C++ - Macro defines directive

At: http://www.learncpp.com/cpp-tutorial/110-a-first-look-at-the-preprocessor/

It mentions a directive called "Macro defines". What do we mean when we say "Macro"开发者_运维百科?

Thanks.

A macro is a preprocessor directive that defines a name that is to be replaced (or removed) by the preprocessor right before compilation.

Example:

#define MY_MACRO1 somevalue
#define MY_MACRO2
#define SUM(a, b) (a + b)

then if anywhere in the code (except in the string literals) there is a mention of MY_MACRO1 or MY_MACRO2 the preprocessor replaces this with whatever comes after the name in the #define line.

There can also be macros with parameters (like the SUM). In that case the preprocessor recognizes the arguments, example:

int x = 1, y = 2;
int z = SUM(x, y);

preprocessor replaces like this:

int x = 1, y = 2;
int z = (x + y);

only after this replacement the compiler gets to compile the resulting code.

A macro is a code fragment that gets substituted into your program by the preprocessor (before compilation proper begins). This may be a function-like block, or it may be a constant value.

A warning when using a function-like macro. Consider the following code:

#define foo(x) x*x

If you call foo(3), it will become (and be compiled as) 3*3 (=9). If, instead, you call foo(2+3), it will become 2+3*2+3, (=2+6+3=11), which is not what you want. Also, since the code is substituted in place, foo(bar++) becomes bar++ * bar++, incrementing bar twice.

Macros are powerful tools, but it can be easy to shoot yourself in the foot while trying to do something fancy with them.

"Macro defines" merely indicate how they are specified (with #define directives), while "Macro" is the function or expression that is defined.

There is little difference between them aside from semantics, however.