I see this interesting question here asking for the possibility of a program without main(). There, I saw eon giving one answer as follows, which works well in C/C++.
#include<stdio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf(" hello ");
}
Can someone explain how the above code works? Isn't really there a main() here or just hiding it from our eyes archly?
Have a close look at the macro: it just spliced together main from the characters in animate and gets them replaced for begin().
After the macro substitution:
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
begin becomes decode(a,n,i,m,a,t,e), which then becomes main. There you have it.
The compiler doesn't see "begin" whatsoever. It's completely substituted by the time it gets to the compiler, because macros are simply text-substitutions. Perhaps a helpful diagram to add on to the other great answers.
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
Take a look at m, and see where m is in the argument list.
decode(s,t,u,m,p,e,d)
^
|
decode(a,n,i,m,a,t,e)
Therefore the first letter is m => m. Then repeat the process, s => a, u => i, t => n.
decode(s,t,u,m,p,e,d)
^ ^ ^
| | |
decode(a,n,i,m,a,t,e)
Then the resulting letters are "pasted" together with token concatenation, and it looks like main as far as the compiler is concerned.
This works because the compiler doesn't really see int begin(){}, The preprocessor will replace begin with main after performing macro substitution and concatenation operator.
First The preprocessor will replace begin with decode(a,n,i,m,a,t,e), after that it will do a rescanning on the replacement list for further replacement, it will find function-like macro decode which will be replaced by it's replacement list after performing the concatenation operator, like this:
m##a##i##n => main
So the compiler will only see the preprocessor output which contain int main(){} and not int begin(){} thus legal code.
Related
I have defined the following macros:
#define START(x) A("start")-
#define ONE(x) -A(#x)
#define END -A("end");
and in my simple main i have:
(START ONE(one)) ONE(two) END
where A("") is a call to constructor of class A. I have made all the operators overload that are necessary, binary and unary minus, yet the compiler outputs that i miss a parenthesis.
What i would like to ask is how exactly the preprocessor replaces my macros into my code because when i run:
(A("start") - - A("one")) - A("two") - A("end");
it compiles without a problem.
(START ONE("one")) ONE("two") END
This will be expanded to this:
(START -A("\"one\"")) -A("\"two\"") -A("end");
The differences between the real result and your expectation is that START is not expanded and "one" and "two" have their quotes escaped.
To get what you want, START should not take arguments and you should not use the stringify operator in ONE:
#define START A("start")-
#define ONE(x) -A(x)
#define END -A("end");
So, it's been a while since I have written anything in C++ and now I'm working on a project using C++11 and macros.
I know that by using the stringify operator I can do this:
#define TEXT(a) #a //expands to "a"
How am I supposed to use the preprocessor for recognizing the tokens like + and * to do this:
#define TEXT(a)+ ??? //want to expand to "a+"
#define TEXT(a)* ??? //want to expand to "a*"
when the input has to be in that syntax?
I have tried doing that:
#define + "+"
but of course it doesn't work. How can I make the preprocessor recognize those tokens?
NOTE:
This is actually part of a project for a small language that defines and uses regular expressions, where the resulting string of the macros is to be used in a regex. The syntax is given and we have to use it as it is without making any changes to it.
eg
TEXT(a)+ is to be used to make the regular expression: std::regex("a+")
without changing the fact that TEXT(a) expands to "a"
First,
#define TEXT(a) #a
doesn't “convert to "a"”. a is just a name for a parameter. The macro expands to a string that contains whatever TEXT was called with. So TEXT(42 + rand()) will expand to "42 + rand()". Note that, if you pass a macro as parameter, the macro will not be expanded. TEXT(EXIT_SUCCESS) will expand to "EXIT_SUCCESS", not "0". If you want full expansion, add an additional layer of indirection and pass the argument to TEXT to another macro TEXT_R that does the stringification.
#define TEXT_R(STUFF) # STUFF
#define TEXT(STUFF) TEXT_R(STUFF)
Second, I'm not quite sure what you mean with TEXT(a)+ and TEXT(a)*. Do you want, say, TEXT(foo) to expand to "foo+"? I think the simplest solution in this case would be to use the implicit string literal concatenation.
#define TEXT_PLUS(STUFF) # STUFF "+"
#define TEXT_STAR(STUFF) # STUFF "*"
Or, if you want full expansion.
#define TEXT_R(STUFF) # STUFF
#define TEXT_PLUS(STUFF) TEXT_R(STUFF+)
#define TEXT_STAR(STUFF) TEXT_R(STUFF*)
Your assignment is impossible to solve in C++. You either misunderstood something or there’s an error in the project specification. At any rate, we’ve got a problem here:
TEXT(a)+ is to be used to make the regular expression: std::regex("a+") without changing the fact that TEXT(a) expands to "a" [my emphasis]
TEXT(a) expands to "a" — meaning, we can just replace TEXT(a) everywhere in your example; after all, that’s exactly what the preprocessor does. In other words, you want the compiler to transform this C++ code
"a"+
into
std::regex("a+")
And that’s simply impossible, because the C++ preprocess does not allow expanding the + token.
The best we can do in C++ is use operator overloading to generate the desired code. However, there are two obstacles:
You can only overload operators on custom types, and "a" isn’t a custom type; its type is char const[2] (why 2? Null termination!).
Postfix-+ is not a valid C++ operator and cannot be overloaded.
If your assignment had just been a little different, it would work. In fact, if your assignment had said that TEXT(a)++ should produce the desired result, and that you are allowed to change the definition of TEXT to output something other than "a", then we’d be in business:
#include <string>
#include <regex>
#define TEXT(a) my_regex_token(#a)
struct my_regex_token {
std::string value;
my_regex_token(std::string value) : value{value} {}
// Implicit conversion to `std::regex` — to be handled with care.
operator std::regex() const {
return std::regex{value};
}
// Operators
my_regex_token operator ++(int) const {
return my_regex_token{value + "+"};
}
// more operators …
};
int main() {
std::regex x = TEXT(a)++;
}
You don't want to jab characters onto the end of macros.
Maybe you simply want something like this:
#define TEXT(a, b) #a #b
that way TEXT(a, +) gets expanded to "a" "+" and TEXT(a, *) to "a" "*"
If you need that exact syntax, then use a helper macro, like:
#define TEXT(a) #a
#define ADDTEXT(x, y) TEXT(x ## y)
that way, ADDTEXT(a, +) gets expanded to "a+" and ADDTEXT(a, *) gets expanded to "a*"
You can do it this way too:
#define TEXT(a) "+" // "a" "+" -> "a+"
#define TEXT(a) "*" // "a" "*" -> "a*"
Two string literals in C/C++ will be joined into single literal by specification.
I would like to define an expression in C++ with macros and I am having quite a bit of trouble.
The expression is :
MATCH string WITH other_string
where string and other_string do not require " "
For example: MATCH r1 WITH string1 is the result i desire.
The purpose of this macro would be to check if r1 string matches with r2.
(I already have the code for the matching)
UPDATE
I would like to call MATCH hello WITH hi
in my main function
int main(){
MATCH hello WITH hi
}
and call my function from this macro to compare them. **Both hello and hi are unquoted arguments and must be treated as variable names
It is always dubious to use macros to make your code look like a different language. It is probably better to consider using a separate parser for your "meta-language" that generates the C++ code for you.
In this case, since C++ syntax requires some way to indicate the end of a statement (close braces or semi-colon) you are in kind of a jam.
Consider your example:
int main () { MATCH hello WITH hi }
Since hi is the last token before the end of main, there is no chance to fix-up the syntax to match C++ requirements.
You can't do what you want, so you have to do something different
If you really intend to embed this syntax into your C++ code, you need sentinel tokens to allow you to fix-up the syntax. My proposed syntax is:
int main () {
BEGIN_MATCHING
MATCH hello WITH hi
MATCH hello WITH hi
END_MATCHING
};
If this syntax is acceptable, then you can use the following macros.
#define BEGIN_MATCHING ((void)0
#define MATCH ); my_function(
#define WITH ,
#define END_MATCHING );
This will cause the code in the proposed syntax example to expand to:
int main () {
((void)0
); my_function( hello , hi
); my_function( hello , hi
);
}
Live Demo
Simply stringify your arguments with #, something like:
#define MATCH_WITH(str1, str2) MATCH #str1 WITH #str2
That way:
MATCH_WITH(testing, testing)
becomes:
MATCH "testing" WITH "testing"
The below code should output 100 to my knowledge of stringification. vstr(s) should be expanded with value of 100 then str(s) gets 100 and it should return the string "100". But, it outputs "a" instead. What is the reason? But, if I call with macro defined constant foo then it output "100". Why?
#include<stdio.h>
#define vstr(s) str(s)
#define str(s) #s
#define foo 100
int main()
{
int a = 100;
puts(vstr(a));
puts(vstr(foo));
return 0;
}
The reason is that preprocessors operate on tokens passed into them, not on values associated with those tokens.
#include <stdio.h>
#define vstr(s) str(s)
#define str(s) #s
int main()
{
puts(vstr(10+10));
return 0;
}
Outputs:
10+10
The # stringizing operator is part of the preprocessor. It's evaluated at compile time. It can't get the value of a variable at execution time, then somehow magically convert that to something it could have known at compile time.
If you want to convert an execution-time variable into a string at execution time, you need to use a function like std::to_string.
Since vstr is preprocessed, the line
puts(vstr(a));
is translated as:
puts("a");
The value of the variable a plays no role in that line. You can remove the line
int a = 100;
and the program will behave identically.
Stringification is the process of transforming something into a string. What your macro stringifies ?
Actually the name of the variable itself, this is done at compilation-time.
If you want to stringify and then print the value of the variable at execution-time, then you must used something like printf("%\n",v); in C or cout << v << endl; in C++.
A preprocessor macro is not the same thing as a function, it does not expand the arguments at runtime and sees the value, but rather processes it at preprocessing stage (which is before compilation, so it doesn't even know the variables dependency).
In this case, you've passed the macro a to stringify, which it did. The preprocessor doesn't care a is also the name of a variable.
I am trying to do something like:
custommacro x;
which would expand into:
declareSomething; int x; declareOtherthing;
Is this even possible?
I already tricked it once with operator= to behave like that, but it can't be done with declarations.
You can elide the parentheses as long as you are willing to accept two additions:
the whole code needs to be wrapped in a block macro
there needs to be something following the echo directive
e.g. thusly:
#define LPAREN (
#define echo ECHO_MACRO LPAREN
#define done )
#define ECHO_MACRO(X) std::cout << (X) << "\n"
#define DSL(X) X
...
DSL(
echo "Look ma, no brains!" done;
)
...
Reasons for this:
There is no way to make a function-like macro expand without parentheses. This is just a basic requirement of the macro language; if you want something else investigate a different macro processor
Therefore, we need to insert the parentheses; in turn we need to have something after the directive, like a done macro, that will expand to a form containinf the necessary close paren
Unfortunately, because the echo ... done form didn't look like a macro invocation to the preprocessor, it wasn't marked for expansion when the preprocessor entered it, and whether we put parens in or not is irrelevant. Just using echo ... done will therefore dump an ECHO_MACRO call in the text
Text is re-scanned, marked for expansion, and expanded again when it is the argument to a function-like macro, so wrapping the entire block with a block macro (here it's DSL) will cause the call to ECHO_MACRO to be expanded on this rescan pass (DSL doesn't do anything with the result: it exists just to force the rescan)
We need to hide the ( in the expansion of echo behind the simple macro LPAREN, because otherwise the unmatched parenthesis in the macro body will confuse the preprocessor
If you wanted to create an entire domain-specific language for such commands, you could also reduce the number of done commands by making the core commands even more unwieldy:
#define LPAREN (
#define begin NO_OP LPAREN 0
#define done );
#define echo ); ECHO_MACRO LPAREN
#define write ); WRITE_MACRO LPAREN
#define add ); ADD_MACRO LPAREN
#define sub ); SUB_MACRO LPAREN
#define NO_OP(X)
#define ECHO_MACRO(X) std::cout << (X) << "\n"
#define WRITE_MACRO(X) std::cout << (X)
#define ADD_MACRO(D, L, R) (D) = (L) + (R)
#define SUB_MACRO(D, L, R) (D) = (L) - (R)
#define DSL(X) DSL_2 X
#define DSL_2(X) X
int main(void) {
int a, b;
DSL((
begin
add a, 42, 47
sub b, 64, 50
write "a is: "
echo a
write "b is: "
echo b
done
))
return 0;
}
In this form, each command is pre-designed to close the preceding command, so that only the last one needs a done; you need a begin line so that there's an open command for the first real operation to close, otherwise the parens will mismatch.
Messing about like this is much easier in C than in C++, as C's preprocessor is more powerful (it supports __VA_ARGS__ which are pretty much essential for complicated macro metaprogramming).
Oh yeah, and one other thing -
...please never do this in real code.
I understand what you're trying to do and it simply can't be done. A macro is only text replacement, it has no knowledge of what comes after it, so trying to do custommacro x will expand to whatever custommacro is, a space, and then x, which just won't work semantically.
Also, about your echo hack: this is actually very simple with the use of operators in C++:
#include <iostream>
#define echo std::cout <<
int main()
{
echo "Hello World!";
}
But you really shouldn't be writing code like this (that is, using macros and a psuedo-echo hack). You should write code that conforms to the syntax of the language and the semantics of what you're trying to do. If you want to write to standard output use std::cout. Moreover, if you want to use echo, make a function called echo that invokes std::cout internally, but don't hack the features of the language to create your own.
You could use for-loop and GnuC statement expression extension.
#define MY_MACRO\
FOR_MACRO(_uniq##__COUNTER__##name,{/*declareSomething*/ },{ /* declareOtherthing */ }) int
#define FOR_MACRO(NAME,FST_BLOCK,SND_BLOCK)\
for(int NAME = ({FST_BLOCK ;0;}); NAME<1 ; NAME++,(SND_BLOCK))
It's "practically hygienic", though this means that whatever you do inside those code blocks wont escape the for-loop scope.