How to copy an array of characters to a string C++

Question

I am writing a function that takes in a string through pass by reference and an array of characters and the size of the array. The string already has characters in it. I am trying to erase the characters in the string and copy the characters from the array. I tried setting up a for loop to copy, but it doesn't work if the string is too small or too big. I can't use strcopy in this function. Need some guidance.

void functionname(string &first, char arr[], int size) {

int i;

    (for i = 0; i < size; i++) {
    first[i] = arr[i];
    }

}

What doesn't work about it? Are you getting errors? What are they? — David G
– David G, Commented Oct 30, 2014 at 2:42
FWIW, you code isn't reliable when the incoming value of first has .size() less than the size argument. Say first is "abc", arr[] is VWXYZ and size is 5... you copy VWX over abc - which is allowed - but then try to copy Z over the first[4] which is (as of C++11) guaranteed to hold a NUL but isn't legal to write to (for C++03 it may not be a NUL and is also undefined behaviour to write), then of course writing the final Z is beyond even any possible NUL and even in C++11 the indexing operation is undefined behaviour, let alone the write.... — Tony Delroy
– Tony Delroy, Commented Oct 30, 2014 at 3:13

thor · Accepted Answer · 2014-10-30 02:42:47Z

2

You can use std::string's default assignment operator = and simply do

first = arr;

answered Oct 30, 2014 at 2:42

thor

22.7k33 gold badges108 silver badges201 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Tony Delroy Over a year ago

just a note: this works if arr holds ASCIIZ text (i.e. textual character terminated by a NUL/0 character that you don't consider part of the text/value), while zzz777's answer happily sets the string to contain the first size characters from the array even if some/all are NUL.

uuu777 · Accepted Answer · 2014-10-30 02:52:57Z

2

std::string has function to do it first.assign(arr, size)

answered Oct 30, 2014 at 2:52

uuu777

8911 gold badge6 silver badges26 bronze badges

Comments

Paul Rooney · Accepted Answer · 2014-10-30 02:53:06Z

1

The below works as std::string overrides the assignment operator (=) for char* to allow direct assignment to the string from a character pointer.

#include <iostream>
#include <string>
#include <cstring>

using namespace std;

void functionname(string &first, char arr[], int size) 
{
    first = arr;
}

int main()
{
    std::string x = "mystring";
    char buff[x.length()];
    strcpy(buff, "other");

    cout << x << endl;
    functionname(x, buff, x.length());

    cout << x << endl;

    return 0;
}

answered Oct 30, 2014 at 2:53

Paul Rooney

21.7k9 gold badges47 silver badges64 bronze badges

Comments

Ryan Henning · Accepted Answer · 2020-09-03 02:08:46Z

0

string has a constructor that copies a character array and takes a count. This will work no matter what contents are inside the arr array, even embedded null characters.

void functionname(string &first, char arr[], int size) {

    first = string(arr, size);

}

Again, the contents of arr are copied into the string, you don't need to keep arr around after this if you don't need it anymore.

edited Sep 3, 2020 at 2:08

answered Nov 3, 2014 at 4:33

Ryan Henning

1364 bronze badges

Collectives™ on Stack Overflow

How to copy an array of characters to a string C++

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related