C++ macro '##' doesn't work after '->' operator

Question

I have a shared_ptr object x, which has get and set methods as follows:

x->a_value(); x->set_a_value(); x->b_value(); x->set_b_value(); 

When i try to define a macro:

#define MAC(type) \   x->set_##type##_value(val);  MAC(a) 

It works fine, but when I do:

#define MAC(type) \   x->##type##_value();  MAC(a) 

it gives the following compile error: pasting formed '->a', an invalid preprocessing token

Answer 1

The preprocessor works on "tokens" - likes names and operators.

The ## operator creates a new token by pasting smaller parts together. In the first example set_##type##_value becomes set_a_value, which is a valid token.

In the second example ->##type##_value would become ->a_value, which is not a valid preprocessor token. It ought to be two tokens.

If you just make the line x->type##_value(); it should work. You get the separate tokens x, ->, a_value, (, ), and ;.

Answer 2

What it says on the tin: ->a is not a single, valid preprocessor token: it's two tokens. You do not need to paste here.

#define MAC(type) \   x->type##_value();