C++ 不打印字符串

Question

Sorry for the noob question, I'm a newbie programmer and transitioning from C to C++. I could easily write a program to reverse a string in C the same way with minor changes but writing this in C++, why does this not print anything:

#include <iostream>
#include <string>

using namespace std;

int main(){
    string s,p;
    getline(cin,s);
    int j=0,i = 0;
    while(s[i]!='\0'){
        i++;
    }
    i--;
    while(i!=-1){
        p[j] = s[i];
        j++;
        i--;
    }
    cout << p << endl;
    return 0;
}

if i replace the p with say p[2], it correctly prints out the reverse 3rd character of the original string, but i cant find a way to print the whole string.

Answer 1

    std::string str{"reverse me"};
    std::string rev{str.rbegin(), str.rend()};
    //or when you are not interested in the orignal string
    std::reverse(str.begin(), str.end());

Giving the constructur of the reverse string the reversed iterators of your input string gives you the string in reversed order.

Answer 2

To fix your string reverse code you just have to resize the string object p :

int main(){
    std::string s = "hello",
           p;
    p.resize(s.size()); // this was causing your problems, p thought it was size 0

    for (int i = s.size() - 1, j = 0; i >= 0; i--, j++)
    {
        p[j] = s[i];
    }

    std::cout << p << std::endl;
    return 0;
}

In addition to this, there is no need to find \\0 in the string, while it will be there, you can just ask std::string what its size() is.

On a side note, while std::string probably allocates some memory by default, just assuming it has enough to store whatever you input is going to be undefined behaviour.

Answer 3

While there are ways to iterate over a std::string and fill the contents of another, the overload of std::basic_string::operator= will replace the content of p (if any) with the content of s in a simple assignment. See std::basic_string::operator=

For example:

#include <iostream>
#include <string>

int main (void) {

    std::string s {}, p {};

    std::cout << "enter string: ";
    if (getline (std::cin, s)) {
        p = s;  /* simple assignment replaced content of p with content of s */
        std::cout << "string in p : " << p << '\n';
    }
}

Example Use/Output

$ ./bin/stringps
enter string: the string s
string in p : the string s

Answer 4

string p; doesn't have enough allocated space for directly accessing by something like p[j]

You can change to initialize p from copying s like below, your code will work.

string s;
getline(cin,s);
string p(s);  // p will be allocated and be the same as s