Is `size_t` always an alias for `vector<int>::size_type` or any other container type?

问题

Let's make the simplest example possible:

Formulation 1:

std::vector<int> vec;
// add 10E11 elements
for(std::size_t n = 0; n < vec.size(); ++n)
    // ...

Formulation 2:

std::vector<int> vec;
// add 10E11 elements
for(std::vector<int>::size_type n = 0; n < vec.size(); ++n)
    // ...

Naturally, unsigned int or any inappropriate data types do not work here and we have to compile x64. My question is: Is there any case in which the first formulation can lead to issues or can we safely always write it in this much shorter notation? I would also be interesting in similar settings if they are trivial to cover (x86, any other container, other applications of size_type).

回答1:

std::vector guarantees that pointers are valid iterators for its entire sequence, because vector::data returns "A pointer such that [data(), data() + size()) is a valid range." That's pointer addition, which is defined over std::ptrdiff_t, which is the signed version of std::size_t.

Also, [iterator.requirements.general]/6 applies:

… for integral values n and dereferenceable iterator values a and (a + n), *(a + n) is equivalent to *(addressof(*a) + n)…

It's possible for vector::size_type to be narrower than std::size_t, for example 32 bits on a 64-bit system. But it's not something I'd worry about.

回答2:

Although it is the case in all common implementation, the standard makes no guarantee on this. What is guaranteed is that std::vector<T>::size_type is an implementation defined unsigned integer type.

References:

23.2.1 General container requirements [container.requirements.general]

X::size_type [is a] unsigned integer type

and 23.3.6.1 Class template vector overview [vector.overview] §2

typedef implementation-defined size_type; // see 23.2

回答3:

While std::vector::size_type is usually std::size_t, there is no guarantee that it must. It's safer to use

for(std::vector<int>::size_type n = 0; n < vec.size(); ++n)

来源：https://stackoverflow.com/questions/41501798/is-size-t-always-an-alias-for-vectorintsize-type-or-any-other-container