C++ Equivalent of strncpy

What is the equivalent of strncpy in C++? strncpy works in C, but fails in C++.

This is the code I am attempting:

string str1 = "hello"; 
stri开发者_运维百科ng str2; 
strncpy (str2,str1,5);

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The equivalent of C's strncpy() (which, BTW, is spelled std::strncpy() in C++, and is to found in the header <cstring>) is std::string's assignment operator:

std::string s1 = "Hello, world!";
std::string s2(s1);            // copy-construction, equivalent to strcpy
std::string s3 = s1;           // copy-construction, equivalent to strcpy, too
std::string s4(s1, 0, 5);      // copy-construction, taking 5 chars from pos 0, 
                               // equivalent to strncpy
std::string s5(s1.c_str(), std::min(5,s1.size())); 
                               // copy-construction, equivalent to strncpy
s5 = s1;                       // assignment, equivalent to strcpy
s5.assign(s1, 5);              // assignment, equivalent to strncpy

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You can use basic_string::copy on a std::string version of your const char*, or use std::copy, which can use pointers as the input iterators.

What do you mean by "strncpy fails in C++", by the way?

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strncpy works on characters array, and it works just fine in C++ as well as in C.
If you are using strncpy with characters array in C++, and it doesn't work, it's probably because of some error in your C++ code: show us if you want some help.

If what you are looking for is a way to copy strings (ie: std::string), string's copy constructor or assignment operator will do what you're looking for:

std::string a = "hello!";

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This will copy the whole string without any risk of buffer overflow. Anyway, as John Dibling says in the comments, it doesn't supply the same semantics of strncpy: it doesn't let you specify how many characters to copy.

If you need to copy up to a certain number of characters there are other ways: std::string offers a constructor that copier up to n characters, or you could use the other ways proposed in other answers.

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strncpy does not take std::string as arguments. The prototype is

char * strncpy ( char * destination, const char * source, size_t num );

and the provided link gives a description and example on how to use it.

I would stick with std::string but if you have to, then use c_str() std::string method to obtain the char * pointer

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strncpy is available from the cstring header in C++. Because it is C specific function with you have to deal with C kind of strings only, that is character arrays, not the string objects of C++

#include<cstring>
using namespace std;
int main(int argc, char *argv)
{
   char *s,*v;
   strncpy(s,v,0);
}

The above will compile properly, but will serve no useful purpose. When you are using C++, try to use the string objects and its copy method of assignment operator to assign to a different string. Copy method takes a parameter where you can specify the number of characters to copy to the target string object.

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The equivalent of strncpy in C++ is strncpy.

You're just doing something wrong in your code, please post your code and the error.