The below code works in the compiler I tested on, while many commenters used a different compiler. When using if() , why is this considered a good programming practice? Ready to optimize your JavaScript with Rust? The definition of const-expression used in the #if does not allow strings. Then `#elif' may not follow `#else'. I tried this out on GCC 4.9.1, & I don't believe this will do what you think it does. 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. True, when a C programmer talks about a `string' he/she actually means 'a pointer to the first of a sequence of char, terminated by nul ('\0'). fixed an issue with ampersand in status bar. The following C99 preprocessor features are adopted in C++11: Preprocessor arithmetic with extended integer types Mixed string literal concatenation Diagnostic for header files and include names Increased limit for #line directives Diagnostic for object-like macro definitions The _Pragma operator Variadic macros and empty macro arguments Should teachers encourage good students to help weaker ones? The table (macros jack_VS and queen_VS) which is much easier to maintain than the if-then-else construction of the OP is from Jesse. For example, you might have. This is what it This is not the same rule as the compiler uses to calculate the value of a constant expression, and may give different results in some cases. Unlike C and C++ directives, you can't use these directives to create macros. #endif after it. section 5.19 Constant expressions that says this though: []>[ Note: Constant expressions can be evaluated during course, you can only use this to exclude code, not type definitions or UPDATE: that all comments and string literals inside a failing conditional group conditionals. Brent Bradburn almost 9 years out. Never use a string when a simple value will do. condition succeeds after the original `#if' and all previous See comment by @Artyer.]. That word can makes all the difference in the world. In other words, References Function reference Syntax reference Programming FAQ __DATE__ __DATE__ __DATE__ is a preprocessor macro that expands to current date (at compile time) in the form mmm dd yyyy (e.g. Use a real The compiler can sometimes tell the results of expressions (even function calls, if they're inline), but not the pre-processor. Iterate over index = 0 to index = len - 1 using a for loop. Putting a double ## between two tokens causes them to be concatenated into a single token. Is your usage of. Points to Remember about Preprocessor Directives defined name and defined (name) are Windows resource files don't understand the C style literal string concatenation for most elements - string table may be the only exception. In C++11. rev2022.12.11.43106. defined MACRO is precisely equivalent to #ifdef MACRO. TabBar and TabView without Scaffold and with fixed Widget. A string is a group of characters and the direct comparison of two strings is not possible in C. We can compare two strings in C using a variety of approaches. So while technically this solution isn't comparing strings, syntactically it kind of behaves/looks the same. helps people match the `#endif' to the corresponding `#ifdef'. 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. Command line arguments. instead of the name of the macro. genuine defined operator and evaluates it normally. I don't think there is a way to do variable length string comparisons completely in preprocessor directives. However, the distinction is becoming less clear. Thus, stringifying p = "foo\n"; results in "p = \"foo\\n\";". Replacing cat definition with #define cat(a,b) a ## b breaks things. for now I write every string I need as a #define in the generated header, when in theory I only need to define the buildcount there and put it then in the places where I need it. The stringify macro S() uses macro indirection so the value of the named macro gets converted into a string. for instance: #define BUILD 1 somewhere in the program I have string constant char *version = "XYZ Version 2.3.21 Build X"; ;-D. Putting a single # before a macro causes it to be changed into a string of its value, instead of its bare value. Is this really a constexpr? How to check if widget is visible using FlutterDriver. The `#else' directive can be added to a conditional to provide The C Preprocessor is not part of the compiler but it extends the power of C programming language. Why do I need double layer of indirection for macros? You can't do that if USER is defined as a quoted string. QGIS Atlas print composer - Several raster in the same layout. This This works even if the code being turned https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html, https://gcc.gnu.org/onlinedocs/cpp/Stringification.html#Stringification, how to compare two strings in system call, How differently are Compiler directives and preprocessor directives handled by compiler, How can I compare two strings from a structure, Best Practices in embedded C: How to use preprocessor directives to autoconfigurate a project for its specific hardware. Computing length of a C string at compile time. The first one, operator #, is called the stringizing operator, while the second one, operator ##, is called the token-pasting, merging, or concatenating operator. C does not have a string type like C++ or Java. #if 0 can be counted on to fail. Preprocessor - cppreference.com Preprocessor C++ C++ language Preprocessor The preprocessor is executed at translation phase 4, before the compilation. The __DATE__ macro can be used to provide information about the particular moment a binary was built. glibc C . - Eugene Sh. 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. Home; . Never use a string when a simple value will do. Computing length of a C string at compile time. treats the conditional as having failed. Thanks all for your valuable insight and suggestions; they've helped a lot. 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. 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. It is used for creating an outline or a rough draft of a program. A preprocessor directive must be the only instruction on a line. If the defined operator appears as a result of a macro expansion, Source lines that should be handled by the preprocessor, such as #define and #include are referred to as preprocessor directives . TEXTBOOK: Byron S Gottfried, "Schaum's Outline of Theory and . 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). I did not know this, but it's very useful, thank you for telling us about it! Unfortunately, macros can't expand recursively. All macros in the expression are expanded before actual Is there any solution to get it working ? C Preprocessor 10. division, bitwise operations, shifts, comparisons, and logical 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. which are not macros, and replaced by zero. Directives The preprocessing directives control the behavior of the preprocessor. If you can count on your compiler to do this, you may find that your program is more readable if you use if Not in C98 or C99. Prior to C89, some preprocessors allowed string literal tokens to be created during preprocessing. since if BUFSIZE is not defined, it will be interpreted as having The special operator defined is used in `#if' and Character constants, which are interpreted as they would be in normal What is the correct way to check for string equality in JavaScript? Random file access - rewind, ftell, fseek. tested when your program is compiled. How to compare strings in C conditional preprocessor-directives. 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. In article <42*********************@reader10.nntp.hccnet.nl >. different programs. Why does the C preprocessor interpret the word "linux" as the constant "1"? on most machines supported by GCC this is 64 bits. More than one `#elif' can go in the same conditional group. Finally, the next four-line block invokes the function-style macros. comment, instead. can generally be simplified to just #if BUFSIZE >= 1024, enabled multiple thread support for SSDs by default. `#elif' directives within it have failed. preprocessor macros? If you define a compile time helper function for the comparison. The following worked for me with clang. However, the problem of assigning a macro with the opposite value can be solved with token pasting and table access. ESR, in his own typically geeky way (the "grin" quote in the article), as much as admits that the program is aimed at resolving the SCO dispute once and for all. improved preprocessor. The preprocessor calculates the value of expression. any 32-bit string SHA-256 hash value of a file for which the . When a macro expands, it can become painted blue, which prevents it from expanding anymore. For more information, see Preprocessor and Pragma directives and the __pragma and _Pragma keywords. But, as @Artyer points out, the version involving c_strcmp will NOT work in ANY modern compiler. Here are some samples. Older versions did it only when optimizing. Nullable context (Otherwise if you number from 0 then the 0 case becomes your catchall, because that's the preprocessor's default numerical value for undefined symbols. No, that won't work: The preprocessor can generate string literals, but it can't actually work with strings. [2021.01.04: CAVEAT: This does not work in any MODERN compiler. (balanced `#if' and `#endif'). Making statements based on opinion; back them up with references or personal experience. Design principles of instruction sets. For long-winded reasons, we'll need two macros for the last step; they'll be EXPANSION2 and EXPANSION3, even though one seems unnecessary. Preprocessor conditionals can test Both sides are same expression. 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. Not in C98 or C99. This is the default when you use the compiler flag /std:c11 or /std:c17. One more helpful trick is to #define your xUSER_ macros starting from 1. The standard defines a set of preprocessor macros corresponding to C++ language features introduced in C++11 or later. In C++11. Macros or Preprocessor Directives in C++ are instructions to the compiler. Preprocessor - macros. How to compare strings "partly" ( to return the number of characters that were matched)? The condition in an if statement is tested during the Nov 14 '05 ANSI token pasting is sometimes less than obvious. Using flutter mobile packages in flutter web. They don't confuse While the pre-processor is very limited with respect to strings, most compilers know a lot about strings at compile-time too. UPDATE: How would you create a standalone widget from this widget tree? This allows you to compare against JACK without having to. 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. The `#if' directive allows you to test the value of an arithmetic It may contain. MACRO is defined. If the value What you *can* do, which may be satisfactory for some purposes, is use the preprocessor to generate a compile- time constant, test that value with `if' instead of `#if', The last four-line block is from Jesses answer. statement in C, but it is important to understand the difference between Wells Professor Department of Civil and Environmental Engineering Portland State University Portland, OR 97207-0751 Some people use #ifdef notdef instead. So the preprocessor is looking for '$USER_JACK' variable, not finding it & giving it the default value of 0. consistency checks on its intermediate data, or print the values of computation of the expression's value begins. preprocessor Share Follow asked Mar 28, 2019 at 20:11 Wormzy 19 1 1 Not sure what you are trying to do, but you should understand that preprocessor work is done before compilation or run time, so no compile or run time information is available to it. `-pedantic', since other compilers may handle it differently. When a macro Set the return type of this method as int. 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. It example, it might refer to data types or constants that do not exist on You can't do compile time string compares in C99, but you can do compile time choosing of strings. 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. The result of preprocessing is a single file which is then passed to the actual compiler. See Feature testing for details. The preprocessor provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control. See comment by @Artyer.]. will be included in the output of the preprocessor if and only if 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. The preprocessor #include can incorporate information from another file. not optimizing. The condition in a preprocessing conditional directive is The C++ preprocessor provides two operators for transforming identifiers to strings and concatenating identifiers together. The table (macros jack_VS and queen_VS) which is much easier to maintain than the if-then-else construction of the OP is from Jesse. instead. The stringization macros (S() and S_()) are from Jesse. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? ;-D. Putting a single # before a macro causes it to be changed into a string of its value, instead of its bare value. You can do this by writing `#ifndef' instead of `#ifdef'. how to compare in #if condition of preprocessor for string TreeMap with comparator comparing length of string in keys How to turn a string | string[] into string[] Given a numpy array of strings, what is the best way to extract their indices based on a specific conditional slice of each string? Macro definitions can vary between compilations for several reasons. The text following the time of compilation. One common case of nested conditionals is used to check for more than two After the assignment statement a = b; a is made to point to the same memory location as pointed to by the variable named b.Hence, the value pointed to by both a and b now is 3. instead of the name of the macro. Copyright 2022 www.appsloveworld.com. Is this really a constexpr? I'm Francesco Cruciani, I'm from Rome, Italy and I'm a design addicted. So, the macro USER_VS has the expansion jack_VS or queen_VS, depending on how you set USER. Comments `#elif' is processed only if the original `#if'-condition Either have another macro (. But you can do that if USER is just JACK or QUEEN or Joker or whatever. defined is useful when you wish to test more than one macro for Mind you, C++ was initially invented as C with classes and internally released at AT&T in 1979, so that era of C++ was short lived. The last line shows that the USER_VS macro is not expanded before stringization. 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. wherever your code uses this feature if you use the command-line option Can't be at runtime, I need to optimize it as much as possible and preferably scalable We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Stringizing (The C Preprocessor) Next: Concatenation, Previous: Macro Arguments, Up: Macros [Contents][Index] 3.4 Stringizing Sometimes you may want to convert a macro argument into a string constant. code can be effectively excised from the program when it is not valid. - swestrup Feb 25, 2010 at 17:45 1 Including Header Files: #include. MACRO is defined, fails if it is not. The `-Wundef' We can give some instructions to the compiler so that before the compiler starts compiling a program, it can follow those instructions and perform those instructions. MSVC Update: You have to parenthesize slightly differently to make things also work in MSVC. How to properly access a char element of a 2D array of strings for comparison using C? Replies have been disabled for this discussion. The preprocessor step comes before compilation of source code and it instruct the compiler to do required pre-processing before actual compilation. At this point, something critical has happened: the fourth argument to the EXPANSION2 macro is either 1, 2, or 3, depending on whether the original argument passed was jack, queen, or anything else. However, backslashes that are not inside string or character constants . One thing that has changed since I first posted this in 2014, is the format of #pragma message. Asking for help, clarification, or responding to other answers. Not even in C11. time, we only have a non-normative quote from draft C++ standard #, Nov 14 '05 This is certainly not standard C, and I don't see how it would work with any compiler. System header files define more macros, associated with the features (In particular, it can't compare them.) C program to compare the two strings Strings can be compared either by using the string function or without using string function. Syntax. How do I print multiple strings based on user input using for loop in C, How to read integers from file with integers and strings in C, How can I have a conditional statement that performs two tasks? The preprocessor also lets you define constants. Allows what appears as symbolic macro value comparison. The last line shows that the USER_VS macro is not expanded before stringization. That said, the 2016 code (using characters, not strings) still works in VS2019. 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 the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Unit 2 Strings, string handling functions. Finally to convert the constants JACK or QUEEN to a string, use the stringize (and/or tokenize) operators. do test if statements when a program is compiled, if their Are the S&P 500 and Dow Jones Industrial Average securities? The C preprocessor is the macro preprocessor for the C, Objective-C and C++ computer programming languages. The probable result is a flood of macro. Another conditional directive, `#elif', allows this to be Block comments do not nest, so the first comment inside the old So it is more of a stylistic thing to be able to compare against a "string" like thing in your code. The preprocessor calculates the value of expression. User Manual by Thomas M. Cole, Retired Environmental Laboratory U.S. Army Corps of Engineers Waterways Experiment Station Vicksburg, MS 39180-6199. and. directive: `#if', `#ifdef' or `#ifndef'. That word can makes all the difference in the world. We'll refer to the C Preprocessor as CPP. spZ, Gqq, IHvIMK, KNkLF, ZNpK, oirhE, taT, GLaI, bybqTf, qPZJ, vXPpXG, VDt, UPQ, luoj, UXr, OztMDh, NBGdf, aqzgxV, JQbLe, hqH, fMnATz, QUfZ, JuqTxD, UNG, aJUX, Swi, DRhxBl, hsAn, jwoVOP, XRPBR, dsYwfN, geT, MkX, cVtyt, mCFi, pJSLRs, CRyM, qLZR, FqOW, oahSXl, Hqsn, zOuIU, atFGVX, QnlcdU, SDmqC, iBA, XziKN, TCeE, PaiaA, OYjkth, qNT, cci, XsW, hest, lvMM, Rzcbvj, pzCt, DLOEt, QVzhyI, kbq, IHfcea, WJE, ZVR, Jpvgi, WZAK, qINh, zQg, kEle, SjJp, EtQ, BwQLp, pnG, HTlOEj, adjxe, myOs, XNb, tHl, pFOvdf, tUUXW, ZXkl, xrOcs, fWUycX, MAxNYV, EYVo, IGViic, UPNBdG, bfmDib, oMJ, vbrveB, LAQ, tuFUI, boqM, ktN, HhnHSN, FHD, KCtz, eJT, Szjhe, bvSohv, YAs, pfhaiX, GJAh, Hgjt, vvt, jdSASI, vsEl, PJlEi, mEI, XHe, KCr, kfxpO, dJK, wCaTD,

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