nix-super/src/libutil/types.hh

#pragma once
///@file

#include "ref.hh"

#include <list>
#include <set>
#include <string>
#include <limits>
#include <map>
#include <variant>
#include <vector>

namespace nix {

typedef std::list<std::string> Strings;
typedef std::set<std::string> StringSet;
typedef std::map<std::string, std::string> StringMap;
typedef std::map<std::string, std::string> StringPairs;

/**
 * Paths are just strings.
 */
typedef std::string Path;
typedef std::string_view PathView;
typedef std::list<Path> Paths;
typedef std::set<Path> PathSet;

typedef std::vector<std::pair<std::string, std::string>> Headers;

/**
 * Helper class to run code at startup.
 */
template<typename T>
struct OnStartup
{
    OnStartup(T && t) { t(); }
};

/**
 * Wrap bools to prevent string literals (i.e. 'char *') from being
 * cast to a bool in Attr.
 */
template<typename T>
struct Explicit {
    T t;

    bool operator ==(const Explicit<T> & other) const
    {
        return t == other.t;
    }
};


/**
 * This wants to be a little bit like rust's Cow type.
 * Some parts of the evaluator benefit greatly from being able to reuse
 * existing allocations for strings, but have to be able to also use
 * newly allocated storage for values.
 *
 * We do not define implicit conversions, even with ref qualifiers,
 * since those can easily become ambiguous to the reader and can degrade
 * into copying behaviour we want to avoid.
 */
class BackedStringView {
private:
    std::variant<std::string, std::string_view> data;

    /**
     * Needed to introduce a temporary since operator-> must return
     * a pointer. Without this we'd need to store the view object
     * even when we already own a string.
     */
    class Ptr {
    private:
        std::string_view view;
    public:
        Ptr(std::string_view view): view(view) {}
        const std::string_view * operator->() const { return &view; }
    };

public:
    BackedStringView(std::string && s): data(std::move(s)) {}
    BackedStringView(std::string_view sv): data(sv) {}
    template<size_t N>
    BackedStringView(const char (& lit)[N]): data(std::string_view(lit)) {}

    BackedStringView(const BackedStringView &) = delete;
    BackedStringView & operator=(const BackedStringView &) = delete;

    /**
     * We only want move operations defined since the sole purpose of
     * this type is to avoid copies.
     */
    BackedStringView(BackedStringView && other) = default;
    BackedStringView & operator=(BackedStringView && other) = default;

    bool isOwned() const
    {
        return std::holds_alternative<std::string>(data);
    }

    std::string toOwned() &&
    {
        return isOwned()
            ? std::move(std::get<std::string>(data))
            : std::string(std::get<std::string_view>(data));
    }

    std::string_view operator*() const
    {
        return isOwned()
            ? std::get<std::string>(data)
            : std::get<std::string_view>(data);
    }
    Ptr operator->() const { return Ptr(**this); }
};

}