How are `#input` type macro strings allocated in C++?

Question

This is kind of a follow-up question to one of my old questions here. I found some code that goes like this:

#define TYPESUPPORT(T)              \
...                                 \
static const char* get_type_name()  \
{                                   \
    return #T;                      \
}                                   \
...

Where T is a variable type (e.g. int). #T is used to get the plaintext name of the macro input.

Is this safe to do? As in, how is #T allocated? Does it allocate a const char* or const char[] string literal, like in my first question, for each macro call or something?

The macro is irrelevant, the preprocessor converts it to a string literal and it's processed just as if you'd written return "int"; — Barmar
– Barmar, Commented Jan 4, 2018 at 20:14
@JesperJuhl when you have about 300 types you need this info (as well as 4 other functions) for, you use macros. — Zeph M
– Zeph M, Commented Jan 4, 2018 at 20:16
@JesperJuhl That is hardly a useful comment. Most metaprogramming stuff cannot be done without some macro infrastructure, for example. — Angew is no longer proud of SO
– Angew is no longer proud of SO, Commented Jan 4, 2018 at 20:16
@Angew To be fair, C++ provides lots of support for metaprogramming using templates and other introspection features, so there's less need for macros. — Barmar
– Barmar, Commented Jan 4, 2018 at 20:18
@JesperJuhl Macros are much less useful in C++ than in C. That doesn’t mean they are completely useless; they still are often the best tool for the job. Each new version of C++ makes that less common, but they are still necessary, and in particular they are necessary here without using compiler-specific functions like abi::__cxa_demangle. — Daniel H
– Daniel H, Commented Jan 4, 2018 at 20:19

Matteo Italia · Accepted Answer · 2018-01-04 20:21:46Z

7

Macros simply perform text-to-text translation. So after TYPESUPPORT(int) is expanded, the function will look like:

static const char* get_type_name()  
{                                   
    return "int";                      
}

And just as explained in the linked question, string literals have static storage duration, so there's no problem with returning this pointer. The fact that it came from expansion of #T is irrelevant at this phase of compilation.

edited Jan 4, 2018 at 20:21

Matteo Italia

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answered Jan 4, 2018 at 20:17

Barmar

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