C++关键字：Placement new

最新推荐文章于 2025-08-20 13:22:19 发布

iteye_19606

最新推荐文章于 2025-08-20 13:22:19 发布

阅读量120

点赞数

文章标签： c/c++

本文介绍C++中Placement new的使用方法及示例，通过对比Operator new展示了Placement new如何减少内存分配开销。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Placement new是C++提供的一个很有意思的特性。new或者mallo函数会在堆里面分配空间，这个分配可能是非常耗时的。Placement new可以为对象提前分配好内存，当应用要使用的对象很多的时候，由于Placement new申请内存使用的时间是常数，这个方法可以节省很多时间。

Placement new的使用语法：

1)提前申请一块buf:

char *buf=new char[N*sizeof(PlacementNew)+sizeof(int)];

2) 创建PlacementNew对象

PlacementNew *placementnew = new (buf) PlacementNew;

3）释放Placement对象

placementnew->~PlacementNew();

4）程序结束时释放buf指向的空间：

delete buf;

下面是示例：

Placement.cpp

先OperatorNew，后PlacementNew

编译：g++ PlacementNew.cpp -o placementnew

#include <iostream>

#include <sys/time.h>

#include <unistd.h>

#include <stdio.h>

#define N 10

class PlacementNew {

public:

int mArray[10000][1000];

};

int getCurrentMillis()

{

struct timeval tv;

struct timezone tz;

gettimeofday (&tv , &tz);

printf("%d s ", tv.tv_sec);

printf("%d us/n",tv.tv_usec);

return 1;

}

int main(int argc, char* argv[])

{

char *buf=new char[N*sizeof(PlacementNew)+sizeof(int)];

PlacementNew *placementnew = NULL;

PlacementNew *operatornew = NULL;

printf("Operator New /n");

getCurrentMillis();

operatornew = new PlacementNew;

getCurrentMillis();

printf("Placement New /n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

printf("Operator New 2/n");

getCurrentMillis();

operatornew = new PlacementNew;

getCurrentMillis();

printf("Placement New 2/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

return 1;

}

Placement.cpp运行结果：

Operator New

1308728379 s 872584 us

1308728379 s 872717 us

Placement New

1308728379 s 872722 us

1308728379 s 872724 us

Operator New 2

1308728379 s 872728 us

1308728379 s 872758 us

Placement New 2

1308728379 s 872763 us

1308728379 s 872765 us

Placement2.cpp

先PlacementNew，后OperatorNew

#include <iostream>

#include <sys/time.h>

#include <unistd.h>

#include <stdio.h>

#define N 10

class PlacementNew {

public:

int mArray[10000][1000];

};

int getCurrentMillis()

{

struct timeval tv;

struct timezone tz;

gettimeofday (&tv , &tz);

printf("%d s ", tv.tv_sec);

printf("%d us/n",tv.tv_usec);

return 1;

}

int main(int argc, char* argv[])

{

char *buf=new char[N*sizeof(PlacementNew)+sizeof(int)];

PlacementNew *placementnew = NULL;

PlacementNew *operatornew = NULL;

printf("Placement New 0/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

printf("Operator New 1/n");

getCurrentMillis();

operatornew = new PlacementNew;

getCurrentMillis();

printf("Placement New 1/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

printf("Operator New 2/n");

getCurrentMillis();

operatornew = new PlacementNew;

getCurrentMillis();

printf("Placement New 2/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

return 1;

}

Placement2.cpp运行结果：

Placement New 0

1308728898 s 256533 us

1308728898 s 256636 us

Operator New 1

1308728898 s 256640 us

1308728898 s 256670 us

Placement New 1

1308728898 s 256675 us

1308728898 s 256677 us

Operator New 2

1308728898 s 256680 us

1308728898 s 256710 us

Placement New 2

1308728898 s 256714 us

1308728898 s 256716 us

增加内存清除

Placement3.cpp

#include <iostream>

#include <sys/time.h>

#include <unistd.h>

#include <stdio.h>

#define N 10

class PlacementNew {

public:

int mArray[10000][1000];

PlacementNew(){

printf("PlacementNew constractor/n");

}

~PlacementNew(){

printf("PlacementNew deconstractor/n");

}

};

int getCurrentMillis()

{

struct timeval tv;

struct timezone tz;

gettimeofday (&tv , &tz);

printf("%d s ", (int)tv.tv_sec);

printf("%d us/n",(int)tv.tv_usec);

return 1;

}

int main(int argc, char* argv[])

{

char *buf=new char[N*sizeof(PlacementNew)+sizeof(int)];

int *intp = new int[10];

intp[0] = 1;

sleep(1);

PlacementNew *placementnew = NULL;

PlacementNew *operatornew = NULL;

printf("Placement New 0/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

//Can not delete it with delete

//delete placementnew;

//placementnew = NULL;

//Delete it with deconstractor

placementnew->~PlacementNew();

printf("Operator New 1/n");

getCurrentMillis();

operatornew = new PlacementNew;

getCurrentMillis();

delete operatornew;

operatornew = NULL;

printf("Placement New 1/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

//delete placementnew;

//placementnew = NULL;

//Delete it with deconstractor

placementnew->~PlacementNew();

printf("Operator New 2/n");

getCurrentMillis();

operatornew = new PlacementNew;

getCurrentMillis();

delete operatornew;

operatornew = NULL;

printf("Placement New 2/n");

getCurrentMillis();

placementnew = new (buf) PlacementNew;

getCurrentMillis();

//delete placementnew;

//placementnew = NULL;

//Delete it with deconstractor

placementnew->~PlacementNew();

delete[]buf;

return 1;

}

参考文献： http://www.ksarea.com/articles/20080124_cc.html