why this works?

[ben]

#include <string>
#include <iostream>

const char* my_concat(const std::string& s1, const std::string& s2)
{
std::string str = s1 + s2;
return str.c_str();
}

int main()
{
using namespace std;
cout << my_concat("hello ", "world");
return 0;
}

Q1: Who deallocates the const char* string returned from my_concat?
Q2: With the local str destroyed, why isn't str.c_str() invalidated?
Q3: In general, how is the C string from string::c_str() allocated and
manged?

ben

[Alf P. Steinbach]

* ben:

#include <string>
#include <iostream>

const char* my_concat(const std::string& s1, const std::string& s2)
{
std::string str = s1 + s2;
return str.c_str();
}

int main()
{
using namespace std;
cout << my_concat("hello ", "world");
return 0;
}

This program exhibits Undefined Behavior because it uses a no longer
valid area of memory. It may seem to "work". Or not.

In general, the result of std::string::c_str() is only valid as long
as the std::string instance hasn't been modified.

Destroying it counts as modifying it.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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