How to use pointer to string in cpp? - c++

Question

I am studying pointers in C++. I have studied call by value and call by reference concept. I am trying to create a function to reverse a string which accepts a pointer to string and the size of string. The code is as follow
void reverse(string* str, int size)
{
int start = 0;
int end = size - 1;
while(start < end)
{
swap(*str[start++], *str[end--]);
}
}
int main()
{
string str = "Something";
reverse(&str, str.length());
cout << "Reversed string: " << str << endl;
return 0;
}
I am getting this error:
error: no match for ‘operator*’ (operand type is ‘std::string’ {aka
‘std::__cxx11::basic_string’})
12 | swap(*str[start++], *str[end--]);
I don't want to use the character array, is there way to do it?
Someone please explain, what's wrong in my code. Thank you.

Answer 1

Here is the simple fix. You don't need to change anything except a few lines.
#include <iostream>
#include <algorithm>
#include <cstring>
void reverse( std::string* str ) // no need to pass size to this function
{
int start = 0;
int end = str->length() - 1; // get the length of str like this
char* ptrToCharArray = const_cast<char*>( str->c_str() ); // gets the pointer to str's internal buffer
while ( start < end )
{
std::swap( ptrToCharArray[start++], ptrToCharArray[end--] ); // no need to use * operator anymore
}
}
int main()
{
std::string str = "Something";
reverse( &str );
std::cout << "Reversed string: " << str << std::endl;
return 0;
}
Output is:
Reversed string: gnihtemoS
Hopefully, this helps you.

Answer 2

Just need a little bit of change in your code
Change this *str[start++] to (*str).at(start++)
void reverse(string* str, int size)
{
int start = 0;
int end = size - 1;
while(start < end)
{
swap((*str).at(start++),(*str).at(end--));
}
}
int main()
{
string str = "Something";
reverse(&str, str.length());
cout << "Reversed string: " << str << endl;
return 0;
}

Answer 3

Note that there is no need to pass the size of the string as an argument to the function. You can use the member function std::string::size for that purpose as shown below:
Version 1: Passing pointer to string as argument
#include <iostream>
#include <algorithm>
void reverse(std::string *str)
{
int n=(*str).size()-1;//dereference the pointer and use size member function on the resulting string object
for(int i=0;i<((*str).size()/2);i++){
//Using the swap method to switch values at each index
std::swap((*str).at(i),(*str).at(n)); //note this can also be written as std::swap((*str)[i],(*str)[n]);
n = n-1;
}
}
int main()
{
std::string myString = "myString";
reverse(&myString);
std::cout<<"Reversed string is: "<<myString<<std::endl;
return 0;
}
In version 1, *(str) gives us a std::string type object. Next we call size member function on this std::string object. Similarly we can call the std::string::at member function on this std::string object.
Version 2: Passing reference to string as argument
#include <iostream>
#include <algorithm>
void reverse( std::string &str)
{
int n=str.size()-1;
for(int i=0;i<(str.size()/2);i++){
//Using the swap method to switch values at each index
std::swap(str.at(i),str.at(n));
n = n-1;
}
}
int main()
{
std::string myString = "myString";
reverse(myString);
std::cout<<"Reversed string is: "<<myString<<std::endl;
return 0;
}