How to define a custom cross-platform size_t type?

Question

std::size_t is commonly used for array indexing and loop counting. By definition, std::size_t is the unsigned integer type of the result of the sizeof operator as well as the sizeof... operator and the alignof operator (since C++11). It's defined in the following headers:

• <cstddef>
• <cstdio>
• <cstdlib>
• <cstring>
• <ctime>
• <cwchar>

To my understanding, the type returned by these operators is implementation-defined.

What I want is to define a custom size_t in order to avoid pulling unnecessary stuff from any of the headers mentioned above in a .cpp file of mine, since in my file I only need std::size_t.

In C++11 and above, I thought I could use the following alias:

using size_t = decltype(sizeof(1));

However, I'd like to define a size_t type for pre-C++11 compilers in a portable/cross-platform way.

So is there a portable way to define size_t for pre-C++11?

Answer 1

Well theoretically, if listing of all possible (unsigned) candidates for size_t doesn't bother you, you could make use of SFINAE:

template <class T, class N = void, bool = sizeof(T) == sizeof(sizeof(T))>
struct TL { 
    typedef typename N::type type;
};

template <class T, class N>
struct TL<T, N, true> {
    typedef T type;
};

typedef TL<unsigned short,TL<unsigned int, TL<unsigned long, TL<unsigned long long> > > >::type SizeT;

[live demo]

---

Edit:

Workaround for compilers which differentiate unsigned long from unsigned long long despite the fact that they're assuming sizeof(unsigned long) == sizeof(unsigned long long):

template <class U>
U *declptrval(U);

template <class U>
char is_exact(U *);

template <class U>
short is_exact(...);

template <class T, class N = void, bool = sizeof(is_exact<T>(declptrval(sizeof(T))))==sizeof(char)>
struct TL { 
    typedef typename N::type type;
};

template <class T, class N>
struct TL<T, N, true> {
    typedef T type;
};

typedef TL<unsigned short,TL<unsigned int, TL<unsigned long, TL<unsigned long long> > > >::type SizeT;

[live demo]