Return a local C-string for function with C++ string return value

Question

The following piece of code compiles and runs without errors and with the expected output:

#include <string>
#include <iostream>
using namespace std;

string getString()
{
    char s[] = "Hello world!";
    return s;
}

int main()
{
    cout << getString() << endl;
}

My question is, will this always work? Ordinarily if you return a C-string that was declared locally you can run into some undefined behavior, but in this case is that still a problem since it is run through the string constructor and (presumably) copied into dynamic memory?

One of the main reason std::string added to standard library - it can be used as an embedded type like int and double, so yes it is fine to return std::string by value. — Slava
– Slava, Commented Aug 1, 2017 at 17:24
Did you know that string litterals have static storage duration: cpp-ref. So if your c-string just point to a string literral like in const char* s="Hello World" you can safely return it from your function, since s is a pointer to an object with static storage duration. This is why std::except::what() or type_info::name() return a c-string — Oliv
– Oliv, Commented Aug 1, 2017 at 21:41
@Oliv that's a neat fact but this is just an example, in my real program the C string is created at runtime — ClydeTheGhost
– ClydeTheGhost, Commented Aug 2, 2017 at 13:40

Community · Accepted Answer · 2020-06-20 09:12:55Z

13

 return s;

That line is equivalent to:

return std::string(s);

And that will make a copy of the string, so it's fine.

reference: http://en.cppreference.com/w/cpp/string/basic_string/basic_string (constructor #5)

Constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s.

Edit: One more detail. You mention

copied into dynamic memory?

And the answer is maybe, perhaps, it doesn't really matter.

The semantics provided by std::string make no specification towards this, it just guarantees that it can be copied/moved around and accessed in a consistent matter. How it acheives this is up to the library implementor.

In fact, popular implementations of std::string use something called the "Small String Optimization". Where strings under a certain length are stored within the string object itself.

edited Jun 20, 2020 at 9:12

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answered Aug 1, 2017 at 17:22

user4442671

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3 Comments

Buster Over a year ago

The temporary std::string object that is returned is constructed (not copied!) from the array s.

user4442671 Over a year ago

I think you are misinterpreting, the constructor of std::string copies the content of the string pointed by s. s is still a string, a c string, but a string nonetheless.

Buster Over a year ago

Yes, fair enough. There's no denying that "a copy of the string" is made, in the sense that the string's bytes will be copied.

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