior. If you are trying to define constants, use const, enum, or enum classes. If you are trying to define multiple versions of the same function, use polymorphism or templates. If you are trying to wrap around arbitrary code, use lambdas.
- monocasa 6y agoI agree that you shouldn't be doing this stuff in production if you don't know the ends and outs of doing this idiomatically. For instance, the lack of parenthesis ala #define SQ(x) ((x)*(x)) set off my C spidey senses. I use them even for constants in pound defines just to be consistent. That being said, there are benefits, like the type safety listed being one of them. No weird cast to void* and go to town on raw memory shenanigans you would have to do to use functions in intrusive collections in C.
- mgerdts 6y agoYou mean: #define SQ(x) ((x)*(x))
- deleted 6y ago[deleted]
- monocasa 6y agoYeah, thanks. That's what I get for trying to type code on my phone.
- throwaway3neu94 6y agoThere's another pitfall: it evaluates a parameter twice. Imagine calling SQ(func_with_side_effects()). If you can assume a compiler with statement expressions and typeof (gcc, clang, probably icc in this case): #define SQ(x) ({ typeof(x) x_ = (x); x_ * x_ }) If not (MSVC++) then best of luck. Anyway I 100% agree people way, way overuse macros. Very often a static inline function is a better trade-off.