将任意元素存储在连续内存中 [英] Store arbitrary elements in contiguous memory
问题描述
我想创建一个数据结构,它将在连续内存中保存N种不同类型的数据。所以在编译时我可以说我要存储3个不同类型的4个元素,在内存中它将看起来像111122223333。
我一直在使用变量模板方法,我认为会做我想要的,但我不知道如何添加元素到添加方法中的每个数组。
template< std :: size_t N,typename ... Args>
class Batch
{
private:
std :: tuple< std :: array< Args,N&数据_;
size_t currentPos_;
public:
template< typename T>
void addToArray(std :: array< T,N& array,const T& value)
{
array [currentPos_] = value;
}
void add(const Args& ... values)
{
// ????
addToArray(/ * array,value * /);
currentPos _ ++;
}
const void * data()
{
& return data_;
}
};
int main()
{
Batched< 3,float,double,int& b;
b.add(1.0f,1.0,1);
b.add(2.0f,2.0,2);
b.add(3.0f,3.0,3);
b.add(4.0f,4.0,4);
return 0;
}
即使我得到这个工作,内存布局是否正确?是否有更好的方法?
我认为这不是一个好主意,
使用 std :: vector< char>
(以及访问由C + +11添加方法 data()
)和好的 memcpy()
,我想你可以简单地做
#include< vector>
#include< cstring>
#include< iostream>
template< typename ... Args>
class Batch
{
private:
std :: vector< char>缓冲;
public:
void addHelper()
{}
template< typename T,typename ... Ts>
void addHelper(T const& v0,Ts ... vs)
{
auto pos = buffer.size();
buffer.resize(pos + sizeof(T));
std :: memcpy(buffer.data()+ pos,& v0,sizeof(T));
addHelper(vs ...);
}
void add(const Args& ... values)
{addHelper(values ...); }
const void * data()
{return buffer.data(); }
void toCout()
{toCoutHelper< Args ...>(0U,buffer.size()); }
template< typename T,typename ... Ts>
typename std :: enable_if<(0U< sizeof ...(Ts)),void> :: type
toCoutHelper(std :: size_t pos,std :: size_t size)
{
if(pos< size)
{
T val;
std :: memcpy(& val,buffer.data()+ pos,sizeof(T));
std :: cout<< - << val<< std :: endl;
toCoutHelper< Ts ...>(pos + sizeof(T),size);
}
}
template< typename T,typename ... Ts>
typename std :: enable_if< 0U == sizeof ...(Ts),void> :: type
toCoutHelper(std :: size_t pos,std :: size_t size)
{
if(pos< size)
{
T val;
std :: memcpy(& val,buffer.data()+ pos,sizeof(T));
std :: cout<< - << val<< std :: endl;
toCoutHelper< Args ...>(pos + sizeof(T),size);
}
}
};
int main()
{
Batch< float,double,int> b;
b.add(1.0f,1.0,1);
b.add(2.0f,2.0,2);
b.add(3.0f,3.0,3);
b.add(4.0f,4.0,4);
b.toCout();
return 0;
}
--- EDIT --- :added一个方法, toCout()
打印(到 std :: cout
)所有存储的值;
Args ...
类型是一些非POD(普通旧数据)类型,则是非常危险的。 在此网页中解释得很好。
我转录相关部分
无法使用memcpy安全复制的类型示例是
std ::串。这通常使用引用计数的
共享指针实现,在这种情况下它将有一个复制构造函数,
使计数器递增。如果使用memcpy
创建副本,那么将不会调用副本构造函数,计数器将是
,其值低于它应为的值。这将很可能
导致包含
字符数据的内存块的过早解除分配。
--- EDIT 3 ---
如果ildjarn指出这个解决方案非常危险,
如果任何人使用以这种方式返回的指针
char const * pv =(char const *)b.data();
size_t pos = {/ * some value here * /};
float f {*(float *)(pv + pos)}; //< - 不对齐访问的风险
在某些体系结构中, float *
在可以杀死程序的未对齐地址中
正确 data()
返回的指针是 toCoutHelper()
中使用的指针,使用`std :: memcpy
char const * pv =(char const *)b.data();
size_t pos = {/ * some value here * /};
float f;
std :: memcpy(& f,pv + pos,sizeof(f));
I am trying to create a data structure, where it will hold N number of different types in contiguous memory. So at compile time I can say I want to store 4 elements of 3 different types, and in memory it will look like 111122223333.
I've been going with a variadic template approach, which I think will do what I want, however I am not sure how to add the elements to each array in the add method.
template<std::size_t N, typename... Args>
class Batch
{
private:
std::tuple<std::array<Args, N>...> data_;
size_t currentPos_;
public:
template<typename T>
void addToArray(std::array<T, N>& array, const T& value)
{
array[currentPos_] = value;
}
void add(const Args&... values)
{
//????
addToArray(/*array, value*/);
currentPos_++;
}
const void* data()
{
&return data_;
}
};
int main()
{
Batched<3, float, double, int> b;
b.add(1.0f, 1.0, 1);
b.add(2.0f, 2.0, 2);
b.add(3.0f, 3.0, 3);
b.add(4.0f, 4.0, 4);
return 0;
}
Even if I get this to work, will the memory layout be correct? Is there a better approach?
I don't think it's a good idea but... I show it just for fun
Using a std::vector<char>
(and the access to the following memory granted by the C++11 added method data()
) and the good-old memcpy()
, I suppose You can simply do as follow
#include <vector>
#include <cstring>
#include <iostream>
template <typename... Args>
class Batch
{
private:
std::vector<char> buffer;
public:
void addHelper ()
{ }
template <typename T, typename ... Ts>
void addHelper (T const & v0, Ts ... vs)
{
auto pos = buffer.size();
buffer.resize(pos + sizeof(T));
std::memcpy(buffer.data() + pos, & v0, sizeof(T));
addHelper(vs...);
}
void add (const Args&... values)
{ addHelper(values...); }
const void * data()
{ return buffer.data(); }
void toCout ()
{ toCoutHelper<Args...>(0U, buffer.size()); }
template <typename T, typename ... Ts>
typename std::enable_if<(0U < sizeof...(Ts)), void>::type
toCoutHelper (std::size_t pos, std::size_t size)
{
if ( pos < size )
{
T val;
std::memcpy( & val, buffer.data() + pos, sizeof(T) );
std::cout << " - " << val << std::endl;
toCoutHelper<Ts...>(pos+sizeof(T), size);
}
}
template <typename T, typename ... Ts>
typename std::enable_if<0U == sizeof...(Ts), void>::type
toCoutHelper (std::size_t pos, std::size_t size)
{
if ( pos < size )
{
T val;
std::memcpy( & val, buffer.data() + pos, sizeof(T) );
std::cout << " - " << val << std::endl;
toCoutHelper<Args...>(pos+sizeof(T), size);
}
}
};
int main()
{
Batch<float, double, int> b;
b.add(1.0f, 1.0, 1);
b.add(2.0f, 2.0, 2);
b.add(3.0f, 3.0, 3);
b.add(4.0f, 4.0, 4);
b.toCout();
return 0;
}
--- EDIT ---: added a method, toCout()
that print (to std::cout
) all the stored values; just to suggest how to use the values.
--- EDIT 2 ---: As pointed by ildjarn (thanks!) this solution is very dangerous if in the Args...
types are some non POD (Plain Old Data) type.
It's explained well in this page.
I transcribe the relevant part
An example of a type that cannot be safely copied using memcpy is std::string. This is typically implemented using a reference-counted shared pointer, in which case it will have a copy constructor that causes the counter to be incremented. If a copy were made using memcpy then the copy constructor would not be called and the counter would be left with a value one lower than it should be. This would be likely to result in premature deallocation of the memory block that contains the character data.
--- EDIT 3 ---
As pointed by ildjarn (thanks again!) with this solution is very dangerous to leave the data()
member.
If anyone use the pointer returned in this way
char const * pv = (char const *)b.data();
size_t pos = { /* some value here */ };
float f { *(float*)(pv+pos) }; // <-- risk of unaligned access
could, in some architecture, cause an access to a float *
in an unaligned address that can kill the program
The correct (and safe) way to recover values from the pointer returned by data()
is the one used in toCoutHelper()
, using `std::memcpy()
char const * pv = (char const *)b.data();
size_t pos = { /* some value here */ };
float f;
std::memcpy( & f, pv + pos, sizeof(f) );
这篇关于将任意元素存储在连续内存中的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!