how to copy const char* to char in c how to copy const char* to char in c

How to convert a std::string to const char* or char*. Has the cause of a rocket failure ever been mis-identified, such that another launch failed due to the same problem? For example, to get the first character of the first argument to your program, you can do e.g. Could a subterranean river or aquifer generate enough continuous momentum to power a waterwheel for the purpose of producing electricity? You might use strncpy if t1->name was a fixed-size array instead (though many people prefer to use strlcpy). What is this brick with a round back and a stud on the side used for? Why did US v. Assange skip the court of appeal? You cannot put a const char * (pointer) to a char variable. It's somewhere else in memory, and a contains the address of that string. Problem with this answer is if s is more than 255 characters there will be no terminating 0 at the end of c. Whether that's important or not is really up to you but 999 times out of 1000 it probably is important. However, you already computed the length of the string once to allocate the memory; there's no sense in doing it again implicitly by calling strncpy. And for the case str0 = str1; I don't understand why it won't work, because str0 points to nothing, while str1 points to a const string literal, so if I now make str0 point to what str1 is pointing to, it should be fine, but it is not. What is the difference between const int*, const int * const, and int const *? Why should C++ programmers minimize use of 'new'? Here, the '1234' does not denote a string. If you'd be able to assign the same pointer to str0 you'd break the const contract; str0 can be modifiable. The owner always needs a non-const pointer because otherwise the memory couldn't be freed. Looking for job perks? (IMHO std::remove (const char*) should be std::remove_file (std::string const&) or at least std::remove_file (const char . I'm trying to copy only a portion of a string (or char *) into another string (or another char *) char * first_string = "Every morning I" char * second_string = "go to the library, eat breakfast, swim." char * final_string; I would like to copy part of the second_string into the first_string. If you need a const char* from that, use c_str(). @Phlucious, because: 1) qPrintable returns const char* not char*, str.toLocal8Bit ().data () returns char*. characters are part of the string object.cont char* stores the address of such a character buffer but does not own it. is very wrong. won't be null terminate if s is longer then 255 bytes, As it's an array you can do sizeof(c) to get its size and use it in via safe string functions that allow you to pass an n to them. What is the Russian word for the color "teal"? Since you manually have to repair the corner case, you could just as well use memcpy in the first place. You can't put character pointers in EEPROM and expect the characters they used to be pointing at to still be there when you read the pointer back into memory. How to combine several legends in one frame? Does a password policy with a restriction of repeated characters increase security? For example: The obvious problem with using an array of constant size is that you need to consider how to handle situation where the input string doesn't fit. Didn't verify this particular case which is the apt one, but initialization list is the way to assign values to non static const data members. how can I compile this code? Can you still use Commanders Strike if the only attack available to forego is an attack against an ally? How a top-ranked engineering school reimagined CS curriculum (Ep. tar command with and without --absolute-names option. Find centralized, trusted content and collaborate around the technologies you use most. In most cases, it is better to create a new char* variable and copy the contents of the const char* to the new variable, rather than modifying the original data. Unfortunately C++ didn't add an array size function until C++ 17 (std::size) so we're left to make our own. char c[] has the same size as a pointer. But if you insist on managing memory by yourself, you have to manage it completely. Thanks for contributing an answer to Stack Overflow! How do I iterate over the words of a string? What are the advantages of running a power tool on 240 V vs 120 V? From Prince Harry's tell-all memoir to King Charles III's ascension to the throne, there has been no shortage of royal family news in the last year. In most cases, it is better to create a new char* variable and copy the contents of the const char* to the new variable, rather than modifying the original data. Also you can not use strings as switch/case labels. So change code to: You need fix how your array is being initialized as you are initializing only one character (and we assume you want full string to be copied). When you have non-const pointer, you can allocate the memory for it and then use strcpy (or memcpy) to copy the string itself. What is the difference between const int*, const int * const, and int const *? "Signpost" puzzle from Tatham's collection. To learn more, see our tips on writing great answers. Doing double the work is not necessarily bad but given the optimal version is simple there's no reason not to use it. Extracting arguments from a list of function calls. Copying the contents of a to b would end up doing this: To achieve what you have drawn in your second diagram, you need to take a copy of all the data which a is pointing to. What is the difference between const int*, const int * const, and int const *? rev2023.4.21.43403. I allocated t1->name = malloc(sizeof(s)) and then used strncpy. Unexpected uint64 behaviour 0xFFFF'FFFF'FFFF'FFFF - 1 = 0? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Not the answer you're looking for? You can implicitly convert char * into const char *. If the const char * were just bytes though, you'd need another way. Why xargs does not process the last argument? C++ : How can I convert const char* to string and then back to char*?To Access My Live Chat Page, On Google, Search for "hows tech developer connect"I promis. The common but non-standard strdup function will allocate new space and copy a string. Thank you, @isal: Then just don't forget to allocate the memory for the string as well and use, Should that be qualified: "strncpy is always wrong. This is valid because std::string overloads the assignment operator and accepts a const char pointer as the right hand value. I prefer to use that term even though it is somewhat ambiguous because the alternatives (e.g. Unexpected uint64 behaviour 0xFFFF'FFFF'FFFF'FFFF - 1 = 0? Find centralized, trusted content and collaborate around the technologies you use most. Getting a "char" while expecting "const char", Using an Ohm Meter to test for bonding of a subpanel. the way you're using it, it doesn't copy the terminating \0. I need to copy a const char * into a const char *, You need to stop thinking of a pointer as "containing" anything. How to Make a Black glass pass light through it? Short story about swapping bodies as a job; the person who hires the main character misuses his body. But I realize my mistake where I was doing malloc(sizeof struct test) and not sizeof *t1. But you can copy the string. Here are three methods you can use: Method 1: Using a const_cast Step 1 - Create a variable of type const char*. That doesn't really matter. e.g. @MarcoA. How to cast the size_t to double or int c++? Is there a generic term for these trajectories? - asveikau Dec 13, 2013 at 7:36 @asveikau: That doesn't help you to pass a char value to something that wants a pointer. How do I profile C++ code running on Linux? Realloc is not resizing array of pointers. In MyEepromArray[12] i enter the following data: and i should change them dynamically through serial. @keanehui1 no. - Zdeslav Vojkovic Sep 28, 2012 at 10:30 I'm receiving a c-string as a parameter from a function, but the argument I receive is going to be destroyed later. Connect and share knowledge within a single location that is structured and easy to search. Is this the real lesson here? How about saving the world? Your wine seems to have got you more rep than my whisky. Otherwise go for a heap-stored location like: You can use the non-standard (but available on many implementations) strdup function from : or you can reserve space with malloc and then strcpy: The contents of a is what you have labelled as * in your diagram. What is the difference between const int*, const int * const, and int const *? Where reinterpret_cast would probably just directly convert to char, without any cast safety. Why does Acts not mention the deaths of Peter and Paul? How do I stop the Flickering on Mode 13h? 2) The pointer to const char* becomes invalid as soon as you hit a semicolon in the statement where qPrintable was used. Not the answer you're looking for? What is the difference between const int*, const int * const, and int const *? Understanding the probability of measurement w.r.t. 565), Improving the copy in the close modal and post notices - 2023 edition, New blog post from our CEO Prashanth: Community is the future of AI. You are getting segmentation fault, because new_name points nowhere. Content Discovery initiative April 13 update: Related questions using a Review our technical responses for the 2023 Developer Survey, C++: How to convert 'const char*' to char, How to convert a std::string to const char* or char*. Step 3 - Use the strcpy() function to copy the const char* to the char*. about your note, is there a reason for the recommended signature of, @JackBauer Yes, because that signature in my NOTE is mentioned in the C standard. But this will probably be optimized away anyway. How about saving the world? In which case you can do the following: How to check if a class is declared in c++? You're getting mixed up between char (character) and char * (string). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Tikz: Numbering vertices of regular a-sided Polygon, Adding EV Charger (100A) in secondary panel (100A) fed off main (200A), What "benchmarks" means in "what are benchmarks for?". How a top-ranked engineering school reimagined CS curriculum (Ep. As you only want to read the string, you want it to be const. How to append text to a text file in c++? allocates space on the stack for 256 bytes and does nothing else. 565), Improving the copy in the close modal and post notices - 2023 edition, New blog post from our CEO Prashanth: Community is the future of AI. Step 2 - Allocate memory for the char* variable. You declared MyEepromArray as an array of pointers to the const strings, it can't be changed by simple way. const char* original = "TEST"; char* copy; copy = original; original points to the start of the string "TEST", which is a string literal and thus points to read-only memory. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. printMe takes an lvalue reference to a mutable pointer to const char. There is no any sense to compare it with a character literal similar to '1234'. A minor scale definition: am I missing something? error: cannot convert 'char**' to 'char*' for argument '1' to 'char* strcpy(char*, const char*)', error: cannot convert 'char**' to 'char*' for argument '1' to 'char* strcpy(char*, const char*)', i don't get that error That they still give you an executable doesn't change what C++ is defined as, how to convert const char [] to char * in c++. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 1 Answer. and want to copy this const char string* to a char*! Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. The const qualifier instructs the compiler to not allow data modification on that particular variable (way over simplified role of const, for more in-depth explanation use your favorite search engine and you should be able to find a bunch of articles explaining const). Why are players required to record the moves in World Championship Classical games? How to Make a Black glass pass light through it? You allocate mem for just 1 char. It is always wrong. Parabolic, suborbital and ballistic trajectories all follow elliptic paths. Without any attempt at completeness or educational direction, here's a version of your code that should work. @gman Abel's answer also (potentially) unnecessarily copies null characters into the buffer when the string is shorter. cont char* stores the address of such a character buffer but does not own it. How is white allowed to castle 0-0-0 in this position? 565), Improving the copy in the close modal and post notices - 2023 edition, New blog post from our CEO Prashanth: Community is the future of AI. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It is useful when you want to pass the contents. OK, that's workable. rev2023.4.21.43403. Please read about RAII to understand why all of the solutions with manual memory management are bad: cppreference , wiki. I believe sizeof is in fact tied to the sizeof a char (regardless of whether a char is 8 bits). Making statements based on opinion; back them up with references or personal experience. "Signpost" puzzle from Tatham's collection. awesome art +1 for that makes it very clear. Find centralized, trusted content and collaborate around the technologies you use most. Parabolic, suborbital and ballistic trajectories all follow elliptic paths. A minor scale definition: am I missing something? pointer to const) are cumbersome. Why are players required to record the moves in World Championship Classical games? Fixed it by making MyClass uncopyable :-). Making statements based on opinion; back them up with references or personal experience. Does the 500-table limit still apply to the latest version of Cassandra? It's always important to understand the trade-offs and implications of the different approaches, and making the right decision will depend on the specific requirements of your program. Not the answer you're looking for? There are a few ways to convert a const char* to a char* in C++. const char* myString = "This is a const char\*"; Step 2 - Use the const_cast operator to convert the const char* to a char*. Remember that converting a const char* to a char* allows you to modify the data, but should be used with caution. It has never been correct C++ to assign a string literal to a. Looking for job perks? characters are part of the string object. Now, you can't write to a location via a const char *. If doesn't have to cover anything complex. Gahhh no mention of freeing the memory in the destructor? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Did the Golden Gate Bridge 'flatten' under the weight of 300,000 people in 1987? What should I follow, if two altimeters show different altitudes? Looking for job perks? It is a multibyte charcater, which is most probably something you don't want. In C, you can allocate a new buffer b, and then copy your string there with standard library functions like this: Note the +1 in the malloc to make room for the terminating '\0'. Here, the destination string is the char* variable and the source string is the const char* variable. Does a password policy with a restriction of repeated characters increase security? Of course, don't forget to free the filename in your destructor. You can either call malloc() and then use strcpy(), or call strdup() which will do both things for you: See this answer for more details on strdup(): https://stackoverflow.com/questions/252782/strdup-what-does-it-do-in-c. You need to allocate space for the new string. Each method has its own advantages and disadvantages, so it is important to choose the right method for your specific use case. gcc 4.8.4 allows it with a deprecation warning, They issue a diagnostic, telling you your program isn't C++. How to Make a Black glass pass light through it? What is Wario dropping at the end of Super Mario Land 2 and why? Otherwise, you can allocate space (in any of the usual ways of allocating space in C) and then copy the string over to the allocated space. rev2023.4.21.43403. Why is char[] preferred over String for passwords? new_name). or use std::string class template library for managing strings. Step 2 - Use the const_cast operator to convert the const char* to a char*. :-S. This answer confused me a little, so I'm clarifying for other readers. The second and the third methods use two library functions strcpy() and memcpy() respectively to copy the content of const char* to char* variable. So: The problem is that you're using strncpy, rather than strcpy. - Some programmer dude Feb 9, 2013 at 19:49 2 Your wine seems to have got you more rep than my whisky. What should I follow, if two altimeters show different altitudes? Why typically people don't use biases in attention mechanism? Your intention was to call std::remove () from <algorithm>, but you inadvertently called std::remove () from <cstdio>. you can copy the String variable, but MyEepromArray [2] needs to point to a char array that it can be copied into. "strdup" is POSIX and is being deprecated. Step 3 - Use the memcpy() function to copy the const char* to the char*. for gcc, at a minimum, use: '-Wall -Wextra -pedantic'. strcpy() function is used for copying the content of one string to another. Not the answer you're looking for? So the C++ way: There's a function in the Standard C library (if you want to go the C route) called _strdup. How do I convert const char* to char[256]? It takes care of copying the string data properly when multiple copies of the object are used (although it doesn't use copy-on-write). Did the Golden Gate Bridge 'flatten' under the weight of 300,000 people in 1987? That would look like the following: Note that the size argument to strncpy should always be the size of the destination, not the source, to avoid writing outside the bounds of the destination buffer. You could change char *str = "C++ Language"; to char str []="C++ Language;" Initializing the pointer directly with constant string is not supported by most compilers. Why did DOS-based Windows require HIMEM.SYS to boot? sizeof (*s) is 1, as it's the same as sizeof (char) which is specified in the C specification to be equal to one. Does the C++ standard allow for an uninitialized bool to crash a program? Embedded hyperlinks in a thesis or research paper. This is the source code which I am testing. Short story about swapping bodies as a job; the person who hires the main character misuses his body. Pointers point to other parts of memory which must, in of themselves, exist. i should change them dynamically through serial. I'm not at the liberty of changing the struct definition. and MyEepromArray[12] is still an array of pointers, char *, not char, MyEepromArray[12] is still an array of pointers, char *, not char, it's correct To prevent possible type overflow you could do this: const char char_max = (char) ( ( (unsigned char) char (-1)) / 2); int i = 128; char c = (i & char_max); // Would always result in positive signed values. @Tronic: Even if it was "pointer to const" (such as, @Tronic: What? You need to pre-allocate the memory which you pass to strcpy. How would you count occurrences of a string (actually a char) within a string? Next I put (char *)string.c_str () but this only causes an unhandled exception. This will waste a lot of cycles if you have large buffers and short strings. You need to add 1 to length after copying in order to copy null character (as strlen returns only number of chars without null character; see more here). Can the game be left in an invalid state if all state-based actions are replaced? Yes, if you read that tutorial carefully you'll see that you can use individual, makes sense. And at the end you might consider using just an array of fixed size that is initialized to maximum path. Asking for help, clarification, or responding to other answers. rev2023.4.21.43403. free() dates back to a time. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. I.e. How do I stop the Flickering on Mode 13h? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. How about saving the world? I have to replace a string value in a specific char* array and then write it in eeprom: Instead it works if I write the code like this: What do you see if you print MyEepromArray after trying to insert the String into it ? Making statements based on opinion; back them up with references or personal experience. What was the actual cockpit layout and crew of the Mi-24A? How to set, clear, and toggle a single bit? Also, keep in mind that there is a difference between. Connect and share knowledge within a single location that is structured and easy to search. What risks are you taking when "signing in with Google"? How to call qdebug without the appended spaces and newline in C++? c_str returns a const char* that points to a null-terminated string. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Instead, you cound use memcpy() or strcpy, (or in your case even strdup() ). For the manual memory management code part, please see Tadeusz Kopec's answer, which seems to have it all right. He also rips off an arm to use as a sword. Does the 500-table limit still apply to the latest version of Cassandra? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, problems with convert const char* to char* in c, https://stackoverflow.com/questions/252782/strdup-what-does-it-do-in-c. Can my creature spell be countered if I cast a split second spell after it? So now what s points to is undefined, If you were not creating the string in that line it would be safe. We already have too many of them, C compilers, not "older compilers". What were the most popular text editors for MS-DOS in the 1980s? this allocates space for a struct test; enough space for the pointer name, but not any memory for the pointer to point to. If total energies differ across different software, how do I decide which software to use? Followed your advice but I get the following error: The length with strlen is OK! By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. What positional accuracy (ie, arc seconds) is necessary to view Saturn, Uranus, beyond? How can I convert const char* to char[256]. Here, I've used an exception, but you can use error handling of your choice, if this is not an option for you. rev2023.4.21.43403. Which was the first Sci-Fi story to predict obnoxious "robo calls"? Here is a fixed version of your code: First of all the standard declaration of main looks like. What is this brick with a round back and a stud on the side used for? - WindyFields Sep 14, 2017 at 3:21 1 You have to decide whether you want your file name to be const (so it cannot be changed) or non-const (so it can be changed in MyClass::func). For example, Now t will be valid until the current scope exits and so will s, As for the copy to an array of 256 characters the arguably optimal solution is. Why did DOS-based Windows require HIMEM.SYS to boot? If you name your member function's parameter _filename only to avoid naming collision with the member variable filename, you can just prefix it with this (and get rid of the underscore): If you want to stick to plain C, use strncpy. P.S. What is the difference between char * const and const char *? Solution: allocate memory for new_name. and want to copy this const char string* to a char*! Why is char[] preferred over String for passwords? - Mark Ransom Dec 8, 2011 at 20:25 Add a comment 4 I'm guessing that the func call is expecting a C-string as it's input. Which language's style guidelines should be used when writing code that is supposed to be called from another language? Something like: (This function actually exists, under the name strcpy_s in C 2011, but Edit: Even better use strdupas Miroslav suggests. filePath: You will have to store the characters, not just a pointer to them. My solution at first to this problem was simply entering in string.c_str (), but that returns a const char * which apparently doesn't work with the function. and I hope it copies all contents in pointer a points to instead of pointing to the a's content. Otherwise, you can allocate space (in any of the usual ways of allocating space in C) and then copy the string over to the allocated space. With it c is not only allocated but also initialized to 0 for all 256 characters. Which was the first Sci-Fi story to predict obnoxious "robo calls"? cannot convert 'const char **' to 'const char*'. Your problem arises from a missing #include directive. Which was the first Sci-Fi story to predict obnoxious "robo calls"? In the first case, you can make filename point to any other const char string, in the second, you can only change that string "in-place" (so keeping the filename value the same, as it points to the same memory location). How to set, clear, and toggle a single bit? Not the answer you're looking for? I think the code crashes. What is the difference between char s[] and char *s? Does the 500-table limit still apply to the latest version of Cassandra? On whose turn does the fright from a terror dive end? What is the EXACT technical difference between "const char *" and "const string". That tells you that you cannot modify the content pointed to by the pointer. i did studied this for hours, just need a hint. The term const pointer usually refers to "pointer to const" because const-valued pointers are so useless and thus seldom used. Is anyone offer some suggestion how to solve that. also wrong. Content Discovery initiative April 13 update: Related questions using a Review our technical responses for the 2023 Developer Survey. -> Never use reinterpret_cast if you can also . An initializer can also call a function as below. Thanks for contributing an answer to Stack Overflow! How to convert a std::string to const char* or char*. - Wander3r Aug 3, 2018 at 9:12 1 Use std::string in C++ - Clonk Aug 3, 2018 at 9:13 Related question: stackoverflow.com/questions/20944784/ - vishal Aug 3, 2018 at 9:18 1 it isn't widely implemented; Microsoft has it, but I've not seen it You will have to store the characters, not just a pointer to them. It is at least as safe (and often safer) and more efficient if done properly. I tried to use strcpy but it requires the destination string to be non-const. n_str is of type std::string and str0 is char*, there's no overloaded operator that allows this. That is the second parameter does not have qualifier const. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA.