2009年10月28日 · #define is a compiler pre processor directive and should be used as such, for conditional compilation etc.. E.g. where low level code needs to define some possible alternative data structures for portability to specif hardware. It can produce inconsistent results depending on the order your modules are compiled and linked.

2015年11月27日 · In the normal C or C++ build process the first thing that happens is that the PreProcessor runs, the preprocessor looks though the source files for preprocessor directives like #define or #include and then performs simple operations with them. in the case of a #define directive the preprocessor does simple text based substitution.

#define will declare HEADERFILE_H once #ifndef generates true. #endif is to know the scope of #ifndef i.e end of #ifndef. If it is not declared, which means #ifndef generates true, then only the part between #ifndef and #endif is executed, otherwise not. This will prevent from again declaring the identifiers, enums, structure, etc...

2011年5月15日 · Note that #define fori(x) for (int i=0; i<=x; i++) should be written #define fori(x) for (int i=0; i<=(x); i++) instead. (Note the additional brackets.) (Note the additional brackets.) The use of brackets may not be obvious in this example, but something wrong will happen if someone put a bitwise operation in argument x.

2012年9月18日 · #include <math.h> #undef OVERFLOW #undef UNDERFLOW #define MATH_H_OVERFLOW _OVERFLOW #define MATH_H_UNDERFLOW _UNDERFLOW You'd just have to ensure that all source code (already-compiled code like libraries won't matter) that wants to use the math.h ones, uses the MATH_H_* constants instead of …

2021年2月12日 · Statements defined using #define are called macros. And macros are used in a multitude of uses. We can use them to conditionally compile sections of code. #ifdef ONE int AddOne(int x) { return x + 1; } #else int AddTwo(int x) { return x + 2; } #endif When we don't need to store constants in a variable. #define MAX_BOUND 1000 #define MIN_BOUND 10

That file can then #ifdef _PASS2/#else to define macros for all the variables that should be different on the two passes. Even though the code gets generated twice, on some micros that will take less space than using the arrow operator with passed-in pointers.

Without boost : define same macro again and compiler HIMSELF will give warning. From warning you can see location of the previous definition.

2018年7月9日 · #define is part of something called the "preprocessor." Essentially, this is the code that is processed before the C document is compiled. Most of the preprocessor code is in a file with a ".h" extension (which is why you may have seen that when importing libraries). The preprocessor language is primitive.

2011年7月12日 · One does not define arrays, or any other thing. You can, however, create multidimensional sequences, as the answers here show. Remember that python variables are untyped, but values are strongly typed. –