How to pass string argument to macro - c++

Question

I am trying to print IT SUCCESS\nET SUCCESS\n using following code but it's failing in compilation with error error: ‘printds’ was not declared in this scope which I know is because it's taking macro input as ds literal. Does anyone know how to do this? The use case is that there are several printXX() functions which should be called based on value passed in macro.
#include <stdio.h>
#define FOO(val) { \
print ## val(); \
}
void printIT() { printf("IT SUCCESS\n"); }
void printET() { printf("ET SUCCESS\n"); }
int main() {
const char* ds = "IT", es = "ET";
FOO(ds); FOO(es);
return 0;
}

Answer 1

You can change
FOO(ds); FOO(es);
to
FOO(IT); FOO(ET);
Because macro substitutions happen before your code is compiled.
But you can define a function called FOO like
#include <stdio.h>
#include <iostream>
using namespace std;
void printIT() { printf("IT SUCCESS\n"); }
void printET() { printf("ET SUCCESS\n"); }
void FOO(const string str)
{
if(str=="IT")
printIT();
else
printET();
}
int main()
{
const char* ds = "IT",*es="ET";
FOO(ds);FOO(es);
return 0;
}

Answer 2

it's taking macro input as ds literal.
Yes, that's to be expected. Preprocessor macros are expanded at compile time. The arguments to function-like macros are the literal source-code tokens that appear between the parentheses in the macro invocation. These have no additional meaning to the preprocessor.
Does anyone know how to do this? The use case is that there are several printXX() functions which should be called based on value passed in macro.
Again, macros are expanded, to source code (approximately), at compile time. The process does not and cannot take into account C++ runtime semantics such as converting variables' identifiers into corresponding values.
If runtime dynamic function dispatch based on variables' values is what you're after then you need an altogether different mechanism. You could use ordinary conditional statements to select between different function calls, for example. If you wanted to be even more dynamic then you could consider preparing a lookup table of function pointers, and using that to select and call appropriate functions.
In comments, you added
I've several methods queryServers, queryNodes, queryTargets which I want to call using above trick.
You may be able to accomplish something similar to what you ask via templates or overloaded functions. These mechanisms, too, operate at compile time, so they have no access to runtime information such as variables' values, but they do know about and rely upon C++ data types.
Alternatively, perhaps you're looking for the Strategy pattern.

Answer 3

The first thing you need to know is that the Macros are preprocessor directives which are a fragment of code with a given name. if you use macro name in your program it will replace that code fragment into that place you use the macro name at compile time first stage called Pre-processing stage.
#include <stdio.h>
#define FOO(val) { \
print ## val(); \
}
void printIT() { printf("IT SUCCESS\n"); }
void printET() { printf("ET SUCCESS\n"); }
int main() {
const char* ds = "IT", es = "ET";
FOO(ds); FOO(es);
return 0;
}
In your code, you try to input ds and es variables into FOO function like macro. But ds and es variables declared in program stack only when you run the program. At the compile-time, it just treats them as only texts. Therefore macro function input it takes as text ds and es and replaced with val. That's why you got the compile time error. The following code fragment I have changed is working as you expected.
#include <stdio.h>
#define FOO(val) { \
print ## val(); \
}
void printIT() { printf("IT SUCCESS\n"); }
void printET() { printf("ET SUCCESS\n"); }
int main() {
const char* ds = "IT", *es = "ET";
FOO(IT); FOO(ET);
return 0;
}
If you are interested you can find more about Macros in the following resources.
GCC online documentation and Article about Macros . Also, you can view the preprocessed code using g++ -E (your cpp file name). Thanks.

Answer 4

Possible solution without MACRO:
void printIT() { printf("IT SUCCESS\n"); }
void printET() { printf("ET SUCCESS\n"); }
void foo(std::string_view s)
{
static const std::map<std::string_view, void(*)()> funcs{
{"IT", &printIT},
{"ET", &printET}
};
auto it = funcs.find(s);
if (it != funcs.end()) {
(*it->second)();
}
}
int main() {
const char* ds = "IT";
const char* es = "ET";
foo(ds); foo(es);
}
Demo