Adding a pointer to a string in C++

Question

I am confused with const pointers in C++ and wrote a small application to see what the output would be. I am attempting (I believe) to add a pointer to a string, which should not work correctly, but when I run the program I correctly get "hello world". Can anyone help me figure out what how this line (s += s2) is working?

My code:

#include <iostream>
#include <stdio.h>
#include <string>

using namespace std;

const char* append(const char* s1, const char* s2){
    std::string s(s1);     //this will copy the characters in s1
    s += s2;               //add s and s2, store the result in s (shouldn't work?)
    return s.c_str();      //return result to be printed
}

int main() {
    const char* total = append("hello", "world");
    printf("%s", total);
    return 0;
}

Answer 1

The variable s is local inside the append function. Once the append function returns that variable is destructed, leaving you with a pointer to a string that no longer exists. Using this pointer leads to undefined behavior.

My tip to you on how to solve this: Use std::string all the way!

Answer 2

you're adding const char* pointer to a std::string and that is possible (see this reference). it wouldn't be possible to make that operation on char* type (C style string).

however, you're returning a pointer to local variable, so once function append returns and gets popped of the stack, the string that your returned pointer is pointing to would not exist. this leads to an undefined behavior.

Answer 3

Class std::string has overloaded operator += for an operand of type const char *

basic_string& operator+=(const charT* s);

In fact it simply appends the string pointed to by this pointer to the contents of the object of type std::string allocating additionly memory if required. For example internally the overloaded operator could use standard C function strcat Conceptually it is similar to the following code snippet.

char s[12] = "Hello ";
const char *s2 = "World";

std::strcat( s, s2 );

Take into account that your program has undefined behaviour because total will be invalid after destroying local object s after exiting function append. So the next statemnent in main

printf("%s", total);

can result in undefined behaviour.