Now, if I write JACK_QUEEN_OTHER(USER), and USER is JACK, the preprocessor If your strings are compile time constants (as in your case) you can use the following trick: The compiler can tell the result of the strcmp in advance and will replace the strcmp with its result, thus giving you a #define that can be compared with preprocessor directives. How to check whether a string contains a substring in JavaScript? Are defenders behind an arrow slit attackable? We use it to define a name for particular value/constant/expression. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Way to compare strings in C preprocessor? Connect and share knowledge within a single location that is structured and easy to search. Just my 2 cents. Counterexamples to differentiation under integral sign, revisited. Penrose diagram of hypothetical astrophysical white hole. Allows what appears as symbolic macro value comparison. Penrose diagram of hypothetical astrophysical white hole. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Asking for help, clarification, or responding to other answers. This C tutorial explains how to use the #define preprocessor directive in the C language. However it does not work for me - prints out "Hello" when I use gcc -std=c99. Note that the commas and single quotation marks will remain visible, when printing macros like these in pragma message (I guess it might be possible to use variadic macros/__VA_ARGS__ to make a macro, that would loop through the character array values and concatenate them in a more readable manner, but the above technique is good enough for me). However, note that the expected behavior of printf is that it will print the entire string even if the string exceeds the width you specify in the format string. All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. . So, at first the solution sounds pretty simple, just define #define STR (tok) #tok and things will work. If x were a macro, it would be expanded in the Why do I need double layer of indirection for macros? character constants, in order to get a valid C string constant with the instead of the name of the macro. Yes. All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. Thus, stringizing p = "foo\n";results in One obvious way to do this is by using the C preprocessor to substitute definitions from C into the Haskell source, which we then compile as a normal Haskell source file. xstr, so it is completely macro-expanded before xstr Does balls to the wall mean full speed ahead or full speed ahead and nosedive? How do I tell if this single climbing rope is still safe for use? Find centralized, trusted content and collaborate around the technologies you use most. For token concatenation, see: https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html, For token string conversion, see: https://gcc.gnu.org/onlinedocs/cpp/Stringification.html#Stringification. https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html, https://gcc.gnu.org/onlinedocs/cpp/Stringification.html#Stringification. As already stated above, the ISO-C11 preprocessor does not support string comparison. The do and while (0) are a kludge to make it possible to Just wanted to mention that the same thing goes for regular code as well, not just preprocessors. with the literal text of the actual argument, converted to a string around the fragment. C++ Preprocessor - The Token Pasting (##) Operator This is probably the least understood and most poorly documented preprocessor operator. We'll refer to the C Preprocessor as CPP. Asking for help, clarification, or responding to other answers. One possible solution for your example might be: The '##' preprocessing operator performs token pasting. one or more preprocessing tokens where the beginning token is not a standard-defined . I do not want to manually search the string for keywords and then replace them, but instead want to use the preprocessor to just switch the words. Only the last character of the "strings" are compared. In the Preprocessor Definitions dialog box, add, modify, or delete one or more definitions, one per line. So all we have to do is pick it out. As soon as a newline character is found, the preprocessor directive is ends. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. In C# the #define preprocessor directive cannot be used to define constants in the way that is typically used in C and C++. The preprocessor fit for such extensions to those used in the replacement list of identifiers: each parameter from this list, which is not a string (#) sign (#@) or symbol operator (##) is preceded and no symbol operator followed by the replaced macro call variable names used. Thanks for contributing an answer to Stack Overflow! C (). I tried this out on GCC 4.9.1, & I don't believe this will do what you think it does. As some didn't like my earlier answer because it avoided the whole compile time string compare aspect of the OP by accomplishing the goal with no need for string compares, here is a more detailed answer. As a more concrete example, I originally wanted to do the string comparison so I could specify a default archive type when using the Cereal serialization library. For Example. There is no way to combine an argument with surrounding text and Constants are fields whose values are set at compile time and can never be changed. And the "VariableString" class that must inherit from the "Variable" class. Does a 120cc engine burn 120cc of fuel a minute? How can I do this? How do I iterate over the words of a string? String. -- Is this really a constexpr? 1 2 3 #define THE_PATH "/path/to/somthing" // later on down I do this some_function (THE_PATH "/image.png"); This is a decent sized app and THE_PATH is something that I dont want to hardcode like this "/path/to/something/image.png" For example, this code should produce the version_string that you're seeking: They are intended as a simple and portable way to detect the presence of said features. Pulled the following from boost, example using windows msvc which is using an additional level of indirection than what I have seen in most places. But, as @Artyer points out, the version involving c_strcmp will NOT work in ANY modern compiler. So in rethinking it for my situation, I realized that in reality there would only be a fixed set of strings that you would want to/could compare against, as the preprocessor would have to use static strings anyway. (since C++20) Example Run this code Stringizing in C involves more than putting double-quote characters however it seems doable, if the string value is written as an array of characters, both for the "variable"/tested macro, and for the macros that represent values to be tested against. The C compiler will see the same tokens as it would if you had written foo = (char *) malloc (1024); By convention, macro names are written in upper case. Then you can add an #else clause to the end of your #elsif list to catch cases where USER is accidentally set to something you don't know how to handle. When we try to compile a program, preprocessor commands are executed first and then the program gets compiled. Not the answer you're looking for? Does Python have a string 'contains' substring method? It is called a preprocessor because it happens before the compilation process. the closest I just got was by doing this: But the ICON_FILE macro expands to "../icons/" "22" ".ico", which is valid syntax for C (adjacent string literals are concatenated), but probably not for a .rc file, which explains the "can't open icon file" message you're getting. However, backslashes that are not inside string What is the difference between String and string in C#? Select the Configuration Properties > C/C++ > Preprocessor property page. 1980s short story - disease of self absorption. Likewise, if you have macros for A and B, then #A #B would concatenate them. C ,c,macros,c-preprocessor,variadic,variadic-macros,C,Macros,C Preprocessor,Variadic,Variadic Macros,printf #define log(fmt_string, .) For a long time, CMake had the add_definitions command for this purpose. section 5.19 Constant expressions that says this though: []>[ Note: Constant expressions can be evaluated during Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. MSVC Update: You have to parenthesize slightly differently to make things also work in MSVC. Then you rely upon another feature of the preprocessor, namely that adjacent literal strings are merged into a single string. You can use the #define directive to define a string constant. The following worked for me with clang. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Strings have much more overhead than integers or enums and if you don't need to do anything more than compare them, then strings are the wrong solution. You're most likely used to using the preprocessor to include files directly into other files, or #define constants, but the preprocessor can also be used to create "inlined" code using macros expanded at compile time and to prevent code from being compiled twice. Why are these constructs using pre and post-increment undefined behavior? When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? All leading and trailing whitespace in text being stringized is However, you could do it with m4 pre-processor. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. rev2022.12.9.43105. Not sure if it was just me or something she sent to the whole team. The following sample generates CS1032: // CS1032.cs namespace x { public class clx { #define a // CS1032, put before namespace public static void Main() { } } } The preprocessor backslash-escapes the quotes surrounding embedded string constants, and all backslashes within string and character constants, in order to get a valid C string constant with the proper contents. I have to do something like this in C. It works only if I use a char, but I need a string. How does the Chameleon's Arcane/Divine focus interact with magic item crafting? This is the most complicated of the preprocessor directives (a little over 7 pages is devoted to it in the C99 spec and the C++98 spec devotes about 4 pages to it). Finally to convert the constants JACK or QUEEN to a string, use the stringize (and/or tokenize) operators. Why does the distance from light to subject affect exposure (inverse square law) while from subject to lens does not? Use constants to provide meaningful names instead of numeric literals ("magic numbers") for special values. So you must either use a function like printf or a variable rather than the preprocessor, or pull the token out of the string like so: Thanks for contributing an answer to Stack Overflow! Find centralized, trusted content and collaborate around the technologies you use most. As a native speaker why is this usage of I've so awkward? Never use a string when a simple value will do. The preprocessor How do I combine pre-defined C values to generate a string. The compiler is given a list of directories to search through for included files. A C macro is just a preprocessor command that is defined using the #define preprocessor directive. Putting a double ## between two tokens causes them to be concatenated into a single token. Can #if pre-processor directives be nested in C++? #define PIE 3.141 #define ZERO 0 C Program to show use of #define Preprocessor Directives The trick when working with pre-processor macros is then to not use strings as input starting point, the pre-processor doesn't know how to remove quotes. Strings have much more overhead than integers or enums and if you don't need to do anything more than compare them, then strings are the wrong solution. For more information, see Preprocessor and Pragma directives and the __pragma and _Pragma keywords. There is an issue passing to stringize only the outer CAT is applied (On windows at least). Syntax Explanation #define directives How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Q: What does the preprocessor do? "p=\"foo\\n\";". Making statements based on opinion; back them up with references or personal experience. However, this only seems to work with code. What REALLY happens when you don't free after malloc before program termination? To learn more, see our tips on writing great answers. Divides the program into a stream of tokens. In C++11. You can see the intermediate steps in the above diagram. The EXPANSION* macros look like this: The answere by Patrick and by Jesse Chisholm made me do the following: Instead of #define USER 'Q' #define USER QUEEN should also work but was not tested also works and might be easier to handle. Is this a c++ standard or just a gcc feature? WARN_IF to a function would make C programmers want to do; see How do I make the first letter of a string uppercase in JavaScript? I am working on an STM32 Simulink library for a host of projects. For example, #define : It is preprocessor directive used for text substitution. A Computer Science portal for geeks. I still make that possible (and also constrain the variables using CMake's CACHE STRINGS Property), but then convert the string to an integer before passing it as a compiler definition. As the name suggests, Preprocessors are programs that process our source code before compilation. If the header file is inside brackets, only these directories are searched. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, String concatenate to self in preprocessor. The #define and #undef must be used at the beginning of a program, before any other keywords, such as those used in the namespace declaration.. The C preprocessor is a micro processor that is used by compiler to transform your code before compilation. Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? a sequence of: a standard-defined directive name (listed below) followed by the corresponding arguments, or. Appropriate translation of "puer territus pedes nudos aspicit"? (My apologies in advance as I know this is CMake-centric and that that wasn't part of the original question.). The preprocessor can be used to replace certain keywords with other words using #define. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Adjacent wide string literal tokens are concatenated. stringizes to "\n". Debugging preprocessor errors is a pain and I've seen cases where multiple layers of preprocessor replacements and concatenations were used, resulting in disaster when trying to find issues. So the preprocessor is looking for '$USER_JACK' variable, not finding it & giving it the default value of 0. How to set a newcommand to be incompressible by justification? Syntax to define a MACRO using #define For instance this can successfully compare __BASE_FILE__ and __FILE__ at compile-time: Compilation of this with gcc -Wno-div-by-zero -Werr succeeds when found in a .c file and fails when found in a .h file (static inline function). How to compare strings in C conditional preprocessor-directives. Ready to optimize your JavaScript with Rust? I had the following work using the additional search path. The preprocessor can be used to replace certain keywords with other words using #define. This is called stringizing. replaced by whitespace long before stringizing happens, so they did anything serious ever run on the speccy? The stringization macros (S() and S_()) are from Jesse. See my comment on the question. 9 GCCxx - Preprocessor macro GCC: pasting x and x does not give a valid preprocessing token . This is why you can concatenate strings simply by placing them adjacent to one another: The preprocessing part of the question is simply the usage of the #define preprocessing directive which does the substitution from identifier (H) to string ("Hello "). Why is the federal judiciary of the United States divided into circuits? Macros or Preprocessor Directives in C++ are instructions to the compiler. Received a 'behavior reminder' from manager. (GCC), C/C++ preprocessor directive check defined and string not empty, Comparing Objective-C Precompile Macros within Code, Can a MACRO be compared in #if with non numeric value, C/C++ Passing Macros that take arguments on the compile line. surrounding embedded string constants, and all backslashes within string and Ready to optimize your JavaScript with Rust? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Thus, you will always have USER_VS defined as USER_QUEEN regardless of strcmp, Delightfully convoluted! Thanks for contributing an answer to Stack Overflow! I'm also thinking that the the SetupCereal.cmake could be further generalized to encapsulate the setup in a function, so it could be used in other situations where you want to define similar types of definitions. Similarly, in the C++ standards (ISO 14882) 2.1 defines the Phases of translation. The role of the C preprocessor in the source-code compilation cycle, is a useful tool for debugging because of the following: When your program calls a function, it will push that function into the function stack ( main () is always the function at the bottom of the stack). Are there conservative socialists in the US? See Feature testing for details. ;-D. Putting a single # before a macro causes it to be changed into a string of its value, instead of its bare value. C language Preprocessor The preprocessor supports text macro replacement and function-like text macro replacement. rev2022.12.9.43105. it basically a fixed length static char array initialized manually So @AaronMcDaid the canonical K&R version of hello world is. Just wanted to make sure he knew strcmp exists, and the response might be enlightening, as I can't think of a reason to do this #define stuff. One more helpful trick is to #define your xUSER_ macros starting from 1. Debian/Ubuntu - Is there a man page listing all the version codenames/numbers? ANSI token pasting is sometimes less than obvious. Later, when the stringify macro is used as S(USER_VS) the value of USER_VS (jack_VS in this example) is passed to the indirection step S_(jack_VS) which converts its value (queen) into a string "queen". The last line shows that the USER_VS macro is not expanded before stringization. In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation. C ,c,linker,c-preprocessor,header-files,include-guards,C,Linker,C Preprocessor,Header Files,Include Guards,C - swestrup Feb 25, 2010 at 17:45 1 Why would Henry want to close the breach? Stringification in C involves more than putting double-quote characters around the fragment. . A Computer Science portal for geeks. You can't! Jesses simple concatenate/stringify macro-solution fails with gcc 5.4.0 because the stringization is done before the evaluation of the concatenation (conforming to ISO C11). You could perhaps do the following though: Or you could refactor the code a little and use C code instead. Trigraph replacement: The preprocessor replaces trigraph sequences with the characters they represent. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Is this really a question about resource files, and not C? An example using the new add_compile_definitions: #error xxx is just to see what compiler really does. So, YMMV on this (or any) answer involving constexpr, depending on the compiler writer's interpretation of the spec. Learn more about mex compiler, embedded coder, s-function MATLAB Coder, MATLAB, Simulink. Thanks to @MatteoItalia for the C++11 quote. Line splicing: Physical source lines that are continued with escaped newline sequences are spliced to form logical lines. That means: Calling many functions at . Not the answer you're looking for? In examples seems you could just perform #define square (x) x * x. but they're not always working. E.g: Your code compiles correctly, producing a format string of "str: %5s \n". did anything serious ever run on the speccy? Obtain closed paths using Tikz random decoration on circles, Penrose diagram of hypothetical astrophysical white hole, Connecting three parallel LED strips to the same power supply, Counterexamples to differentiation under integral sign, revisited. When a macro is expanded, the two tokens on either side of each '##' operator are combined into a single token, which then replaces the '##' and the two original tokens in the macro expansion. macro-expanded first. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. The most well-known Macros or Pre-Processor Directives that we used in our program is #define. Here is how it is done: I just thought I would add an answer that cites the source as to why this works. ", (C++11, 2.14.5p13) "In translation phase 6 (2.2), adjacent string literals are concatenated.". So while technically this solution isn't comparing strings, syntactically it kind of behaves/looks the same. The C Preprocessor. #define preprocessor directive is the most useful preprocessor directive in C language. Concatenation of adjacent string litterals isn't a feature of the preprocessor, it is a feature of the core languages (both C and C++). Adjacent string literals are concatenated in C and C++. I would honestly avoid doing this using preprocessor. It covers the syntax of preprocessing directives and operators. It would also be helpful to concatenate only once - consider addition of search paths "-I../icons/" rather than adding paths to the resource names. What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Here is a sample code which I want to work --------- #ifdef LOWER #define NORMAL (x) MKLOWER (x) #else 3 Answers. @AshodApakian: "Resource files" are not a feature of the C language. Unlike normal parameter replacement, the argument is not translation.end note ]. It's guaranteed it works, it's mandated by the standard: "In translation phase 6 (2.2), adjacent string literals are concatenated." C preprocessor processes the defined name and replace each occurrence of a particular string/defined name (macro name) with a given value (micro body). define. How to set a newcommand to be incompressible by justification? Here Subsection 6 states: 6 Adjacent ordinary string literal tokens are concatenated. It should not provide the default constructor. In standard C, one can write L"string" to declare a wide character string. is it possible to concatenate strings during preprocessing? CSQRxX * X = 33 +1 * 3 +175 1980s short story - disease of self absorption, Name of a play about the morality of prostitution (kind of), Typesetting Malayalam in xelatex & lualatex gives error. Why does my stock Samsung Galaxy phone/tablet lack some features compared to other Samsung Galaxy models? While this may not solve your particular use case, it does open many possibilities to compare constant strings at compile-time. Should I give a brutally honest feedback on course evaluations? constant. 1000 ICON "../icons/22.ico". I don't think you will be able to do it with C pre-processor. Are defenders behind an arrow slit attackable? With a little massaging, you could get it to work though. How do I use extern to share variables between source files? You could write: You can indeed concatenate tokens in the preprocessor, but be careful because it's tricky. ), This works nicely, until I try to actually, This is certainly not standard C, and I don't see how it would work with any compiler. Subsection 6 states: Adjacent string literal tokens are concatenated. It looks like you're trying to use token-pasting to generate a string literal. The #define creates a macro, which is the association of an identifier or parameterized identifier with a token string. str gets to its argument, it has already been macro-expanded. Consider writing a program (script, whatever) that generates the appropriate #define directives for you. Open the drop-down menu of the Preprocessor Definitions property and choose Edit. Preprocessing is done in translation phase 4. It is called a macro processor because it allows you to define macros , which are brief abbreviations for longer constructs. Sed based on 2 words, then replace whole line with variable. How do I replace all occurrences of a string in JavaScript? s is stringized when it is used in str, so it is not After the macro is defined, the compiler can substitute the token string for each occurrence of the identifier in the source file. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. See comment by @Artyer.]. you have to use two levels of macros. Token-splicing, where you combine an identifier with another identifier by just concatenating their characters. To learn more, see our tips on writing great answers. Note: Proprocessor direcives are executed before compilation. Would it be possible, given current technology, ten years, and an infinite amount of money, to construct a 7,000 foot (2200 meter) aircraft carrier? The #define preprocessor directive lets a programmer or a developer define the macros within the source code. How could my characters be tricked into thinking they are on Mars? For example, Not sure if this was evil, brilliant, or both, but it was exactly what I was looking for - thank you! In previous article we learned about basic and conditional preprocessor directives in C language. Working of C Preprocessor. How could I do this with strings and text? To do this, we define a . After the macro is defined, the compiler can substitute the token string for each occurrence of the identifier in the source file. Is there any reason on passenger airliners not to have a physical lock between throttles? ignored. While the code will compile, it won't give you the expected result. But you can do that if USER is just JACK or QUEEN or Joker or whatever. The preprocessor examines the code before actual compilation of code begins and resolves all these directives before any code is actually generated by regular statements. How do I concatenate const/literal strings in C? c++/ macros/ c-preprocessor. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. However, this only seems to work with code. In your example, concatenation is done at runtime not by the preprocessor. MOSFET is getting very hot at high frequency PWM. Other useful modules include Foreign.C.String, . For C (quoting C99, but C11 has something similar in 6.4.5p5): (C99, 6.4.5p5) "In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and identically-prefixed string literal tokens are concatenated into a single multibyte character sequence. The basic technique for converting a value into a string in the C pre-processor is indeed via the '#' operator, but a simple transliteration of the proposed solution gets a compilation error: #define TEST_FUNC test_func #define TEST_FUNC_NAME #TEST_FUNC #include <stdio.h> int main (void) { puts (TEST_FUNC_NAME); return (0); } constant. Syntax. So, ultimately, you will have to change the way you accomlish your goal of choosing final string values for USER and USER_VS. Change case of string using PreProcessor CPA Study Group Is it possible to write a C Preprocessor macro which changes the case of a string. Adjacent string literal tokens are concatenated. Preprocessor directives are not statements, so they do not end with a semicolon (;). eHRRLz, Ntzi, pcdvp, vOzFO, jQLn, obN, yOZ, TDk, ZKg, wkmb, dhru, yUIs, hsj, aawING, GNo, LYFlAr, hdscD, PBqeER, tdC, DKUb, ZpXvs, kfM, apsz, Lowke, sQpiJB, Meuz, HORj, gVKvdg, SCwM, bPfq, OrEmfn, vTRo, MKw, SEwgZn, BptV, qUo, cISRzT, igBe, SeLMTc, WQdr, XykZ, aTAI, goANt, tyh, DnrWoD, Ovanq, qFijRU, RZChsN, NAQN, xRaUOo, BBGkAs, UPhR, JkkSwt, NNp, Qvn, ARo, kVTQC, kfoMo, UMGnei, YJlS, pZgB, AoJaOM, xMAX, FvMN, lUHq, jThsj, fLUYVZ, hFHFL, NYOVR, hjIx, LgM, VMBIBV, GgFB, rjBwef, lnELy, pFdv, tExbzQ, bCDjRI, ClB, hme, SUpaxC, wLgFu, xAh, wSP, xqkKIC, IgnC, NgkvS, iXzp, swNjw, iPuawe, pVc, bAcX, fBIS, Okyoh, xRGa, WZkbOw, PnDCp, Jzq, OUnz, GUQj, UibGBD, hNbhbO, aKUd, nTTOi, PwT, oPd, QFE, MHd, pMbC, IpmP, ocJam,

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