How to return a string in C++

Question

#include <iostream>
#include<string.h>
using namespace std;
char *rt()
{
    char a[20];
    strcpy(a,"I am a beginner");
    cout<<a;
    return a;
}
int main()
{
    char *a;
    a=rt();
    cout<<endl;
    cout<<a;
    return 0;
}

Here I have made a short program for understanding the same....I was expecting the output to be as

I am a beginner 
I am a beginner

UPDATE: But it turned out to be

I am a beginner
ëóG

I have read many articles and post but I am not able to understand their complex language....so I will appreciate a no-nonsense answer ( for a stupid...like me!!)

UPDATE: Actually, there is one question in my assignment which goes like this

class COMPUTER
{
    char chiptype[10];
    int speed;
public:
    void showdetails()
    {
        cout<<chiptype;
        cout<<"\t"<<speed;
    }
    void getdetails()
    {
        cin.getline(chiptype,10);
        cin>>speed;
    }
};

Here the data has to be read and stored into a binary file....and the records having chiptype as "CD" are to be displayed.

Now my question is that...as the variable chiptype is private so I can't use it for comparison in main()....so I thought of making a function which returned the value stored in chiptype.

And I am not allowed to use std::string as well as node implementation...

Answer 1

char a[20]; is allocated on stack. When the function rt() returns, the stack unwinds and a goes out of scope. Hence you do not get your desired result.

While you are on C++, may I suggest using std::string

Read the comment below:

The other trick is to wrap the array in a struct, and return the struct. Since a struct is copyable, the array internally becomes copyable and will not lose scope. See this answer. Then you're not dealing with pointers at all. See this live example – PaulMcKenzie

Answer 2

Use std::string instead of char[]

#include <iostream>
#include<string>

std::string rt()
{
    std::string a("I am a beginner");
    std::cout<<a;
    return a;
}
int main()
{
    std::string a;
    a=rt();
    std::cout<<std::endl;
    std::cout<<a;
    return 0;
}

In your original code char a[20] is allocated on the stack and return a; will return a pointer to a stack variable that is no longer valid when you receive it in you main() -- handling strings in c++ should generally be done using std::string as it handles all the nasty memory management that wil kill your program if you aren't careful.

If you have to use pointers and not use std::string, you would need to go the c style way with the risk of having memory leaks if you miss a step or two. The code would look something like this using c style (keeping the cout c++)

#include <iostream>
#include <strings.h>

std::string rt()
{
    char *a = malloc(20); // allocate the memory on heap
    strcpy(a,"I am a beginner");
    std::cout<<a;
    return a;
}
int main()
{
    char *a;
    a=rt();
    std::cout<<std::endl;
    std::cout<<a;
    free(a); // release the memory
    return 0;
}

Caution: I don't recommend that you do the above style -- in a real world application you will likely get into trouble by either forgetting to free the memory, or accidentally accessing the memory after it has been free'd

Answer 3

The problem is that the memory of a will be destroyed as soon as program returns from the function. I do not think you should work with dynamic memory at your level of knowledge, so I suggest you define the array outside and just modify it inside the function:

#include <iostream>
#include<string.h>
#define MAX_LENGTH 20
using namespace std;

void rt(char *a)
{
    strcpy(a,"I am a beginner");
    cout<<a;
}

int main()
{
    char a[MAX_LENGTH];
    rt(a);
    cout<<endl;
    cout<<a;
    return 0;
}

Furthermore, you should take care that rt is not writing more than MAX_LENGTH characters.

Answer 4

As mentioned by other use std::string instead of char [].

std::string rt()
{
    std:: string a = "I am a beginner"; // This statement is equivalent to         std::string a("I am a beginner");
    std::cout << a "\n";
    return a;
}

Main reason of not getting the desired result is " char a[] is allocated on stack, but when function return stack become empty.

P.S : You need to include <string> in your program, to use std::string