c string assignment copy

• 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 Visit the blog

Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Get early access and see previews of new features.

How to copy a string of std::string type in C++?

I used the strcpy() function and it only works if I use C-string arrays like:

but whenever I use

I get this error:

functions.cpp: no matching function for call to strcpy(std::string&, std::string&)

How to copy 2 strings of string data type in C++?

• 17 Just a=b; :D ____ –  PiotrNycz Commented Oct 1, 2012 at 18:24
• You should use the std::string type and not use strcpy but copy assignment. –  Basile Starynkevitch Commented Oct 1, 2012 at 18:24

4 Answers 4

You shouldn't use strcpy() to copy a std::string , only use it for C-Style strings .

If you want to copy a to b then just use the = operator.

• and what of strcat () ? is it a=a+"image"; ? –  Muhammad Arslan Jamshaid Commented Oct 1, 2012 at 18:28
• 7 for strcat you can use a+="image" –  Caesar Commented Oct 1, 2012 at 18:30
• 1 All of strcpy, strcat, strtok etc. are functions of string.h which operator only on c strings i.e. strings that are mere character arrays terminated by a null. If for convenience you have a string object and you still want to use some string.h function, you can call str.c_str() on the string object which basically returns a c string for the contents of the object. However, this is seldom needed because string object has a pretty good api –  fkl Commented Oct 1, 2012 at 19:17
• @Caesar, please do answer this : link ; Thanks. –  uss Commented Oct 7, 2015 at 12:29
• If for example I would wish for a copy of string that was "on the stack" (ie. say passed by function call) to be put into a struct that lives on the heap, will this still hold? –  Samarth Ramesh Commented Oct 3, 2021 at 19:58

strcpy is only for C strings. For std::string you copy it like any C++ object.

If you want to concatenate strings you can use the + operator:

You can even do many at once:

Although "hello" + " world" doesn't work as you might expect. You need an explicit std::string to be in there: std::string("Hello") + "world"

• and what is the alternative of strcat for std::string ? –  Muhammad Arslan Jamshaid Commented Oct 1, 2012 at 18:28
• 1 @Programer You can combine strings using + : std::string a = "text"; std::string b = "image"; a = a+b; –  bames53 Commented Oct 1, 2012 at 18:29
• 1 @PiotrNycz He asks for copying without specifying copy assignment or copy construction, probably because that distinction doesn't make sense for C strings. New C++ programmers typically don't understand the difference between initialization and assignment anyway, and the syntax is deliberately similar. I don't see any reason for the distinction to matter here. –  bames53 Commented Oct 1, 2012 at 18:34
• @bames53 OP has two string objects of two distinct values, then wants to overwrite one sting by value of other, This is assignment. But this is your answer not mine. I just thought you overlooked this. –  PiotrNycz Commented Oct 1, 2012 at 18:38
• 1 @mLstudent33 No, the std::string copy is entirely independent. –  bames53 Commented Jun 5, 2020 at 7:05

strcpy example:

Output: str1: Sample string

A simple = operator should do the job.

• does that mean it's not possible to cast from one type string to other type? –  user1710917 Commented Oct 2, 2012 at 21:46

Caesar 's solution is the best in my opinion, but if you still insist to use the strcpy function, then after you have your strings ready:

You can try either:

Just call either the data() or c_str() member functions of the std::string class, to get the char* pointer of the string object.

The strcpy() function doesn't have overload to accept two std::string objects as parameters. It has only one overload to accept two char* pointers as parameters.

Both data and c_str return what does strcpy() want exactly.

• " It has only one overload". Is this really true? Isn't it the case that it's no overloads? Or that's how I understood the concept of overload. –  RichieHH Commented Aug 26, 2021 at 2:21
• Should you have the urge to use this, take a cold shower and just don't. The size of the underlying buffer is only guaranteed to be the size of the c-string. Fitting "image" into "text" might work without causing horrible things to happen, but WILL result in the string "imag", since the length of std::string is stored and not evaluated by the terminating character. Plus the second variant will probably not even compile, because c_str() returns a const char * . –  pogojotz Commented Aug 16, 2022 at 12:03

Your Answer

Reminder: Answers generated by artificial intelligence tools are not allowed on Stack Overflow. Learn more

Sign up or log in

Post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .

Not the answer you're looking for? Browse other questions tagged c++ string copy or ask your own question .

• The Overflow Blog
• Community Products Roadmap Update, July 2024
• Featured on Meta
• We spent a sprint addressing your requests — here’s how it went
• Upcoming initiatives on Stack Overflow and across the Stack Exchange network...
• Policy: Generative AI (e.g., ChatGPT) is banned
• The [lib] tag is being burninated
• What makes a homepage useful for logged-in users

Hot Network Questions

• Can you Constrain the Noise Modifier?
• Don't make noise. OR Don't make a noise
• Siunitx: spread table content accross page
• Do you always experience the gravitational influence of other mass as you see them in your frame?
• Are US enlisted personnel (as opposed to officers) required, or allowed, to disobey unlawful orders?
• Questions about mail-in ballot
• Everything has a tiny nuclear reactor in it. How much of a concern are illegal nuclear bombs?
• How can I align things in a multiline equation (ConTeXt)
• Optimizing Pi Estimation Code
• Read an article about connecting a hot tub to a heat pump, looking for details
• How do I drill a 60cm hole in a tree stump, 4.4 cm wide?
• Does Salesforce Pro Suite allows you to do apex?
• Should I ask permission from my former PI after finishing my contract when I'm sole author of the paper?
• Product of expectation of two random variables
• How shall I find the device of a phone's storage so that I can mount it in Linux?
• Connect multiple menu switch in Geometry Nodes
• What is the meaning of @ and & in PennyLane?
• Can you access the list of network devices from the command line?
• Reduce the column padding in tabular environment
• Why does independent research from people without formal academic qualifications generally turn out to be a complete waste of time?
• Sci-fi book series about a teenage detective, possibly named Diamond, with a floating AI assistant
• Understanding Polynomial Rolling Hash Function by Modular Arithmetic
• Plane to train in Copenhagen
• Can a country refuse to deliver a person accused of attempted murder?

• Data Structures
• Linked List
• Binary Tree
• Binary Search Tree
• Segment Tree
• Disjoint Set Union
• Fenwick Tree
• Red-Black Tree
• Advanced Data Structures

Different ways to copy a string in C/C++

Copying a string is a common operation in C/C++ used to create a duplicate copy of the original string. In this article, we will see how to copy strings in C/C++.

Methods to Copy a String in C/C++

1. using strcpy().

We can use the inbuilt function strcpy() from <string.h> header file to copy one string to the other. strcpy() accepts a pointer to the destination array and source array as a parameter and after copying it returns a pointer to the destination string.

Example: Program to copy the string using strcpy() function

Complexity Analysis

• Time Complexity : O(N)
• Auxiliary Space : O(1)

2. Using memcpy()

The memcpy () function is also used to copy from source to destination no matter what the source data contains and is defined in <string.h> header and memcpy() require a size parameter to be passed.

The main difference is that memcpy() always copies the exact number of specified bytes. strcpy() and other str methods, on the other hand, will copy until it reads a NULL (‘\0’) byte, and then stop after that. strcpy() is not intended to be used with zero-terminated C-strings.

memcpy() is hardware optimized and copies faster and works with any type of source data (like binary or encrypted bytes). strcpy() should never be used unless for any specific reason, and if you know the lengths of the strings, memcpy() is a better choice .

Example: Program to copy the string using memcpy function

3. Using Loops

The idea is to use a for loop to copy the content of the first character array to the second character array one by one.

Example: Program to copy string using loops

4. Using Pointers

The idea is to copy the contents of the string array to another array using pointers and print the resultant string by traversing the new pointer. We run a loop until the null character ‘\0’ is found in the source string and copy each character from the source string to the destination string using the dereferencing operator *.

Example: Program to copy the string using pointers

5. Using Pointers and Post-Increment Operator

The idea is to use a while loop to assign the content of string array1 to string array2 one by one and increment using the post-increment operator to move to the next character. The loop continues until the null character ‘\0’ is found.

Example: Program to copy a string using pointers and post-increment operator

6. Using sprintf()

Instead of printing the string in the output buffer, we can store it in a specified char buffer or destination string in sprintf () to copy the string.

Example: Program to copy the string using sprintf function

7. strncpy()

We can use the inbuilt function strncpy() from <string.h> header file to copy one string to the other. The strcnpy() function accepts a pointer to the destination array and source array as a parameter and the maximum number of characters to be copied and after copying, it returns a pointer to the destination string.

Example: Program to copy the string using strncpy function

8. Using strdup()

We can use the inbuilt function strdup() from the string.h header file to duplicate a string by allocating memory for a copy of the string using malloc, and then copying the string into the newly allocated memory. The function returns a pointer to the newly allocated copy of the string.

Example: Program to copy the string using strdup function

• Auxiliary Space : O(N)

9. Using strndup()

The strndup() function is similar to strdup(), but it can copy at most n bytes.

Example: Program to copy the string using strndup function

10. Using std::string class (=) Operator

The easiest way to copy a string is to use the assignment operator (=) of the std::string class to copy the contents of one string to another.

Note : There are no inbuilt classes in C, hence ‘=’ operator is only available in C++.

Example: Program to copy the string using std::string class (=) Operator

11. Using std::string::assign() Method

We can use the std::string::assign() method to copy a string. This method takes a string as an argument and assigns its value to the string object on which the method is called.

Note : This method is only available in C++, not in C.

Example: Program to copy the string using std::string::assign() method

12. Using memmove() Function

We can use memmove() function to copy a memory block from one location to another. It is declared in <string.h> header file.

Example: Program to copy the string using memmove() function.

13. Using strcat() Function

We can use strcat() function to append a copy of the source string in the destination string and a terminating Null character. It is declared in string.h (For C) and cstring(For C++) header file.

Note : The strcat() function can be used to copy the source string to a destination string only if the destination string is empty, as the strcat function is used to concatenate one string at the end of the another string.

Example: Program to copy the string using strcat() function

14. Using strncat() Function

We can use strncat() function to append not more than n characters from the string pointed to by src to the end of the string pointed to by dest plus a terminating Null-character. It is declared in string.h (For C) and cstring(For C++) header file.

Note : The strcat() function can be used to copy the source string to a destination string only if the destination string is empty, as the strcat function is used to concatenate a specified number of characters from one string at the end of the another string.

Example: Program to copy the string using strncat() function

15. Using std::string::c_str()

std::string::c_str() is a member function of the C++ string class which returns a pointer to an array of characters (null-terminated) containing the same sequence of characters as the string object. The returned pointer is valid until the string object is destroyed or modified. This method is used to copy the contents of the string object into a character array, which can be used to access the contents of the string.

Example: Program to copy the string using std::string::c_str() method

• Time Complexity : O(1)

16. Using std::string::append() Method

std::string::append() is a C++ method used to copy a string into another string. It takes the string to be copied as an argument and appends it at the end of the calling string.

Note : This function is only available in C++. In C, the strcat() function can be used to copy a string into another string.

Example: Program to copy the string using std::string::append() method

17. Using strtok() Method

The strtok() method is used in C/C++ to tokenize a string. It takes two parameters, the first is a pointer to the string to be tokenized, and the second is a pointer to a set of delimiting characters. The method then returns a pointer to the token found in the string, which can then be used to copy it.

Example: Program to copy the string using strtok() method

Note: In all the above methods the size of the destination array must be greater than the length of the source string to copy all the characters.

Please Login to comment...

Similar reads.

• cpp-strings-library
• Technical Scripter 2020
• Program Output
• Technical Scripter

Improve your Coding Skills with Practice

What kind of Experience do you want to share?

• <cassert> (assert.h)
• <cctype> (ctype.h)
• <cerrno> (errno.h)
• C++11 <cfenv> (fenv.h)
• <cfloat> (float.h)
• C++11 <cinttypes> (inttypes.h)
• <ciso646> (iso646.h)
• <climits> (limits.h)
• <clocale> (locale.h)
• <cmath> (math.h)
• <csetjmp> (setjmp.h)
• <csignal> (signal.h)
• <cstdarg> (stdarg.h)
• C++11 <cstdbool> (stdbool.h)
• <cstddef> (stddef.h)
• C++11 <cstdint> (stdint.h)
• <cstdio> (stdio.h)
• <cstdlib> (stdlib.h)
• <cstring> (string.h)
• C++11 <ctgmath> (tgmath.h)
• <ctime> (time.h)
• C++11 <cuchar> (uchar.h)
• <cwchar> (wchar.h)
• <cwctype> (wctype.h)

Containers:

• C++11 <array>
• <deque>
• C++11 <forward_list>
• <list>
• <map>
• <queue>
• <set>
• <stack>
• C++11 <unordered_map>
• C++11 <unordered_set>
• <vector>

Input/Output:

• <fstream>
• <iomanip>
• <ios>
• <iosfwd>
• <iostream>
• <istream>
• <ostream>
• <sstream>
• <streambuf>

Multi-threading:

• C++11 <atomic>
• C++11 <condition_variable>
• C++11 <future>
• C++11 <mutex>
• C++11 <thread>
• <algorithm>
• <bitset>
• C++11 <chrono>
• C++11 <codecvt>
• <complex>
• <exception>
• <functional>
• C++11 <initializer_list>
• <iterator>
• <limits>
• <locale>
• <memory>
• <new>
• <numeric>
• C++11 <random>
• C++11 <ratio>
• C++11 <regex>
• <stdexcept>
• <string>
• C++11 <system_error>
• C++11 <tuple>
• C++11 <type_traits>
• C++11 <typeindex>
• <typeinfo>
• <utility>
• <valarray>

class templates

• basic_string
• char_traits
• C++11 u16string
• C++11 u32string
• C++11 stold
• C++11 stoll
• C++11 stoul
• C++11 stoull
• C++11 to_string
• C++11 to_wstring
• string::~string
• string::string

member functions

• string::append
• string::assign
• C++11 string::back
• string::begin
• string::c_str
• string::capacity
• C++11 string::cbegin
• C++11 string::cend
• string::clear
• string::compare
• string::copy
• C++11 string::crbegin
• C++11 string::crend
• string::data
• string::empty
• string::end
• string::erase
• string::find
• string::find_first_not_of
• string::find_first_of
• string::find_last_not_of
• string::find_last_of
• C++11 string::front
• string::get_allocator
• string::insert
• string::length
• string::max_size
• string::operator[]
• string::operator+=
• string::operator=
• C++11 string::pop_back
• string::push_back
• string::rbegin
• string::rend
• string::replace
• string::reserve
• string::resize
• string::rfind
• C++11 string::shrink_to_fit
• string::size
• string::substr
• string::swap

member constants

• string::npos

non-member overloads

• getline (string)
• operator+ (string)
• operator<< (string)
• >/" title="operator>> (string)"> operator>> (string)
• relational operators (string)
• swap (string)

std:: string ::assign

Return Value

Iterator validity

Exception safety.

22.5 — std::string assignment and swapping

Learn C practically and Get Certified .

Popular Tutorials

Popular examples, reference materials, learn c interactively.

Sort Elements in Lexicographical Order (Dictionary Order)

Copy String Without Using strcpy()

• Concatenate Two Strings

Find the Length of a String

Remove all Characters in a String Except Alphabets

• Count the Number of Vowels, Consonants and so on

Find the Frequency of Characters in a String

C Tutorials

• String Manipulations In C Programming Using Library Functions
• C Programming Strings

C Program to Copy String Without Using strcpy()

To understand this example, you should have the knowledge of the following C programming topics:

As you know, the best way to copy a string is by using the strcpy() function. However, in this example, we will copy a string manually without using the strcpy() function.

The above program copies the content of string s1 to string s2 manually.

Sorry about that.

Related Examples

cppreference.com

Std::basic_string<chart,traits,allocator>:: assign.

Replaces the contents of the string.

[ edit ] Parameters

[ edit ] Return value

[ edit ] complexity, [ edit ] exceptions.

              propagate_on_container_move_assignment :: value ||

If the operation would result in size ( ) >   max_size ( ) , throws std::length_error .

If an exception is thrown for any reason, this function has no effect ( strong exception safety guarantee ).

[ edit ] Example

[ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

• conditionally noexcept
• Recent changes
• Offline version
• What links here
• Related changes
• Upload file
• Special pages
• Printable version
• Permanent link
• Page information
• In other languages
• This page was last modified on 29 May 2024, at 15:49.
• Privacy policy
• About cppreference.com
• Disclaimers

Copy assignment operator

A copy assignment operator of class T is a non-template non-static member function with the name operator = that takes exactly one parameter of type T , T & , const T & , volatile T & , or const volatile T & . For a type to be CopyAssignable , it must have a public copy assignment operator.

Explanation

• Typical declaration of a copy assignment operator when copy-and-swap idiom can be used.
• Typical declaration of a copy assignment operator when copy-and-swap idiom cannot be used (non-swappable type or degraded performance).
• Forcing a copy assignment operator to be generated by the compiler.
• Avoiding implicit copy assignment.

The copy assignment operator is called whenever selected by overload resolution , e.g. when an object appears on the left side of an assignment expression.

Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type ( struct , class , or union ), the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T & T :: operator = ( const T & ) if all of the following is true:

• each direct base B of T has a copy assignment operator whose parameters are B or const B & or const volatile B & ;
• each non-static data member M of T of class type or array of class type has a copy assignment operator whose parameters are M or const M & or const volatile M & .

Otherwise the implicitly-declared copy assignment operator is declared as T & T :: operator = ( T & ) . (Note that due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.)

A class can have multiple copy assignment operators, e.g. both T & T :: operator = ( const T & ) and T & T :: operator = ( T ) . If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword default . (since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification (until C++17) exception specification (since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

Deleted implicitly-declared copy assignment operator

A implicitly-declared copy assignment operator for class T is defined as deleted if any of the following is true:

• T has a user-declared move constructor;
• T has a user-declared move assignment operator.

Otherwise, it is defined as defaulted.

A defaulted copy assignment operator for class T is defined as deleted if any of the following is true:

• T has a non-static data member of non-class type (or array thereof) that is const ;
• T has a non-static data member of a reference type;
• T has a non-static data member or a direct or virtual base class that cannot be copy-assigned (overload resolution for the copy assignment fails, or selects a deleted or inaccessible function);
• T is a union-like class , and has a variant member whose corresponding assignment operator is non-trivial.

Trivial copy assignment operator

The copy assignment operator for class T is trivial if all of the following is true:

• it is not user-provided (meaning, it is implicitly-defined or defaulted) , , and if it is defaulted, its signature is the same as implicitly-defined (until C++14) ;
• T has no virtual member functions;
• T has no virtual base classes;
• the copy assignment operator selected for every direct base of T is trivial;
• the copy assignment operator selected for every non-static class type (or array of class type) member of T is trivial;

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove . All data types compatible with the C language (POD types) are trivially copy-assignable.

Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used . For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove ). For non-union class types ( class and struct ), the operator performs member-wise copy assignment of the object's bases and non-static members, in their initialization order, using built-in assignment for the scalars and copy assignment operator for class types.

The generation of the implicitly-defined copy assignment operator is deprecated (since C++11) if T has a user-declared destructor or user-declared copy constructor.

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move ), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment ).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.