How do I pass an index of a stl string to a function in c++?

Question

I am currently stuck with a slight problem where I want to swap contents in a std::string.

#include <iostream>
#include <string>

void swap(char* t1, char* t2);    // function parameter is wrong syntax
int main(){
    std::string message = "ABC";
    swap(message[0], message[1]); // parameter probably wrong here
    return 0;
}



void swap(char * t1, char * t2){
 return; 
}

GOAL: I wish to do a simple swap of the contents in index 0 and 1 such that after swapping it, the message "ABC" becomes "BAC". As you can see, I tried doing it like I was using a normal array, but it seems this logic doesn't work with strings. I understand that if I switch to

char a[] = "ABC";

it would work, but I wanna try it using the string class.

It wouldn't work with a char array either. – Benjamin Lindley May 04 '15 at 20:51 — Benjamin Lindley, May 04 '15 at 20:51

score 8 · Accepted Answer · answered May 04 '15 at 20:43

The type of message[n] is char. So the signature of your swap function should be

swap(char& a, char& b);

But you should use std::swap instead.

#include <iostream>
#include <string>
#include <utility>

int main(){
    std::string message = "ABC";
    std::swap(message[0], message[1]);
    std::cout << message << std::endl;
}

Vlad from Moscow · Answer 2 · 2015-05-04T21:03:46.673

0

Try the following

#include <iostream>
#include <string>

void swap( char *c1, char *c2 )
{
    char tmp = *c1;
    *c1 = *c2;
    *c2 = tmp;
}

void swap( char &c1, char &c2 )
{
    char tmp = c1;
    c1 = c2;
    c2 = tmp;
}

int main()
{
    std::string message = "ABC";

    swap( message[0], message[1] );
    swap( &message[1], &message[2] );

    return 0;
}

If you want to use standard utilities then the program above can look the following way

#include <iostream>
#include <string>
#include <utility>    

int main()
{
    std::string message = "ABC";

    std::swap( message[0], message[1] );
    message[1] = std::exchange( message[2], message[1] );

    return 0;
}

edited May 04 '15 at 21:03

answered May 04 '15 at 20:45

Vlad from Moscow

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I'm currently using codeblocks and it's not compiling for me. – Belphegor May 04 '15 at 20:47
@user3718584: I can't see why not. – Lightness Races in Orbit May 04 '15 at 20:50
1

@user3718584 I think you made a typo. What error did the compiler issue? – Vlad from Moscow May 04 '15 at 20:50
Okay, I fixed it, but now it compiles and it gives me this message from code blocks everytime I run it "It seems that this project has not been built yet. Do you want to build it now?" – Belphegor May 04 '15 at 20:58
ANyways, it runs fine on my Linux OS when I switched to it. I'll have to figure out waht is wrong with codeblocks later. thx – Belphegor May 04 '15 at 21:00

How do I pass an index of a stl string to a function in c++?

2 Answers2