C ++预处理器连接操作

Question

I would like to use the C++ preprocessor concatenator.

#include <iostream>
#include <string>

#define GetBlack(colorName) (color.##colorName)

struct Color
{
    int black = 0;
};

int main()
{
  Color color;
  int c = color.black;
  int d = GetBlack(black); 
}

The error I get is

5:35: error: pasting "." and "black" does not give a valid preprocessing token
16:11: note: in expansion of macro 'GetBlack'

C++Shell:
http://cpp.sh/3547x

Any tips?

Answer 1

You don't actually want concatenation here. You are just pasting a preprocessing "token" into place, so just use:

#define GetBlack(colorName) (color.colorName)

The macro argument name colorName is expanded in place. Then

int d = GetBlack(black);

will expand into

int d = color.black;

Concatenation is for merging two "tokens" together into one, see here .

For instance, if the member was named black_value , but you just wanted to have to call GetBlack(black) , the macro definition would be something like:

#define GetBlack(colourName) colour.colourName##_value;