Using C++ macros to construct a method name to call

Question

I have a class Foo from a third party library that has a lots of setters with a string argument named as set_parameter_name(std::string value). And I need to configure an instance of the class using a simple parser that returns a needed value of the required parameter like this:

std::string value = parser.parse("parameter_name");

So the code looks like:

Foo foo = Foo();
Parser parser = Parser();
foo.set_a(parser.parse("a"));
foo.set_b(parser.parse("b"));
// lots of strings
foo.set_z(parser.parse("z");

This code is pretty ugly so I'm trying to replace it with a C++ macro. The idea is using a list of names to generate code automatically by preprocessor but I cannot imagine how to do that.

If I deal with the parameters as strings, I cannot convert it to set_param_name in macro:

#define PARAM_LIST "a", "b", "c", "d" // lots of elements

#define PARAM_SETTER(foo, param, parser) ({ \
foo.set_#param(parser.parse(param)); \ // <- as I understand, it's impossible to create a method name from a string value.
})

Foo foo = Foo();
Parser parser = Parser();
for (auto param : std::string[]{PARAM_LIST}) PARAM_SETTER(foo, param, parser);

Alternatively, I can define the list as variables, but in this case I don't know how to save a variable name when it passed to macro:

#define PARAM_LIST a, b, c, d

#define PARAM_SETTER(foo, param, parser) ({ \
foo.set_#param(parser.parse("#param"); \
})

Foo foo = Foo();
Parser parser = Parser();
std::string PARAM_LIST;
for (auto param : std::string[]{PARAM_LIST}) PARAM_SETTER(foo, param, parser); // the problem is variable name to stringify is just 'param'

The 3rd method is possibly using VA_ARGS in macro but I don't know how.

a##b is token glue to make ab, and #c is string-ify to make a string "c", so: foo.set_##param(parser.parse(#param); — Eljay
– Eljay, Commented Sep 17 at 14:25
"pretty ugly so I'm trying to replace it with a C++ macro" - a macro usually makes thins more ugly and there's usually always a better way. Avoid macros unless they are the only viable option. — Jesper Juhl
– Jesper Juhl, Commented Sep 17 at 14:42
@Elijay In both cases #param in macro will be replaced with "param" because macro called using variable named "param" in a loop. And therefore set_##param will be replaced to set_param. — Grigory
– Grigory, Commented Sep 17 at 14:47
Besides of that you are going to create a debugging hell, I'd hardly believe your PARAM_SETTER is going to work like that in a loop. This means your loop is going to generate four different functions in one single instruction. — armagedescu
– armagedescu, Commented Sep 17 at 14:48

Jarod42 · Accepted Answer · 2025-09-17 15:04:16Z

10

With X MACRO,

#define LIST_OF_VARIABLES \
    X(a) \
    X(b) \
    X(c) \
    X(z) \

you might do:

#define X(var) foo.set_##var(parser.parse(#var));
LIST_OF_VARIABLES
#undef X

Demo

answered Sep 17 at 15:04

Jarod42

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1 Comment

Grigory Sep 17 at 15:21

Thanks, I think X macro is what I actually want.

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