empty string plus an int? - c++

Question

Here is part of my code:
//num is an int
string s = "" + num;
When I run this in Xcode on a Macbook, s will be assigned to a strange string.
Can any one explain this to me?
I am really confused.
Thanks.

Answer 1

I assume string refers to the std::string type, declared in the standard header <string>. You haven't given that as context - technically, from information you have given it could be some preceding typedef or a macro.
The explanation as to why
string s = "" + num;
gives a "strange string" is that "" is represented in memory as a const array of one char that has the value zero. In the expression "" + num, the "" is converted to a pointer (equal to the address of that char with value zero), and + num then gives the address of some location in memory, num characters after.
If num is non-zero, that memory address may not exist, or (if it exists) might contain arbitrary data.
In any event, that pointer is passed to the constructor of std::string (in order to construct s). That constructor starts at the address given, and keeps copying data into the std::string until it happens to find a character with value zero. The characters in between could be anything - they are whatever happens to be at that memory location.
Formally, the C++ standard describes all this (accessing data via a bad pointer) as undefined behaviour. That means the C++ standard says nothing about what result is permitted, so any result is allowed. It could result in strange data in your string. The operating system might detect your program accessing memory it shouldn't, and forceably terminate your program. It could reformat your hard drive and reinstall your operating system.
Assuming you just want to write num to a string (e.g. num with value 42 results in a string like "42"), then the usual technique is to convert num to a std::string. For example;
std::string s = to_string(num); // C++11 or later
or
#include <sstream> // pre_C++11 (albeit valid in C++11)
std::ostringstream ostr;
ostr << num;
std::string s(ostr.str());

Answer 2

string s = std::to_string(num);
Try this one.

Answer 3

The idea about std::to_string() is good, but it unfortunately only works with C++11 and I have seen compilers that do not fully support that aspect though they support C++11. If you can use it, use it.
If it does not work try this:
#include <sstream>
...
int num = 5;
float rNum = 63.2;
std::stringstream ss;
ss << num;
ss << rNum;
std::string s;
ss >> s;
cout << s << endl;
You are going to get the float and int values formatted to one string. Actually I think you are better off with stringstreams if you have multiple variables get formatted. Pull them out as string with ss >> s and you are done!

Answer 4

The reason you were getting undesirable answer is the arithmetic addition of num to the empty string s, which is basically like pointer addition in C language.
See the examples for better explanation:
Eg. 1:
#include <iostream>
#include <string>
using namespace std;
int main() {
int num = 0;
string s = "1234567890" + num;
cout << s << endl;
return 0;
}
OUTPUT: 1234567890
Eg. 2:
#include <iostream>
#include <string>
using namespace std;
int main() {
int num = 1;
string s = "1234567890" + num;
cout << s << endl;
return 0;
}
OUTPUT: 234567890
Eg. 3:
#include <iostream>
#include <string>
using namespace std;
int main() {
int num = 4;
string s = "1234567890" + num;
cout << s << endl;
return 0;
}
OUTPUT: 567890
Eg. 4:
#include <iostream>
#include <string>
using namespace std;
int main() {
int num = 8;
string s = "1234567890" + num;
cout << s << endl;
return 0;
}
OUTPUT: 90
so on..
Since, here the string s is empty and the num has garbage value which leads to undefined result.
Better use to_string() before appending num to the empty string s.
Example:
#include <iostream>
#include <string>
using namespace std;
int main() {
int num = 1000;
string s = "" + to_string(num);
cout << s << endl;
return 0;
}