CODEWITHSUNDEEP
c++string-view
NoSenseEtAl
NoSenseEtAl

Reputation: 30006

Elegantly concantenating std::string_view and string literal into a std::string

Assume I have the following function:

std::string greeting(const std::string_view name){
    return std::string(name) + " we wish you a very nice day and week and month and year";
}

It works fine, but I want to avoid more than 1 memory allocation, assuming name is too long to fit into SSO buffer. So I came up with this:

std::string greeting_efficient(const std::string_view name){
    constexpr std::string_view suffix = " we wish you a very nice day and week and month and year";
    std::string result;
    result.reserve(name.size()+suffix.size());
    result+=name;
    result+=suffix;
    return result;
}

AFAIK it works but it is quite ugly, is there a easier way to do this?

I am fine with using C++20/23 if solution needs it.

Upvotes: 1

Views: 1188

Answers (2)

Ted Lyngmo
Ted Lyngmo

Reputation: 117288

I'm not sure if it's more elegant or not, but this should assure that there's only one allocation:

  • Initialize the std::string with the correct size instead of default constructing it and reserving space. Expected effects:
    • (+) It makes the one allocation directly at construction.
    • (+) The string's internal size counter will not be changed in the rest of the process.
    • (-) It initializes the memory with the character of your choice.
  • Copy name directly into the string's memory.
  • Copy suffix directly into the string's memory.
    • Both copy operations needed should be using the fastest copying available. Two memcpys or memmoves will likely be the result.
#include <algorithm>

std::string greeting(std::string_view name){
    static constexpr std::string_view suffix = " we wish you a very nice day"
                                               " and week and month and year";

    std::string rv(name.size() + suffix.size(), '\0'); // one allocation

    std::copy(suffix.begin(), suffix.end(),
              std::copy(name.begin(), name.end(), rv.begin()));

    return rv;
}

Upvotes: 1

WBuck
WBuck

Reputation: 5503

You could use std::format which is a C++20 feature. Or you could pull in the fmt library which is what std::format is based off of if your compiler hasn't implemented std::format yet.

#include <iostream>
#include <format>
#include <string>


static std::string greeting(std::string_view name)
{
    return std::format("{} have a very nice day", name);
}

int main() 
{
    std::cout << greeting("Bill") << '\n';
}

You could also use an std::stringstream as well, but I'm unsure how efficient it is.

static std::string greeting(std::string_view name) 
{
    std::ostringstream os{};
    os << name << " have a very nice day";
    return os.str();
}

Upvotes: 3

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