CODEWITHSUNDEEP
c++arraysstringcopystrncpy
user2537688
user2537688

Reputation: 87

Copy length of characters from array to std::string

I am trying to copy 5 characters from a character array into a std::string

char name[] = "Sally Magee";
std::string first;
copy(name, name + 5, first.begin()); //from #include <algorithm>
std::cout << first.c_str();

However I get the string plus a whole bunch of unprintable characters that I do not want. Any ideas? Thanks.

Upvotes: 5

Views: 9904

Answers (2)

jogojapan
jogojapan

Reputation: 69967

What the std::copy algorithm does is to copy one source element after the other, and advance the destination iterator after each element.

This assumes that

  • either the size of the destination container has been set large enough to fit all the elements you copy,
  • or you use an iterator type that increases the size of the destination container each time you make an assignment to it.

Therefore, if you want to use the std::copy algorithm, there are two ways of solving this:

  1. Resize the string before making the copies:

    #include <iostream>
#include <string>
#include <algorithm>

int main()
{
  char source[] = "hello world";

  std::string dest;
  dest.resize(5);
  std::copy(source,source+5,begin(dest));

  std::cout << dest << std::endl;

  return 0;
}

  2. Using a back-insert iterator instead of the standard one:

    #include <iostream>
#include <string>
#include <algorithm>
#include <iterator>

int main()
{
  char source[] = "hello world";

  std::string dest;
  std::copy(source,source+5,std::back_inserter(dest));

  std::cout << dest << std::endl;

  return 0;
}

However, as pointed out by others, if the goal is simply to copy the first 5 characters into the string at initialization time, using the appropriate constructor is clearly the best option:

std::string dest(source,source+5);

Upvotes: 1

billz
billz

Reputation: 45410

Just do

char name[] = "Sally Magee";
std::string first(name, name + 5);
std::cout << first << std::endl;

see std::string constructor link

Upvotes: 18

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