I know that in C/C , if you write a string literal, this is actually placed into read-only memory with static (lifetime of the program) storage. So, for example:

void foo(const char* string) {
    std::cout << static_cast<void const*>(string) << std::endl;
}

int main() {
    foo("Hello World");
}

should print out a pointer to someplace in read-only memory.

Here's my question, let's say I want to write a copy-on-write String class which has an optimization for static data like this. Instead of copying the whole string into dynamically-allocated memory (which is expensive), why not just keep a pointer to the static data instead. Then, if a write actually does need to occur, then I can make a copy at that point.

But how can I tell if a string is static or something like:

int main() {
    char[] myString = "Hello World";
    foo(myString);
}

In this case, myString is located in the stack and not the heap, thus it's lifetime is not static.

My first thought was a special constructor for std::string_view, but I'm not sure that std::string_view implies a string with static lifetime either...

CodePudding user response：

The easiest way I can think of is to do it with a user-defined literal operator.

struct cow_string { /* stuff */ };

cow_string operator "" _cow( const char*, size_t );

Then, when someone does:

cow_string betsy = "moo"_cow;

the argument to operator""_cow is guaranteed to be a static string. I mean, barring pathological cases like someone invoking operator"" directly by name (which I honestly don't even know if it is possible; if it is and you do it and it breaks stuff, well, you deserve whatever happens).