Small String Optimizations

Remember when I said that "strings are std::vector<char> in disguise"?

In practice, very smart folks realized that you may store small strings inside the already allocated memory.

Given that the size of a std::string is 24 bytes on a 64-bits platform (to store data pointer, size and capacity), some very cool tricks allow us to store statically up to 23 bytes before you need to allocate memory.

That has a huge impact in terms of performance!

For the curious minds, here there are some details about the implementation:

• SSO-23
• CppCon 2016: “The strange details of std::string at Facebook"

According to your version of the compiler, you may have less than 23 bytes, that is the theoretical limit.

Example

const char* SHORT_STR = "hello world";

void ShortStringCreation(benchmark::State& state) {
  // Create a string over and over again.
  // It is just because "short strings optimization" is active
  // no memory allocations
  for (auto _ : state) {
    std::string created_string(SHORT_STR);
  }
}

void ShortStringCopy(benchmark::State& state) {
  // Here we create the string only once, but copy repeatably.
  // Why is it much slower than ShortStringCreation?
  // The compiler, apparently, outsmarted me
  std::string x; // create once
  for (auto _ : state) {
    x = SHORT_STR; // copy
  }
}

const char* LONG_STR = "this will not fit into small string optimization";

void LongStringCreation(benchmark::State& state) {
  // The long string will trigger memory allocation for sure
  for (auto _ : state) {
    std::string created_string(LONG_STR);
  }
}

void LongStringCopy(benchmark::State& state) {
  // Now we do see an actual speed-up, when recycling
  // the same string multiple times
  std::string x;
  for (auto _ : state) {
    x = LONG_STR;
  }
}

As you may notice, my attempt to be clever and say "I will not create a new string every time" fails miserably if the string is short, but has a huge impact if the string is allocating memory.