30th January 2023, 2 min read

Move or copy your strings? Possible performance impacts

You sometimes want to add a string to an existing data structure. For example, the C++17 template ‘std::optional’ may be used to represent a possible string value. You may copy it there, as this code would often do…

std::string mystring;
std::optional<std::string> myoption;
myoption = mystring;

Or you can move it:

std::string mystring;
std::optional<std::string> myoption;
myoption = std::move(mystring);

In C++, when ‘moving’ a value, the compiler does not need to create a whole new copy of the string. So it is often cheaper.

I wrote a little benchmark to assess the performance difference. It is a single test, but it should illustrate.

Firstly, for relatively long strings (a phrase or a sentence), the move is 5 times to 20 times faster.

Secondly, for short strings (a single word), the move is 1.5 times to 3 times faster but the absolute difference is small (as small as a fraction of a nanosecond). Your main concern should be with long strings.

My results illustrate that moving your sizeable data structure instead of copying them is beneficial.

But that’s not the fastest approach: the fastest approach is to just hold a pointer. Copying an address is unbeatably fast. A slightly less optimal approach is to use a lightweight object like an std::string_view: copying or creating an std::string_view is cheaper than doing the same with a C++ string.