Select a method as (default) template parameter - c++

Question

I'm developing a container template class. This code needs to interface with existing C code and needs to stay binary compatible, so I can not use i.e. std::vector or similar.
The problem that I have is that it needs to support different allocation strategies, and I don't know how to provide the allocator as a template argument. I created an SSCCE to illustrate how far I got (which of course doesn't compile, because if it would, I wouldn't need to ask this question :)).
#include <iostream>
#include <cstring>
#include <type_traits>
typedef unsigned int uint_t;
typedef signed int int_t;
template <typename T, typename S, typename _allocator = _virtual>
class Container
{
public:
Container(S nItems = 0, S nMaxItems = 0, T *pArray = NULL)
{
mItems = nItems;
mMaxItems = nMaxItems;
mArray = pArray;
}
void adjustMalloc(uint_t nClusterSize)
{
if(mItems == mMaxItems)
{
mArray = (T *)realloc(mArray, (mMaxItems+nClusterSize)*sizeof(T));
mMaxItems += nClusterSize;
}
}
void adjustAligned(uint_t nClusterSize)
{
if(mItems == mMaxItems)
{
mArray = (T *)_aligned_realloc(mArray, (mMaxItems+nClusterSize)*sizeof(T), 16);
mMaxItems += nClusterSize;
}
}
void adjustVirtual(uint_t nClusterSize)
{
if(mItems == mMaxItems)
{
mArray = VirtualAlloc(mArray, (mMaxItems+nClusterSize)*sizeof(T), MEM_RESERVE, PAGE_NOACCESS);
mMaxItems += nClusterSize;
}
}
void adjust(uint_t nClusterSize)
{
if (std::is_same<_allocator>::value == _virtual)
adjustVirtual(nClusterSize);
else if(std::is_same<_allocator>::value == _aligned)
adjustAligned(nClusterSize);
else if(std::is_same<_allocator>::value == _malloc)
adjustMalloc(nClusterSize);
else
{
// Cause a static compiler error, how?
}
}
bool add(T *pItem)
{
if(find(pItem) == NULL)
{
adjust(100);
mItems++;
return true; // added
}
return false;
}
T *find(T *pItem)
{
T *p = mArray;
for(S i = 0; i < mItems; i++, p++)
{
if(*p == *pItem)
return p;
}
return NULL;
}
private:
S mItems;
S mMaxItems;
T *mArray;
};
class Record
{
public:
bool operator==(const Record &oRecord)
{
if(Id != oRecord.Id)
return false;
if(strcmp(Name, oRecord.Name) != 0)
return false;
return true;
}
int Id;
char Name[10+1];
};
int main(int argc, char *argv[])
{
Record rec;
rec.Id = 0;
strcpy(rec.Name, "Test");
Container<Record, uint_t> records; // Default using malloc
records.add(&rec);
if(records.find(&rec) == NULL)
std::cerr << "Not found" << std::endl;
Container<Record, uint_t, _virtual> vrecords; // VirtualAlloc allocator used.
vrecords.add(&rec);
if(records.find(&rec) == NULL)
std::cerr << "Not found" << std::endl;
return 0;
}
I'm using Visual Studio 2010 so it's not 100% C++11.
The VirtualAlloc is provided just as (another) example and will not work as it is shown here.

Answer 1

I found a solution for my problem. However, I get warnings
warning C4127: conditional expression is constant
in the adjust() method for the if(std::is_same... and I was wondering if this is normal or if I can get rid of it, other than disabling it.
#include "stdafx.h"
#include "windows.h"
#include <iostream>
#include <cstring>
#include <type_traits>
#pragma warning (push)
//#pragma warning (disable : 4127)
typedef unsigned int uint_t;
typedef signed int int_t;
typedef struct { const static bool _virtual_allocator = true; } _virtual_type;
typedef struct { const static bool _aligned_allocator = true; } _aligned_type;
typedef struct { const static bool _malloc_allocator = true; } _malloc_type;
template <typename T, typename S, typename _allocator = _aligned_type>
class Container
{
public:
Container(S nItems = 0, S nMaxItems = 0, T *pArray = NULL)
{
mItems = nItems;
mMaxItems = nMaxItems;
mArray = pArray;
}
void adjustMalloc(uint_t nClusterSize)
{
if(mItems == mMaxItems)
{
mArray = (T *)realloc(mArray, (mMaxItems+nClusterSize)*sizeof(T));
mMaxItems += nClusterSize;
}
}
void adjustAligned(uint_t nClusterSize)
{
if(mItems == mMaxItems)
{
mArray = (T *)_aligned_realloc(mArray, (mMaxItems+nClusterSize)*sizeof(T), 16);
mMaxItems += nClusterSize;
}
}
void adjustVirtual(uint_t nClusterSize)
{
if(mItems == mMaxItems)
{
mArray = (T *)VirtualAlloc((LPVOID)mArray, (mMaxItems+nClusterSize)*sizeof(T), MEM_RESERVE, PAGE_NOACCESS);
mMaxItems += nClusterSize;
}
}
void adjust(uint_t nClusterSize)
{
if (std::is_same<_allocator, _virtual_type>::value)
adjustVirtual(nClusterSize);
else if(std::is_same<_allocator, _aligned_type>::value)
adjustAligned(nClusterSize);
else if(std::is_same<_allocator, _malloc_type>::value)
adjustMalloc(nClusterSize);
else
{
// Cause a static compiler error, how?
}
}
bool add(T *pItem)
{
if(find(pItem) == NULL)
{
adjust(100);
mItems++;
return true; // added
}
return false;
}
T *find(T *pItem)
{
T *p = mArray;
for(S i = 0; i < mItems; i++, p++)
{
if(*p == *pItem)
return p;
}
return NULL;
}
private:
S mItems;
S mMaxItems;
T *mArray;
};
#pragma warning (pop)
class Record
{
public:
bool operator==(const Record &oRecord)
{
if(Id != oRecord.Id)
return false;
if(strcmp(Name, oRecord.Name) != 0)
return false;
return true;
}
int Id;
char Name[10+1];
};
int main(int argc, char *argv[])
{
Record rec;
rec.Id = 0;
strcpy(rec.Name, "Test");
Container<Record, uint_t> mrecords;
mrecords.add(&rec);
if(mrecords.find(&rec) == NULL)
std::cerr << "Malloc Not found" << std::endl;
Container<Record, uint_t, _aligned_type> arecords;
arecords.add(&rec);
if(arecords.find(&rec) == NULL)
std::cerr << "Aligned Not found" << std::endl;
Container<Record, uint_t, _virtual_type> vrecords;
vrecords.add(&rec);
if(vrecords.find(&rec) == NULL)
std::cerr << "Virtual Not found" << std::endl;
return 0;
}