Std string constructor size. std::string s4 ("A character sequence", 6); .

Std string constructor size I'd imagine most std::strings have 3 pointers (begin, end, capacity) and a variable for size. #include <iostream> #include <cassert> using namespace std; // NEVER write this in a header file. These types and functions are provided as a set of member typedef-names and functions in the template parameter traits std::string has a constructor that takes an initializer_list argument. What happens if you just do std::string operationalReason;?That should have the same effect as the two examples you provided. The behavior is undefined if: **[ first, last ) is not a valid range; It does not model Moving a std::string is reasonably cheap, amounting to a few assignments for the pointer, the size, and the allocator -- so this produces negligible performance overhead while also being a simple and maintainable approach. you have to think whether you really want to use the size constructor. It is implicitly convertible from std::string, but can also be explicitly constructed from contiguous data owned somewhere else, avoiding the unnecessary copying std::string imposes. This doesn't invoke the Geeks copy constructor at all. First of all, std::string has so-called Small String Optimization (SSO), which means that for very short (or empty) strings, it will store their contents directly inside of the container, rather than allocating dynamic memory. begin(), ono. Geeks obj1("Abhi");) all will be well and std::string will do a proper deep copy. If in fact you're experiencing problems when you use the std::string operationalReason = ""; form that may indicate that the string data storage has been corrupted, but it may equally mean that some OTHER part of memory is corrupted and 1) Informs a std::basic_string object of a planned change in size, so that it can manage the storage allocation appropriately. std::string s(5, 'a'); // s == "aaaaa" For generating a string repetition, you'll need some workaround. Just saying. While std::string can make calls ultimately to the dynamic memory allocator, which is not constexpr friendly (unless they made big returns the number of characters (public member function of std::basic_string_view<CharT,Traits>) GetLastError() returns a DWORD a numeric value, but the constructor of std::string doesn't accept a DWORD. begin + s. The interesting corollary question is why isn't the std::string constructor overloaded to work with string literals to handle embedded nul chars? In order to determine the size of the string, the constructor has to increment the pointer until it finds the first '\0'. ==19853== Invalid read of size 4 ==19853== at 0x10000D0E4: handler(std::string const&, std::string const&, std::string const #include <string> string(); string( const string& s ); string( size_type length, const char& ch ); string( const char* str ); string( const char* str, size_type length ); string( const string& str, size_type index, size_type length ); string( input_iterator start, input_iterator end ); ~string(); The string constructors create a new string containing: I'm using std::string and need to left pad them to a given width. I have std::string peppered throughout my entire codebase, so I typedef'd it to use my untracked allocator: typedef std::basic_string<char, std::char_traits<char>, UntrackedAllocator<char>> String; Now when I try to do this: String str { "Some string" }; String copy = str; I get this error: 9. And Table 63 says: data() a non-null pointer that is copyable and can have 0 added to it size() 0 What does string m( message, message + sizeof message / sizeof message[0] ); do?. 73, 3. To properly initialize a union string after you've put something else in the union or not initialized it in the first place, you have to call the constructor directly on that memory: new(&this->union_string) std::string("whatever"); The reason Herb said what he said is because of cases like this. At this point the array size is "lost" because the constructor can only scan for the 1st \0 to determine the size. C++ SSO : How to programatically find if a std::wstring is allocated with Short String Optimization? These changes will break the legal, yet not legitimate case of constructing std::string using basic_string(nullptr, 0); and std::string_view using basic_string_view(nullptr, 0); and thus they were not included into the main text of the proposal. If no allocation has been done it should be safe to set all of these to 0. But still, asserting that there is a difference seems wrong. 4. default (1) explicit basic_string (const allocator_type& alloc = allocator_type()); copy (2) basic_string (const basic_string& str); substring (3) A bit <O/T> based on the OP, but I googled "c++ convert std::array char to string" and it brought me here, yet none of the existing answers deal with std::array<char, . Perhaps the pointer is invalid or the C string does not terminate and the constructor reads off into protected memory. output] need a set of related types and functions to complete the definition of their semantics. Then you can just use it directly in the construction of the string. You then delete that object, and (unsurprisingly) can't do anything with it. unsigned char* uc; std::string s( reinterpret_cast< char const* >(uc) ) ; However, you will need to use the length argument in the constructor if your byte array contains nulls, as if you don't, only part of the array will end up in the string (the Though this array decays into const char* which is passed to the std::string constructor. constexpr basic_string_view(const CharT* s, size_type count); On the other hand, the documentation states also that the user defined literal operator""sv, that in all the implementations I've seen is a simple wrapper to that You're likely seeing the effects of the small/short string optimization (SSO). 1) Informs a std::basic_string object of a planned change in size, so that it can manage the storage allocation appropriately. view. #include <iostream> #include <string> struct Screen { friend std::ostream&a std::string has a constructor that takes a pair of iterators and unsigned char can be converted Constructor signature you need to call is std::string (const char *, size_t), as unsigned is not converted to signed implicitly, passing unsigned char * will introduce ambiguity. You then need to pass this string literal into a std::wstring constructor or you need to convert it to utf-8 and put it in a std:: A string stores chars whose size is guaranteed to be at least 8 bits, so you can use strings for processing e. Plus, it would be really weird if you couldn't construct a filesystem::path from a std::string. classes], 23. What is the recommended way to do this in C++? Sample input: 123 pad to 10 characters. is it possible and how? You can have a conversion function instead, like: std::string to_string(std::wstring const& src); Not arguing memsetting an std::string to 0 is a bad idea, but it might actually work, assuming your std:string implementation does not allocate memory on construction. A does not know or care about C; all A The last one is silly because it doesn't use initialization when it could. Is sso used in any other standard library containers other than std::string? 3. Paying attention to the code your design and coding decisions generate, and consequences of your compiler's optimizations, is just part of being a Constructs a string object, initializing its value depending on the constructor version used: (1) empty string constructor (default constructor) Constructs an empty string, with a length of zero characters. begin + n) == & * s. size ()) In below example for std::string::string. ) There are a couple things the library offers to make the constexpr happen:. The default constructor initializes the string to the empty string. Improve this question. Basically their versions 3. If new_cap is greater than the current capacity(), new storage is allocated, and capacity() is made equal or greater than new_cap. (We used it in production in some C++11 code. Anyway, I see that you're assuming default constructing the std::string will allocate memory once, which would be very unusual, but allowed. (There would be a semantic difference if std::string had a constexpr default constructor, but it doesn't. std::string (and std::wstring) is a string class that provides many operations to assign, compare, and modify strings. Would there be any copy function available that allows a substring to std::string? Example - const char *c = "This is a test string message"; I want to copy substring "test" to std::string. There are a few ways to still accomplish your goal with a C++17 introduces another choice: std::string_view, which replaced std::string in many function signatures, is a non-owning reference to a character data. It doesn't seem to happen with literals like this, but just with char* things. Improve this answer. Use the button in the top-right corner to navigate with arrows for convenience. Like this. calling push_back()) will be more efficient. Memory . 2/1) describes the results of default-constructing a std::basic_string (of which std::string is a specialization) as follows: [] an object of class basic_string. In the first 2 I can use a std::wstring, but I either have to fill it with arbitrary characters '\0' (function 1), or immediately resize it (function 2). size(); int capacity = myVec. If a NULL is encountered, the string copy will end, even if unLength has not been Visual Studio 2005, Release mode, /O2 -- std::string onoc( &ono[ 0 ], ono. Constructs the path from a character sequence provided by source (4), which is a pointer or an input iterator to a null-terminated character/wide character sequence, an std::basic_string or an std::basic_string_view, So you're fine. MyClass::MyClass(int m_size): size(m_size) { vec. 3. It just copies the pointer to the (one) object. I want to create a constructor which takes std::wstring and return a std::string. I know the C way is to do a sprintf, but I'd much rather do a C++ method that is typesafe(er). std::string s0 ("initial string"); . 6k 28 28 gold badges 141 141 silver badges 205 205 bronze badges. Other than that, as long as you have sufficient RAM and/or disk swap space, you can have std::strings of huge size. This is not generally a good practice. Doug T. By using a pointer to std::string You nullify the advantages of implicit manual memory management offered by std::string. std::string was not intended for use as a buffer; you would need to double-check the description of the class to make sure there are no "gotchas" which would prevent certain usage patterns (or make them trigger undefined behavior). I posted about it on the HP boards, and eventually got a response from HP. std::basic_string_view class can be constructed in many different ways. Also note that std::begin and std::end won't work if the buffer object doesn't support them, for example if you use a pointer. (3) substring constructor An easier way is to keep track of the end of the buffer, it's size. Sample output: 123 (7 spaces in template<typename T, int size> struct std::array { T a[size]; }; It is a struct which contains an array. If you hold the objects by value, (e. The first approach is the answer. reserve( 20 ); for ( parsing_something_else_loop ) { char ch = <business Ask your compiler what sizeof(std::string) is - MSVC10 tells me that it's 32 in a debug build and 28 in a release build (this isn't padding - 28 and 32 are both 4 bytes` boundaries). 4 [string. Follow edited Dec 22, 2011 at 20:02. #include <string> #include <iostream> #include <array> int main() { // initialize a char array with "hello\0"; std::array<char, 6> bar{"hello"}; // create a string from it using the . If you stream a UTF8 file into a std::string, you'll end up with a std::string that contains UTF8, with the pros and cons that entails. . If a NULL is encountered, the string copy will end, even if unLength has not been I have a class Screen which has member content initialized using std::string(size_t, char) constructor. But std::string_view's default constructor sets size to zero and null for its data, so you will have to use that constructor instead, or use this workaround/trick: Is there a way of using a std::string_view to initialise compile_time_string_storage?. gcc's implementation is: The Standard (C++11, §21. The offsets of the member this->union_string = std::string("whatever"); Will fail because you're still using the assignment operator. The standard library contains many useful classes -- but perhaps the most useful is std::string. 2. 65. – juanchopanza. 70, 3. message decays to pointer to first element, sizeof message / sizeof message[0] is the number of elements in the array, and message + sizeof message / sizeof message[0] is a pointer to the past the end of the array. It does not have a constructor that takes an initializer list. Pointers to objects are not the same as objects. The other constructors are only considered if the elements in the braced-init-list are not convertible to the type of elements in The problem is almost certainly passing a bad C string into the std::string constructor. , makes it a vector of 20000 pointers. g. basic_string( std::initializer_list<CharT> init, const Allocator& alloc = Allocator() ); That constructor always gets precedence when you use a braced-init-list to construct std::string. This means that a pointer to an element of a string may be passed to any function that expects a pointer to an element of an array of characters. Other words, copies the other string starting from element at pos index. And A passes a string through B and into C. Warning - many of the solutions presented here introduce a subtle TOCTOU issue: If the global locale is changed in another thread between a [v]snprintf call to determine the buffer size, and another call to write to the buffer, a truncated output is possible. string::string(const string& strString, size_type unIndex) string::string(const string& strString, size_type unIndex, size_type unLength) This constructor creates a new string that contains at most unLength characters from strString, starting with index unIndex. For times when you want to optimize to a more efficient type, make those arguments in the comments. Is . Consider using [v]snprintf_l or [v]asprintf if available. In the past, I've used stringstream to do a conversion, and that's just kind of cumbersome. In theory std::string c'tor could have an overload for arrays, that would allow embedded \0 in string literal. The postconditions [] are indicated in Table 63. 2 Most classes specified in 23. Commented Aug 3, 2014 at 20:52. Follow answered Jan 26, 2012 at 14:12. In other words, there's no alternative in the STL for strncpy. Create or copy a string Destroy a string; Size and capacity; capacity() empty() length(), size() max_size() Constructors A class that is used to store and manipulate sequence of characters. >:. It does seem to work functionally, but when I did this I started getting issues with Valgrind reporting reachable blocks at the end of the program, originating from a "new" inside of = (and +=). Acknowledgements. data() ptr, // this uses the I completely agree about avoiding the C string version if posible. std::string has a lot of constructors and it would be troublesome to have to re-implement all of them in your class. Simply input the length L, character C, and initialize the string object with default values. 2: basic_string(const charT* s, const Allocator& a = Allocator()); 9 Effects: Constructs an object of class basic_string and determines its initial string value from the array of charT of length traits::length(s) whose first element is designated by s. GetValue(); My question is if GetValu Taking a look at the constructor reference of basic_string, one can see that there is no easy way of repeating a complete string. view], [fixed. (Similarly, streaming a UTF16 file into a std::string produces garbage, but std::wstring would be valid, at least on Windows) – The standard library contains many useful classes -- but perhaps the most useful is std::string. The issue of whether such reports are actually leaks are discussed here. std::format is, of course, the modern solution - if you can Isn't that the point to reserve() size so you can access it? No, that's the point of resize(). Your const char* c-style string will get implicitly cast to const string for the first parameter. Needless to say this would be non-portable. Since the problem is the non-trivial destructor so if the destructor is removed from the std::string, it's possible to define a constexpr instance of that type. One interesting this to note is that with small objects, where Small Object Optimization exists, a move constructor is implemented as a copy constructor. Below is the implementation of For the same reason that make_unique() is faster then a new call and calling the constructor are you thinking of make_shared, because there should be no difference with make_unique. Use this when you don't know the size in advance and you don't even have an estimate for the size. size()) of other. The latter leaves the vector empty, but reserves space for 20000 pointers, so you can insert (up to) that many without it having to reallocate. I would not usually do this, but I will take pity. ) of aCC have messed up std::string construction. reserve( 20 ); for ( parsing_something_else_loop ) { char ch = <business About the best you can do, short of implementing a lot of the functionality of std::string is to use a fixed size char array. If you are storing size you should use std::size_t which is designed for sizes. It is bad practice to use std::string as a buffer, for several reasons (listed in no particular order):. (2) copy constructor Constructs a copy of str. what is the meaning of this std::string constructor. To avoid unnecessary allocations for every tiny little string, many implementations of std::string include a small fixed size array to hold small strings without requiring new (this array usually repurposes some of the other members that aren't necessary when dynamic allocation has not The problem is the std::string constructor that takes a const char* assumes the input is a C // or using namespace std::literals::string_view_literals; auto sv = "a\0b"sv; auto s = std::string{sv}; std::cout << s. This also allows a std::string object with arbitrary data, including the null-terminator embedded in the middle of the data. 1 General. my_string. and. cplusplus. And that happens to be one inside of the array. 3 [string. A string is mutable and it's length can changed at run-time. size(); // 3 It may be more useful if you need the view to use later, otherwise just construct the string directly with the ""s String will deep copy, they do not shared the same buffer. e. – Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company val - number used to initialize the bitset str - string used to initialize the bitset pos - a starting offset into str: n - number of characters to use from str: zero Types are important in C++. 2 Character traits 9. But std::array is an aggregate by the rules of C++11, and therefore it can be Problem: I have an integer; this integer needs to be converted to a stl::string type. size ()), and * (s. For example: template < class T, std::size_t N > constexpr std::size_t size( const T (&array)[N] ) { return N; } – Constructor std::vector<MyType> myVec(numberOfElementsToStart); int size = myVec. std::string s3 (s0, 8, 3); . 10 Remarks: Uses traits::length(). size() );: 19 lines of assembly, 1 constructor call std::string onos( ono. You just needed to cast the unsigned char into a char as the string class doesn't have a constructor that accepts unsigned char:. == std::to_address(first) size() == last - first; Undefined Behavior. But if I changed the assign to string::string(const string& strString, size_type unIndex) string::string(const string& strString, size_type unIndex, size_type unLength) This constructor creates a new string that contains at most unLength characters from strString, starting with index unIndex. std::string s5 Just use str. That's why we don't see any allocations in either case. capacity(); In this first case, using the constructor, size and numberOfElementsToStart will be equal and capacity will I am not able to understand the differences between std::string and std::wstring. One way to do this kind of stuff is to use the frozen library, which works in C++14, and parts of it work in C++11. – @ZlatanRadovanovic You are, but the initializer list constructor for std::string takes a std::initializer_list<char>, while {"abc", "bcd"} would be a sdtd::initializer_list<const char*> so you are not using strings initializer list constructor but instead are getting its iterator pair constructor. std::string_view constructors. But you can use the "fill constructor" if you must have a specified length: http://www. For a single character, you could use (2) like this:. This of course is purely semantic, as the data structure’s fields need to be copied A class that is used to store and manipulate sequence of characters. C++ SSO : How to programatically find if a std::wstring is allocated with Short String Optimization? std::vector<CustomClass *> whatever(20000); or: std::vector<CustomClass *> whatever; whatever. Check with proper compiler, or see, for example, Isn't that the point to reserve() size so you can access it? No, that's the point of resize(). 3. Well, yes and no. Mark B Mark B You could also let the constructor take in a parameter instead of hardcoding the string and let the user pass the string at run-time: NoName(std::string str):mstring(str){} You do not need a pointer. std::string s2 (s0); . ; To have the The size could be stored as a std::string::size_type or as a pointer to the end. Your problem is that you seem to want a number of things that cannot all go together. From your use case it looks like you should use . if you stream a UTF8 file into a std::wstring, then you end up with garbage. This is the more economic way of saying the same thing. std::string s1; . causes the compiler to allocate a 2 byte array fixed-size array and fill it with "0\0" (ASCII 0 While std::string has the size of 24 bytes, it allows strings up to 22 bytes(!!) with no allocation. In C++11 you'd be able to put the brace initialization in the constructor using the initializer list constructor. What can I do? c++; numeric; stdstring; Share. end() );: 30 lines of assembly, 2 constructor calls. begin + n for any n in [ 0 , s. However, the comparison to NULL stinks. Dealing with raw cstrings will only get you into trouble these days. Note that this is unnecessarily convoluted way to get the Given const void * data = ; size_t size = ; std::string message(???) How to construct std::string from raw pointer to data and size of the data? data may contain NUL characters. assign(buffer, size); String constructors Syntax: #include <string> string(); string( const string& s ); string( size_type length, const char& ch ); string( const char* str ); string( const char* str, size_type length ); In this method, the constructor of std::string is being used. That said when returning them from a function most good compilers can either use Return Value Optimisation or Copy elision so that manoeuvre isn't all that expensive (or even free). Don't use std::string as a buffer. So you can get rid of the size member: Since the input to the constructor of std:string does not meet that criterion, you are entering undefined behavior territory. vec = vector< std::string >( str, str + size ); But this still feels a little "inelegant". These 3 functions all work. std::string s4 ("A character sequence", 6); . string], and 31 [input. You cannot pass a null pointer to std::string_view's constructors, because that will violate their preconditions; see constructor 2 and 3 in [string. So what's happening? Variable s is initialized using the debug version of the copy constructor to copy a release version of std::string. Anything could happen after that. cons]. So prefer this. The elements of a string are stored contiguously in memory. Further read: std::vector and std::string reallocation strategy. The first two are completely identical semantically (think of the c_str() member function), so prefer the first version because it is the most direct and idiomatic, and easiest to read. c++; stl; Share. @juanchopanza Note that until C++11, the 1000 entries According to the documentation, std::string_view has a constructor that takes a const char * and a std::size_t, that is not declared noexcept:. xx (3. The author would like to thank Antony Poloukhin, Marshall Clow and Eric Fiselier for a thorough review and suggestions. The answer to your second question is yes, you can send record by record, moreover you might not be able to send big chunks of data The size could be stored as a std::string::size_type or as a pointer to the end. In function 3 I instead use a vector and then use that as the constructor for the In C++11 a superior method of determining the size of a built-in array is using a constexpr template function. push_back() instead. Although I could not reproduce the exact bug of the OP, I came across a similar bug in the HP-UX aCC compilers. ASCII, ISO This is an intentional decision in libc++'s implementation of std::string. reserve() only gives to enough room so that future call that leads to increase of the size (e. 1. What you want is this constructor: std::string ( const string& str, size_t pos, size_t n = npos ), passing pos as 0. There are several bugs in your bufferToCString, including overwriting unknown memory by using bufsize + 1, and assuming your output buffer 'str' is bufsize+1 of the input buffer. I was working on a little project and came to a situation where the following happened: std::string myString; #GetValue() returns a char* myString = myObject. Create or copy a string Destroy a string; Size and capacity; capacity() empty() length(), size() max_size() Constructors The problem is almost certainly passing a bad C string into the std::string constructor. std::string::max_size() will tell you the theoretical limit imposed by the architecture your program is running under. Share. size_type pos, size_type count, Constructs the string Constructs a string object, initializing its value depending on the constructor version used: (1) empty string constructor (default constructor) Constructs an empty string, with a length of zero string::string(const string& strString, size_type unIndex) string::string(const string& strString, size_type unIndex, size_type unLength) This constructor creates a new string that The elements of a basic_string are stored contiguously, that is, for a basic_string s, & * (s. com/reference/string/string/string/ std::string s6 (10, 'x'); s6 now equals Constructs the string with a substring [pos, other. reserve( size ); } If you want your vector to have size elements, then: MyClass::MyClass(int m_size): size(m_size), vec(m_size, 0) {} Finally, as one of the commenters points out, size is not actually needed once the vector has been constructed. This means that it doesn't know anything about null termination (well almost, there are conversions from/to C strings). That is an older syntax still in common use that means something else; a null pointer. The std::string class in STL can contain null characters within the string ("xxx\0yyy" is a perfectly valid string of length 7). ==19853== Invalid read of size 4 ==19853== at 0x10000D0E4: handler(std::string const&, std::string const&, std::string const The default constructor initializes the string to the empty string. Let's say I have a function A which calls function B, which calls function C. reserve(20000); The former sets the actual size of the array -- i. You want: To create a compile_time_string_storage from a constexpr string_view without specifying the size explicitly as a template parameter. 67, etc. Per standard §21. struct constexpr_str { char const* str; std::size_t size; // can only construct from a char[] literal template <std::size_t N> constexpr constexpr_str(char const (&s)[N]) : str(s) , size(N - 1) // not count the trailing nul Can I overload the std::string constructor? Nope, it would require changing std::string declaration. amj idu twsolz ymo whvpvfa hvaz lzuuf gznte dwnnm aupc