C++ 一个字符串的代理类（String View)

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;
};