What is SSO

 

Just briefly, SSO stands for Short String Optimization. It’s usually implemented as a small buffer (an array or something similar) occurring in the same storage as the string object. When the string is short, this buffer is used instead of a separate dynamic memory allocation.

See a simplified diagram below:

The diagram illustrates two strings and where they “land” in the string object. If the string is long (longer than N characters), it needs a costly dynamic memory allocation, and the address to that new buffer will be stored in ptr. On the other hand, if the string is short, we can put it inside the object in the buf[N]. Usually, buf and ptr might be implemented as union to save space, as we use one or the other, but not both simultaneously.

Let’s start with a basic test and see what’s the stack size of std::string using sizeof():

int main() {
    return sizeof(std::string);
}

Run at Compiler Explorer

GCC and MSVC show 32, while the libc++ implementation for Clang returns 24!

And now, it’s time to check the length of that short string; how can we check it? We have several options:

• at runtime
• constexpr since C++20
• constinit since C++20
• and we can always look into real implementation and check the code :)

Let’s start with the first obvious option:

Checking length at runtime

 

To check the length of the small buffer, we can write a new() handler and simply watch when new is used when creating a string object:

#include 
#include 

void* operator new(std::size_t size) {
	auto ptr = malloc(size);
	if (!ptr)
		throw std::bad_alloc{};
	std::cout