带有 #define 的 C++ 类型别名

Question

I'm aware we can use typedef or using for making type aliases. However, what about using #define ? For example:

#define integer int

Should I ever use this instead of using or typedef ?

Answer 1

Should I ever use this instead of using or typedef?

Probably not. I do not know any case where a macro would be in any way better, but there are reasons why type aliases are objectively better. Most importantly, type aliases are enclosed by scopes.

Answer 2

There is no reason to use a macro when you can not use a macro. Macros are not aware of scopes or namespaces. Macros are replaced before actual compilation takes place, hence macros are not C++ entities. Consider for example:

#define integer int

struct Foo
{
    int integer;
};

GCC emits the following error message:

<source>:5:5: error: multiple types in one declaration
    5 |     int integer;
      |     ^~~
<source>:1:17: error: declaration does not declare anything [-fpermissive]
    1 | #define integer int
      |                 ^~~
<source>:5:9: note: in expansion of macro 'integer'
    5 |     int integer;
      |         ^~~~~~~

because of the macro, the message is unnecessarily verbose. While reading I have to follow the indirection via the macro to understand what is going on.

On the other hand, this:

using integer = int;

struct Foo
{
    int integer;
};

Compiles without problems. Foo has a member called integer . This is just one case out of millions where using a macro causes confusion.

See here for what makes the difference.