I need to replace
GET("any_name")
with
String str_any_name = getFunction("any_name");
The hard part is how to trim off the quote marks. Possible? Any ideas?
How about:
#define UNSAFE_GET(X) String str_##X = getFunction(#X);
Or, to safe guard against nested macro issues:
#define STRINGIFY2(x) #x
#define STRINGIFY(x) STRINGIFY2(x)
#define PASTE2(a, b) a##b
#define PASTE(a, b) PASTE2(a, b)
#define SAFE_GET(X) String PASTE(str_, X) = getFunction(STRINGIFY(X));
Usage:
SAFE_GET(foo)
And this is what is compiled:
String str_foo = getFunction("foo");
Key points:
Use ## to combine macro parameters into a single token (token => variable name, etc)
And # to stringify a macro parameter (very useful when doing "reflection" in C/C++)
Use a prefix for your macros, since they are all in the same "namespace" and you don't want collisions with any other code. (I chose MLV based on your user name)
The wrapper macros help if you nest macros, i.e. call MLV_GET from another macro with other merged/stringized parameters (as per the comment below, thanks!).
One approach is not to quote the name when you call the macro:
#include <stdio.h>
#define GET( name ) \
int int##name = getFunction( #name ); \
int getFunction( char * name ) {
printf( "name is %s\n", name );
return 42;
}
int main() {
GET( foobar );
}
In answer to your question, no, you can't "strip off" the quotes in C++. But as other answers demonstrate, you can "add them on." Since you will always be working with a string literal anyway (right?), you should be able to switch to the new method.
Related
I have the following defines:
#define STRING_OBJECT_1 "bird"
#define STRING_OBJECT_2 "dog"
#define STRING_OBJECT_3 "cat"
#define STRING_OBJECT_4 "human"
#define STRING_OBJECT_5 "cow"
#define STRING_OBJECT_6 "snake"
#define STRING_OBJECT_7 "penguin"
#define STRING_OBJECT_8 "monkey"
I want to get numbered STRING_OBJECT only using STRING_OBJECT_ + "(number string)", so basically not directly type STRING_OBJECT_1.
Is there anyway to use for loop with string macro in C++?
Is there anyway to use for loop with string macro in C++?
No, there isn't.
Macros are processed before source code is compiled to create object code.
The values of variables in a for loop are set at run time. Hence, they cannot make use of macros.
Your best bet is to augment your code with an array variable and use the array variable in the for loop.
#define STRING_OBJECT_1 "bird"
...
#define STRING_OBJECT_8 "monkey"
std::string object_array[] = {STRING_OBJECT_1, ..., STRING_OBJECT_8};
for ( int i = 0; ... )
{
do_something(object_array[i]);
}
No, You can't do this. macros not part of the C/C++ language.
Macros are replaced by the preprocessor by their value compiles time. There is no way you had be able to change the macro at runtime.
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 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.
I have a macro defined in C something like this:
#define SOME_FIELD(_A_,_B_,_C_) \
MyObj[ ## _A_ ## ].somePTR = \
(DWORD_PTR) (buff_ ## _C_ ## _C_ ## _ ## _B_ ## );
What i can understand that for index A we are getting some value for "somePTR". My question is, What is ## <name> ## notation for and with this how value of somePTR is calculated??
I am new to such a macro so a descriptive explanation would be very helpful.
That is called token concatenation. It allows you to glue arguments together.
For your example, SOME_FIELD(Param1,Param2,Param3); expands like this:
MyObj[Param1].somePTR = (DWORD_PTR) (buff_Param3Param3_Param2);
It's easy enough to try this out yourself by using your compiler's pre-processor. You don't generally need to go to the trouble of writing a fully-fledged C program—the pre-processor can generally be invoked by itself.
That's preprocessor token pasting
http://msdn.microsoft.com/en-us/library/09dwwt6y(v=vs.80).aspx
It will copy the actual parameter token as a string literal, so read it like
// preprocessor_token_pasting.cpp
#include <stdio.h>
#define paster( n ) printf_s( "token" #n " = %d", token##n )
int token9 = 9;
int main()
{
paster(9);
}
the ## is concatenation primitive, it's used to create nwe symbols.
It's useful to create names in macro:
#define GENERIC_GETTER(f,g) (g->member_ ## f )
GENERIC_GETTER(a,b) will create (b->member_a) (new symbol created). If you don't use sharp-sharp, it would create (b->member_ a) (not glued together)
Normally, the ## operator concatenates two tokens: it requires a legal
token on the left and a legal token on the right, and results in a new
token. In your case, the first line in the macro
(MyObj[ ## _A_ ## ].somePtr = \) is
illegal, and results in undefined behavior. Most implementations just
concatenate the strings, then retokenize once they've finished all of
the substitutions, so it will work, but it's not guaranteed. And as far
as I can tell here, it's not necessary. In the second line, on the
other hand, you are generating a new token. If you invoke the macro:
SOME_FIELD(x,y,z);
it will expand to:
MyObj[x].somePtr = (DWORD_PTR)(buff_zzy);
(I might add that the use of symbols like _A_, _B_ and _C_ is also
undefined behavior. A symbol starting with an underscore followed by a
capital letter is in the namespace of the implementation.)
For a lot of function calls in a C app that needs some degree of debugging I wanted to add a macro to ease the typing that I had to do.
right now I am calling a function like this:
aDebugFunction(&ptrToFunction, __LINE__, "ptrToFunction", param1, param2, etc)
So I thought lets write a macro that does the first 3 parameters for me, like this:
#define SOMEDEFINE(x) &x, __LINE__, "x"
However, as most of you will immediately know, this won't work it won't replace "x" with the name that x has been given but will just pass "x" as 3rd parameter.
My knowledge of this preprocessor macro happening stuff is quite limited and thus my googling-ability is also quite useless due to not knowing where to search for exactly.
I hope one of you guys/girls could give me either a solution or point me in the right direction.
You need to use the # convert token to string command of the preprocessor. You should define your second macro this way:
#define SOMEDEFINE(x) &x, __LINE__, # x
Or if x can also be a macro call, and you want the string to contains the expansion of the macro, you need to use an auxiliary macro:
#define TOKEN_TO_STRING(TOK) # TOK
#define STRINGIZE_TOKEN(TOK) TOKEN_TO_STRING(TOK)
#define SOMEDEFINE(x) &x, __LINE__, STRINGIZE_TOKEN(x)
For example, if you have the following code:
#define SHORT_NAME a_very_very_very_long_variable_name
SOMEDEFINE(SHORT_NAME)
Then, with the first macro, it will expand to
&a_very_very_very_long_variable_name, __LINE__, "SHORT_NAME"
While, with the second macro, it will expand to:
&a_very_very_very_long_variable_name, __LINE__, "a_very_very_very_long_variable_name"
You can actually do a lot better than that: #define SOMEDEFINE( X, ... ) aDebugFunction( &(X), __LINE__, #X, __VA_ARGS__ )
You could then simply call into this code like this: SOMEDEFINE( ptrToFunction, param1, param2, etc )
And that would effectively call: aDebugFunction( &( ptrToFunction ), __LINE__, "ptrToFunction", param1, param2, etc )