How to create and return string from function?

Question

Would like to generate a string from a function, in order to format some data, so the function should return a string.

Tried to do the "obvious", shown below, but this prints garbage:

#include <iostream>
#include <string>

char * hello_world()
{
    char res[13];
    memcpy(res, "Hello world\n", 13);
    return res;
}

int main(void)
{
    printf(hello_world());
    return 0;
}

I think this is because the memory on the stack used for the res variable, defined in the function, is overwritten before the value can be written, maybe when the printf call uses the stack.

If I move char res[13]; outside the function, thus makes it global, then it works.

So is the answer to have a global char buffer (string) that can be used for the result?

Maybe doing something like:

char * hello_world(char * res)
{
    memcpy(res, "Hello world\n", 13);  // 11 characters + newline + 0 for string termination
    return res;
}

char res[13];

int main(void)
{
    printf(hello_world(res));
    return 0;
}

Or you can add using namespace std. It is the C++ Standard Library which is a collection of classes and functions. The built in C++ library routines are kept in the standard namespace. That includes stuff like cout, cin, string, vector, map, etc. — user9379842
– user9379842, Commented May 24, 2018 at 11:38

MSalters · Accepted Answer · 2018-05-24 10:31:36Z

9

Don't bother with that early-20th century stuff. By the end of the previous century we already had std::string, and that's straightforward:

#include <iostream>
#include <string>

std::string hello_world()
{
    return "Hello world\n";
}

int main()
{
    std::cout << hello_world();
}

answered May 24, 2018 at 10:31

MSalters

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1 Comment

EquipDev Over a year ago

Thanks for the suggestion; accepted the other answer, since it is closer to the actual application.

nvoigt · Accepted Answer · 2018-05-24 10:34:08Z

5

You are programming c. That's not bad, but your question is about c++ so this is the solution for the question you asked:

std::string hello_world()
{
    std::string temp;

    // todo: do whatever string operations you want here
    temp = "Hello World";

    return temp;
}

int main()
{
    std::string result = hello_world();

    std::cout << result << std::endl;

    return 0;
}

answered May 24, 2018 at 10:34

nvoigt

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Comments

mahesh · Accepted Answer · 2018-05-24 10:51:44Z

2

Best solution would be to use std::string. However, if you must use an array, then it is best to allocate it in the calling function (in this case, main()):

#include <iostream>
#include <cstring>

void hello_world(char * s)
{
    memcpy(s, "Hello world\n", 13);
}

int main(void)
{
    char mys[13];
    hello_world(mys);
    std::cout<<mys;
    return 0;
}

answered May 24, 2018 at 10:51

mahesh

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Comments

Eduardo Fernandes · Accepted Answer · 2018-05-24 12:26:30Z

1

Still, if you want to write a pure C code, will can do something like that.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *HelloWorld(char *s, int size)
{
        sprintf(s, "Hello world!\n");
        return s;
}

int main (int argc, char *argv[])
{
        char s[100];

        printf(HelloWorld(s, 100));

        return 0;
}

edited May 24, 2018 at 12:26

answered May 24, 2018 at 12:10

Eduardo Fernandes

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2 Comments

Frax Over a year ago

What is the memset for? It doesn't matter. Also, sprintf will cause a buffer overflow if size is too small.

Eduardo Fernandes Over a year ago

You are absolutely right, it there for no reason. Let me edit it.

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