C++ 一个字符串的代理类（String View)

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在学习muduo/base源码的时候看到一个StringPiece类，是个string view，记录下来。

关于string view，看http://stackoverflow.com/questions/20803826/what-is-string-view。

简单摘抄下来就是

The purpose of any and all kinds of “string reference” and “array reference” proposals is to avoid copying data which is already owned somewhere else and of which only a non-mutating view is required. The string_view in question is one such proposal; there were earlier ones called string_ref and array_ref, too.

The idea is always to store a pair of pointer-to-first-element and size of some existing data array or string.

string view解释过来就是个窗口，你通过这个窗口查看代理的字符串的内容，移动指针和改变长度就可以移动窗口，而且，自身不需要存储字符串，只读，不可修改，生命期应和代理的字符串一致。

muduo的StringPiece代码如下，个人做了一点修改：

class StringPiece {public:    StringPiece()        : _ptr(nullptr),          _length(0)    {    }    StringPiece(const char* str)        : _ptr(str),          _length(strlen(_ptr))    {    }    StringPiece(const unsigned char* str)        : _ptr(reinterpret_cast<const char*>(str)),          _length(strlen(_ptr))    {    }    StringPiece(const std::string& str)        : _ptr(str.data()),          _length(static_cast<size_t>(str.length()))    {    }    StringPiece(const char* offset, size_t len)        : _ptr(offset),          _length(len)    {    }    const char* data() const { return _ptr; }    size_t size() const { return _length; }    bool empty() const { return _length == 0; }    const char* begin() const { return _ptr; }    const char* end() const { return _ptr + _length; }    void clear()    {        _ptr = nullptr;         _length = 0;     }    void set(const char* buffer, size_t len)    {        _ptr = buffer;        _length = len;    }    void set(const char* str)    {        _ptr = str;        _length = strlen(str);    }    void set(const void* buffer, size_t len)    {        _ptr = reinterpret_cast<const char*>(buffer);        _length = len;    }    char operator[](size_t i) const { return _ptr[i]; }    void remove_prefix(size_t n)    {        assert(n <= _length);        _ptr += n;        _length -= n;    }    void remove_suffix(size_t n)    {        assert(n <= _length);        _length -= n;    }    bool operator==(const StringPiece& x) const    {        return ((_length == x._length) &&                 (memcmp(_ptr, x._ptr, _length) == 0));    }    bool operator!=(const StringPiece& sp) const    {        return !(*this == sp);    }    std::string asString() const     {         return std::string(data(), size());    }     void copyToStdString(std::string* target) const    {        assert(target != nullptr);        target->assign(_ptr, _length);    }    bool startWith(const StringPiece& x) const    {        return ((_length >= x._length) &&                 (memcmp(_ptr, x._ptr, x._length) == 0));    }    int compare(const StringPiece& x) const     {        int r = memcmp(_ptr, x._ptr, _length < x._length ? _length : x._length);        if (r == 0)        {            if (_length < x._length) r = -1;            else if (_length > x._length) r = +1;        }        return r;    }private:    const char* _ptr;    size_t _length;};

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