StringPool

A performant and memory efficient storage for immutable strings with C++17. Supports all standard char types: char, wchar_t, char16_t, char32_t and C++20's char8_t.

Motivation

Standard C++ string classes - std::string, std::wstring etc. - aren't very efficient when it comes to memory usage and allocations. Due to small string optimization a lot of space can be wasted when storing huge amounts of long strings, that don't fit the capacity of the small string buffer. A common allocation strategy that std::basic_string uses (doubling the capacity when extending the storage) can lead to almost 50% waste of memory unless std::basic_string::shrink_to_fit() is called.

StringPool was created to provide a way of storing strings that don't change throughout program execution without excessive memory usage. Furthermore, it combats memory fragmentation by storing strings together, in blocks.

StringPool doesn't do string interning

StringPool doesn't perform any string comparisons, neither it differentiates between two strings - each call to StringPool<>::add() gives you a brand new view of the string.

Use cases

• localization (translation) strings
• storing filenames and paths of files packed in a VPK

Getting started

1. Include StringPool.h in your project.
2. Create a pool object:

StringPool<char> pool;

3. Add some strings: (don't forget to save values returned from StringPool<>::add())

std::vector<std::string_view> strings;
strings.push_back(pool.add("foo"));
strings.push_back(pool.add("bar"));

Example usage

#include <cassert>
#include <cstring>
#include <string_view>
#include <vector>

#include "StringPool.h"

int main()
{
    // StringPool of null-terminated strings
    {
        StringPool<char, true> pool;

        std::vector<std::string_view> strings;
        strings.push_back(pool.add("one"));

        // string views passed to StringPool<>::add() don't have to point to a null-terminated string
        constexpr std::string_view s{ "two three" };
        strings.push_back(pool.add(s.substr(0, 3)));
        strings.push_back(pool.add(s.substr(4)));

        // strings added to a null-terminated pool can be used with C API
        assert(std::strcmp(strings[0].data(), "one") == 0);
        assert(std::strcmp(strings[1].data(), "two") == 0);
        assert(std::strcmp(strings[2].data(), "three") == 0);

        // string_view can be skipped, and a raw const char* can be used
        const char* str = pool.add("just a pointer").data();
        assert(std::strcmp(str, "just a pointer") == 0);
    }

    // StringPool of not null-terminated strings, uses less memory (1 byte per string) by dropping C compatibility
    {
        StringPool<char, false> pool;

        std::vector<std::string_view> strings;
        strings.push_back(pool.add("one"));

        // string views passed to StringPool<>::add() don't have to point to a null-terminated string
        constexpr std::string_view s{ "two three" };
        strings.push_back(pool.add(s.substr(0, 3)));
        strings.push_back(pool.add(s.substr(4)));

        // strings added to this pool can't be passed to C functions
        assert(strings[0] == "one");
        assert(strings[1] == "two");
        assert(strings[2] == "three");
    }

    // StringPool uses default block capacity of 8192 characters, but a custom value can be specified
    {
        constexpr auto myCustomCapacity = 1'000'000;
        StringPool<wchar_t, false> bigPool{ myCustomCapacity };

        const auto something = bigPool.add(L"something");
        StringPool<wchar_t, false> tooSmallPool{ something.length() / 2 };

        // if you try to add a string exceeding default block capacity, StringPool will allocate a new block capable of storing the string
        const auto stillAdded = tooSmallPool.add(something);
        assert(stillAdded == L"something");
    }
}

C++20

StringPool supports char8_t type introduced in C++20 standard out of the box.

Thread safety

• To add strings to a pool (StringPool<>::add()) from multiple threads you have to provide synchronization yourself.
• Once added to pool, strings are read-only therefore can be safely accessed from multiple threads. That means you can add new strings to the pool and access existing ones in parallel.

For an example use of StringPool accross multiple threads check Examples/Threaded.cpp.

Exceptions

StringPool is exception-neutral meaning that while it doesn't throw any exception itself, exceptions may be emitted by STL algorithms or containers used in the implementation (std::bad_alloc etc.).