Why is my string null while if I can access an individual char?

I am new to C++ and I recently learned pointers and references through u-demy lectures. After lectures, I am solving a problem to reverse a string using a separate function. I came up with a code but for some reason, I can access an individual char using [] notation (like reverse[0] is equal to !) but when I print the whole string I get an empty string? What is causing this issue?

Thank you for your time in advance!

Here I have pasted my code:

#include <iostream>
// using namespace std;

std::string reverse_string(const std::string &str) {
    std::string reversed;

    size_t size {str.length()};
    for(int i{}; i < size/2; i++) {
        reversed[i] = str[(size-1)-i];
        reversed[(size-1)-i] = str[i];
    }
    
    return reversed;
}

int main() {
    // Write C++ code here
    std::string input = "Hello, World!";
    std::string reversed = reverse_string(input);
    std::cout << reversed << std::endl;  //I get nothing, BUT
    std::cout << reversed[0] << std::endl //here I do get a '!'
    return 0;
}

The original issue is that no space is ever reserved for std::string reversed in function reverse_string(). This creates and returns an empty std::string.

The operator[] performs no bounds checking. That's your first instance of undefined behavior.

Think of the number in brackets as a memory offset. If you don't switch them on, there are no checks to see if the offset is still within the chunk of memory owned by your string.

What we see in main() is a secondary problem. reverse is an empty string (because that's what reverse_string() always returns, see above). And reverse[0] (another out-of-bounds access and thus undefined behavior) is "!" because you incorrectly set it to this value.

To fix the problem, initialize reversed with a capacity of str.size() or simply as a copy of str.

To prevent such problems, turn on all compiler warnings and do a debug build.

I get the following error when running your code (after adding string header)

usr/src/debug/gcc/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.h:1259: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]: Assertion '__pos <= size()' failed.
Aborted (core dumped)

Usually you'd get Segmentation fault, but here as you can see from the error message,
Assertion '__pos <= size()' failed,
some assertion failed. Thus, the program was aborted.

Invalid memory access is a common cause of Segmentation fault. It is undefined behavior.

Specifically in this case, the index is not within the range of the string object.

Replace the [] operator with .at(), which throws when the index is out of range. [] operator does not.

Debug the program either using a debugger (recommended) or manually to find out where this might be happening.