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c++stringperformancestdconstruct

Is it cheap to construct a const std::string from a const char* + length?

How expensive is it to execute

const std::string s(my_const_char_ptr, my_length);

? Is there copying involved? If not, how many instructions can I expect from typical standard library implementations? Few enough to have this in performance-critical code?

... or must I get a GSL implementation and use string_view?

Solution

  • You need to measure it on your target system and compilation settings if you want to know the exact answer. But for what's happening under the hood:

    • If my_length is large enough (or the standard library's std::string implementation doesn't use small string optimization — which is rare), then there will be a dynamic memory allocation.

    • In any case, there will be an O(n) character-by-character copy from *my_const_char_ptr to the std::string's buffer.