c++ - 如何实现STL容器中对象的快速释放？

Question

TL/DR: How can a large std::vector<std::string> achieve such fast deallocation when compiled with release settings on Visual Studio 2012 RC?

我写了一个类strung其行为类似于 std::string作为练习，实现基本的复制和移动语义。

class strung
{
private:
    size_t length_;
    char* data_;

public:
    // -------- Constructors --------

    strung() : length_(0), data_(nullptr) {};

    strung(const char* c_str)
    {
        length_ = strlen(c_str);
        data_ = new char[length_];
        ::std::copy(c_str, c_str + length_, data_);
    };

    inline explicit strung(size_t length) : length_(length)
    {
        data_ = new char[length_];
    };

    strung(size_t length, char value) : length_(length)
    {
        data_ = new char[length_];
        ::std::fill(data_, data_ + length_, value);
    };

    // -------- Copy/move-constructors --------

    strung(const strung& old)
    {
        data_ = new char[old.length_];
        ::std::copy(old.data_, old.data_ + old.length_, data_);
        length_ = old.length_;
    };

    strung(strung&& old)
    {
        data_ = old.data_;
        length_ = old.length_;
        // Even though it is a rvalue, its destructor will still be called,
        // so we would like to prevent our data from being freed.
        old.data_ = nullptr;
    };

    // -------- Assignment operators --------

    inline strung & operator =(const strung& old)
    {
        if (this != &old)
        {
            delete[] data_;
            data_ = new char[old.length_];
            ::std::copy(old.data_, old.data_ + old.length_, data_);
            length_ = old.length_;
        }
        return *this;
    };

    strung & operator =(strung&& old)
    {
        if (this != &old)
        {
            delete[] data_;
            data_ = old.data_;
            length_ = old.length_;
            old.data_ = nullptr;
        }
        return *this;
    };

    // -------- Array operators (no bounds checking by design) --------

    inline char& operator[](size_t pos)
    {
        return data_[pos];
    };

    inline const char& operator[](size_t pos) const
    {
        return data_[pos];
    };

    // -------- Insertion operator for `ostream`s --------

    inline friend ::std::ostream &operator<<(::std::ostream &out, const strung& source)
    {
        out.write(source.data_, source.length_);
        return out;
    };

    // -------- Various functions --------

    inline const size_t length() const
    {
        return length_;
    }

    // -------- Poor man's iterators --------

    char* begin()
    {
        return data_;
    };

    char* end()
    {
        return data_ + length_;
    };

    // -------- Destructor --------

    inline ~strung()
    {
        delete[] data_;
    };
}; 

我尝试比较 std::string 的性能和 strung使用此代码:

double time(const std::function<void(void)> &func)
{
    using namespace std::chrono;
    auto t1 = high_resolution_clock::now(); 
    func();
    auto total = duration_cast<nanoseconds>(high_resolution_clock::now()-t1);
    return static_cast<double>(total.count()) / 1000000.;
}

template<typename T>
void test(const int num)
{
    double allocation_time, full_time;

    full_time = time([&] {
        std::vector<T> container;

        allocation_time = time([&] {
            container.reserve(num);

            for (int i=0; i < num; i++)
            {
                container.emplace_back(rand() % 10 + 1,'\0');

                for (char &chr : container.back())
                    chr = ('A' + rand() % ('Z' - 'A' + 1) );
            }
        });
    });

    std::cout << "Full time: " <<  full_time << " miliseconds" << std::endl 
        << "Allocation time: " << allocation_time << " miliseconds" << std::endl 
        << "Deallocation time: " << full_time - allocation_time << " miliseconds" << std::endl;
}

int main()
{
    std::cout << "-------- std::string --------" << std::endl;
    test<std::string>(500000);
    std::cout << "-------- strung --------" << std::endl;
    test<strung>(500000);
    return EXIT_SUCCESS;
}

结果如下:

调试 (x86-64)

-------- std::string --------
Full time: 51050.9 miliseconds
Allocation time: 1853.11 miliseconds
Deallocation time: 49197.8 miliseconds
-------- strung --------
Full time: 52404 miliseconds
Allocation time: 4886.28 miliseconds
Deallocation time: 47517.7 miliseconds

版本 (x86-64):

-------- std::string --------
Full time: 113.007 miliseconds
Allocation time: 107.006 miliseconds
Deallocation time: 6.0004 miliseconds
-------- strung --------
Full time: 47771.7 miliseconds
Allocation time: 356.02 miliseconds
Deallocation time: 47415.7 miliseconds

---

分配速度是可以理解的，因为我并没有真正对类进行太多优化，但释放速度更有趣。

对调试设置的测试表明，对于 std::string 两者，重新分配同样复杂。和 strung (虽然仍然很慢)，但是对发布设置的测试会为 std::string 重新分配非常非常快，同时 strung保持完全相同。什么是std::string考虑到strung，实现如此快速的释放的析构函数几乎是微不足道的。

起初我以为std::string被优化为 nop，所以根本不执行释放，但是当我删除 strung 时的析构函数，后者仍然快得多，所以情况可能并非如此。

我希望我的释放速度更快，那么我该怎么做才能达到类似的释放速度？

Answer 1

Microsoft 的 std::string 实现使用了一种叫做“小字符串优化”的东西。这意味着 std::string 实际上包含一个 15 个字符的字符串(char[16])。如果给定一个短于 16 个字符的字符串，那么它会将其存储在该内存中。所以在这些情况下没有进行动态内存分配。

您的strung 总是 动态分配字符串。这意味着它的析构函数将始终释放它。 std::string，如果足够小，则两者都不做。