CODEWITHSUNDEEP
c++
Husain Alhamali
Husain Alhamali

Reputation: 823

return char array from function that passes parameters

I'm working with a piece of C++ code that reads lines from a txt file and then assign each line to to an array called lines. Then it calls a function that converts each element in the lines array to a char array and then return the resulted char array. This step is where I stuck. How could I return a char array from the function toChar and assign the returned array to another array so I can use it as I need? (the rest of the code should use each returned char array to write it in a pipe, this not important right now but just to clarify why I need to learn to return an array from a function)

Here is the code I'm using:

#include <fstream>
#include <iostream>
#include <string.h>
#include <unistd.h>
#include <cstdlib>


using namespace std;

char * toChar(string line);

int main()
{
string lines[0] = "line1";
char* a = toChar(lines[0]);



return 0;
}

char * toChar(string line)
{
    char a[1024];
    strcpy(a, line.c_str());
    return a;
}

Please note that in this code I'm trying to shrink the code so I'm assigning a simple string value to the array

when I try to compile this code, the error below appears:

warning: address of local variable 'a' returned

any help or suggestion is greatly appreciated..

Upvotes: 0

Views: 2763

Answers (3)

g24l
g24l

Reputation: 3145

First use a const & in passing the string to the function to avoid the unnecessary copying, and be able to use temporaries, e.g. toChar("I am temp");

The are the following alternatives:

(1) Return std::string 

 std::string toChar(std::string const & line)
 {
     char a[1024];
     strcpy(a, line.c_str());
     return a;
 }

Of course it is assumed that line is smaller than 1024 chars with the null termination symbol

(2) Return an allocated array the 

char * toChar(std::string const & line)
{
  char * a = new char[1024];
  strcpy(a, line.c_str());
  return a;
}

but you will have to actually manage it and delete it later.

(3) Allocate the array and pass it to the function

void toChar(string const & line, char a[])
{
 strcpy(a, line.c_str());
}

I imagine that you actually want to extract a C-string from an std::string , or some part of it. The proper way to do it is (3).

Upvotes: 3

dkg
dkg

Reputation: 1775

If you want to access to the internal storage of your string you can call line.c_str() which returns a const char *.

Hence you won't need you function toChar that creates a local array that will go out of scope at the end of your function.

You could simply do :

int main()
{
    string lines[0] = "line1";
    const char* a = lines[0].c_str();


    return 0;
}

But I advise you to keep manipulating std::string as they will handle string better than a simple char *. And if you want a copy, just do it : std::string mycopy = lines[0].

Upvotes: 1

Jepessen
Jepessen

Reputation: 12445

The warning is correct and practically is an error. You're declaring the char array locally so it will be deleted after going out of scope of that function and its address will be no more valid.

Since you're using c++ avoid char array and use a std::string.

Upvotes: 1

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