How to use pointer to string in cpp?-CodePudding

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.

CodePudding user response：

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.

CodePudding user response：

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;
    }

CodePudding user response：

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;
}