Require Help Resolving error C2664 - c++

Question

I have the following code which is giving me this error
main.cpp(41): error C2664: 'std::pair std::make_pair(_Ty1,_Ty2)' : cannot convert argument 1 from 'Handle' to 'unsigned int &'
My example program is
#include <vector>
#include <utility>
typedef unsigned int u32;
typedef u32 Handle;
struct File
{
File()
: ch(0),
pageIdx(0)
{
}
Handle ch : 8;
u32 pageIdx;
};
int main() {
std::vector<std::pair<Handle, u32> > toTrim;
toTrim.reserve(64);
File* m_pFirstPage = new File();
File* pRef = m_pFirstPage;
toTrim.push_back(std::make_pair(pRef->ch,pRef->pageIdx));
return 0;
}
When I try to static cast i.e
toTrim.push_back(std::make_pair(static_cast<unsigned int>(pRef->ch), pRef->pageIdx));
I get the following error
main.cpp(41): error C2664: 'std::pair std::make_pair(_Ty1,_Ty2)' : cannot convert argument 1 from 'unsigned int' to 'unsigned int &'
Can someone please help me resolve it and explain what I am doing wrong.

Answer 1

What is happening is that you are specifying bit fields with the : 8 notation.
More info at:
http://www.tutorialspoint.com/cprogramming/c_bit_fields.htm
That creates a pseudo-char field on your handle variable of 8 bits instead of 32 as typedef u32 Handle defines. std::make_pair requires passing it's arguments by reference.
Since Handle ch : 8 is different type from Handle you can't pass it by reference because is considered undefined behavior cast a variable that is pass by reference.
More info at:
How to cast a variable member to pass it as reference argument of a function
If you need the : 8 field you may use an extra variable to create the pair correctly.
#include <vector>
#include <utility>
typedef unsigned int u32;
typedef u32 Handle;
struct File
{
File()
: ch(0),
pageIdx(0)
{
}
Handle ch : 8; //Different type than just Handle
u32 pageIdx;
};
int main() {
std::vector<std::pair<Handle, u32> > toTrim;
toTrim.reserve(64);
File* m_pFirstPage = new File();
File* pRef = m_pFirstPage;
unsigned int ch_tmp = pRef->ch; //<-Extra variable here
toTrim.push_back(std::make_pair(ch_tmp, pRef->pageIdx));
return 0;
}