為了回應
COdE fr3@K大大的Coscup2013講稿
“C++: 誰說只能硬著來?”，我回家用g++49 -std=c++11實驗了，是否用
std::move 搭配 RValue-Ref in constructor可否達到
ZERO COPY。
#include <utility>
#include <algorithm>
#include <string>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
using namespace std;
class mystr{
public:
int len;
char* buf;
mystr()
: len(0) , buf(NULL)
{
printf("trivia ctor\n");
}
mystr(int l)
: len(0) , buf(NULL)
{
printf("length-spec ctor, l=%d\n", l);
buf = (char*)malloc(len);
len = l;
memset(buf, 0, len);
}
mystr(const mystr& rhs){
printf("copy ctor const& \"%s\", len=%d\n", rhs.buf,
rhs.len);
buf = strdup(rhs.buf);
len = rhs.len;
}
~mystr(){
if(buf){
printf("dtor free (\"%s\"), len=%d\n", buf, len);
free(buf);
buf = NULL;
len = 0;
}
else{
printf("dtor nothing to free\n");
}
}
mystr(const char* rhs)
: len(0), buf(NULL)
{
printf("ctor from char* \"%s\"\n", rhs);
buf = strdup(rhs);
len = strlen(buf);
}
mystr(mystr&& rhs)
: len(0), buf(NULL)
{
printf("ctor RRValue; swap: buf=\"%s\" len=%d \n", rhs.buf,
rhs.len);
std::swap(buf, rhs.buf);
std::swap(len, rhs.len);
}
mystr& operator=(const mystr& rhs){
printf("assignment copy: \"%s\"\n", rhs.buf);
if(buf)
free(buf);
buf = strdup(rhs.buf);
len = rhs.len;
return *this;
}
mystr& operator+=(const mystr& rhs){
printf("concat \"%s\" + \"%s\", len=(%d + %d)\n", buf,
rhs.buf, len, rhs.len);
char* newbuf = (char*)malloc(len + rhs.len + 1);
memcpy(newbuf, buf, len);
memcpy(newbuf+len, rhs.buf, rhs.len);
newbuf[len+rhs.len]=0;
free(buf);
buf = newbuf;
len += rhs.len;
return *this;
}
};
mystr foo(mystr rhs){
printf("etner foo\n");
mystr ret("foo ");
ret += rhs;
printf("[foo] return \"%s\"\n", ret.buf);
return ret;
}
int main(){
mystr a("apple");
mystr fooapple = foo( std::move(a) );
return 0;
}
執行結果
passing 到 foo() 函式時，若是使用std::move()，的確是
ZERO COPY。看
ctor "apple"的個數，只有一開始在
main()的那唯一一個，確定是
zero copy。
ctor from char* "apple"
ctor RRValue; swap: buf="apple" len=5
etner foo
ctor from char* "foo "
concat "foo " + "apple", len=(4 + 5)
[foo] return "foo apple"
dtor free ("apple"), len=5
dtor free ("foo apple"), len=9
dtor nothing to free
