C ++相当于strncpy

Question

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"; 
string str2; 
strncpy (str2,str1,5);

Answer 1

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

Answer 2

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?

Answer 3

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!";

---

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.

Answer 4

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.

Answer 5

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

Answer 6

There is no built-in equivalent. You have to roll your own strncpy.

#include <cstring>
#include <string>

std::string strncpy(const char* str, const size_t n)
{
    if (str == NULL || n == 0)
    {
        return std::string();
    }

    return std::string(str, std::min(std::strlen(str), n));
}

Answer 7

The equivalent of strncpy in C++ is strncpy .

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